Can List have multiple columns?

In today’s post we’ll learn to subset multiple columns from a Pandas dataframe. We’ll look into three methods:

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Data Preparation
Select multiple columns in a Python DataFrame
Select Python columns by condition
Drop multiple columns
Video liên quan

Slicing using the “brackets notation”
Using the loc indexer
Using the iloc indexer

Data Preparation

Go ahead and import the pandas library into your Python development environment and create your DataFrame.

import pandas as pd 'read the data from a csv file data = pd.read_csv('hr.csv') 'Preview the DataFrame print(data)

Here’s our DataFrame:

	language	avg_salary	candidates
1	C#	85.0	82.0
2	Java	72.0	78.0
3	JavaScript	81.0	78.0
4	Ruby	82.0	87.0

Select multiple columns in a Python DataFrame

Using the brackets notation:

When using this technique we’ll subset the DataFrame using a list containing the required column labels.

cols = ['language', 'candidates'] data[cols]

Here’s the result:

	language	candidates
1	C#	82.0
2	Java	78.0
3	JavaScript	78.0
4	Ruby	87.0

Using the loc indexer:

We can achieve the same result by passing the column labels as a parameter to the loc indexer. Here’s the code:

cols = ['language', 'candidates'] data.loc[:,cols]

Using the iloc indexer

We use the iloc indexer to slice one or several distinct columns ranges out of a DataFrame by index.

First let’s take a look at the DF columns index.

data.columns

Here’s our DataFrame index:

Index(['language', 'avg_salary', 'candidates'], dtype='object')

We are interested in the first two columns, so we’ll slice the index accordingly:

data.iloc[:,0:2] # alternatively, we can use the following code: data.iloc[:,range(2)]

Select Python columns by condition

You might want to subset your data according to specific logic related to the column labels or index.

Here’s a quick example that allows to achieve the same data subset we seen before:

filt = data.columns.str.contains('language') | data.columns.str.contains('avg') #pass the boolean filter to a loc indexer data.loc[:,filt]

Drop multiple columns

A somewhat related topic is how to delete specific columns from your DataFrame. We have obviously covered it in a previous tutorial. You can find it here.

Additional learning:

The axis labeling information in pandas objects serves many purposes:

Identifies data (i.e. provides metadata) using known indicators, important for analysis, visualization, and interactive console display.
Enables automatic and explicit data alignment.
Allows intuitive getting and setting of subsets of the data set.

In this section, we will focus on the final point: namely, how to slice, dice, and generally get and set subsets of pandas objects. The primary focus will be on Series and DataFrame as they have received more development attention in this area.

Note

The Python and NumPy indexing operators [] and attribute operator . provide quick and easy access to pandas data structures across a wide range of use cases. This makes interactive work intuitive, as there’s little new to learn if you already know how to deal with Python dictionaries and NumPy arrays. However, since the type of the data to be accessed isn’t known in advance, directly using standard operators has some optimization limits. For production code, we recommended that you take advantage of the optimized pandas data access methods exposed in this chapter.

Warning

Whether a copy or a reference is returned for a setting operation, may depend on the context. This is sometimes called chained assignment and should be avoided. See Returning a View versus Copy.

See the MultiIndex / Advanced Indexing for MultiIndex and more advanced indexing documentation.

See the cookbook for some advanced strategies.

Object selection has had a number of user-requested additions in order to support more explicit location based indexing. pandas now supports three types of multi-axis indexing.

.loc is primarily label based, but may also be used with a boolean array. .loc will raise KeyError when the items are not found. Allowed inputs are:
- A single label, e.g. 5 or 'a' (Note that 5 is interpreted as a label of the index. This use is not an integer position along the index.).
- A list or array of labels ['a', 'b', 'c'].
- A slice object with labels 'a':'f' (Note that contrary to usual Python slices, both the start and the stop are included, when present in the index! See Slicing with labels and Endpoints are inclusive.)
- A boolean array (any NA values will be treated as False).
- A callable function with one argument (the calling Series or DataFrame) and that returns valid output for indexing (one of the above).
See more at Selection by Label.
.iloc is primarily integer position based (from 0 to length-1 of the axis), but may also be used with a boolean array. .iloc will raise IndexError if a requested indexer is out-of-bounds, except slice indexers which allow out-of-bounds indexing. (this conforms with Python/NumPy slice semantics). Allowed inputs are:
- An integer e.g. 5.
- A list or array of integers [4, 3, 0].
- A slice object with ints 1:7.
- A boolean array (any NA values will be treated as False).
- A callable function with one argument (the calling Series or DataFrame) and that returns valid output for indexing (one of the above).
See more at Selection by Position, Advanced Indexing and Advanced Hierarchical.
.loc, .iloc, and also [] indexing can accept a callable as indexer. See more at Selection By Callable.

Getting values from an object with multi-axes selection uses the following notation (using .loc as an example, but the following applies to .iloc as well). Any of the axes accessors may be the null slice :. Axes left out of the specification are assumed to be :, e.g. p.loc['a'] is equivalent to p.loc['a', :, :].

Object Type	Indexers
Series	s.loc[indexer]
DataFrame	df.loc[row_indexer,column_indexer]

As mentioned when introducing the data structures in the last section, the primary function of indexing with [] (a.k.a. __getitem__ for those familiar with implementing class behavior in Python) is selecting out lower-dimensional slices. The following table shows return type values when indexing pandas objects with []:

Object Type	Selection	Return Value Type
Series	series[label]	scalar value
DataFrame	frame[colname]	Series corresponding to colname

Here we construct a simple time series data set to use for illustrating the indexing functionality:

In [1]: dates = pd.date_range('1/1/2000', periods=8) In [2]: df = pd.DataFrame(np.random.randn(8, 4), ...: index=dates, columns=['A', 'B', 'C', 'D']) ...: In [3]: df Out[3]: A B C D 2000-01-01 0.469112 -0.282863 -1.509059 -1.135632 2000-01-02 1.212112 -0.173215 0.119209 -1.044236 2000-01-03 -0.861849 -2.104569 -0.494929 1.071804 2000-01-04 0.721555 -0.706771 -1.039575 0.271860 2000-01-05 -0.424972 0.567020 0.276232 -1.087401 2000-01-06 -0.673690 0.113648 -1.478427 0.524988 2000-01-07 0.404705 0.577046 -1.715002 -1.039268 2000-01-08 -0.370647 -1.157892 -1.344312 0.844885

Note

None of the indexing functionality is time series specific unless specifically stated.

Thus, as per above, we have the most basic indexing using []:

In [4]: s = df['A'] In [5]: s[dates[5]] Out[5]: -0.6736897080883706

You can pass a list of columns to [] to select columns in that order. If a column is not contained in the DataFrame, an exception will be raised. Multiple columns can also be set in this manner:

In [6]: df Out[6]: A B C D 2000-01-01 0.469112 -0.282863 -1.509059 -1.135632 2000-01-02 1.212112 -0.173215 0.119209 -1.044236 2000-01-03 -0.861849 -2.104569 -0.494929 1.071804 2000-01-04 0.721555 -0.706771 -1.039575 0.271860 2000-01-05 -0.424972 0.567020 0.276232 -1.087401 2000-01-06 -0.673690 0.113648 -1.478427 0.524988 2000-01-07 0.404705 0.577046 -1.715002 -1.039268 2000-01-08 -0.370647 -1.157892 -1.344312 0.844885 In [7]: df[['B', 'A']] = df[['A', 'B']] In [8]: df Out[8]: A B C D 2000-01-01 -0.282863 0.469112 -1.509059 -1.135632 2000-01-02 -0.173215 1.212112 0.119209 -1.044236 2000-01-03 -2.104569 -0.861849 -0.494929 1.071804 2000-01-04 -0.706771 0.721555 -1.039575 0.271860 2000-01-05 0.567020 -0.424972 0.276232 -1.087401 2000-01-06 0.113648 -0.673690 -1.478427 0.524988 2000-01-07 0.577046 0.404705 -1.715002 -1.039268 2000-01-08 -1.157892 -0.370647 -1.344312 0.844885

You may find this useful for applying a transform (in-place) to a subset of the columns.

Warning

pandas aligns all AXES when setting Series and DataFrame from .loc, and .iloc.

This will not modify df because the column alignment is before value assignment.

In [9]: df[['A', 'B']] Out[9]: A B 2000-01-01 -0.282863 0.469112 2000-01-02 -0.173215 1.212112 2000-01-03 -2.104569 -0.861849 2000-01-04 -0.706771 0.721555 2000-01-05 0.567020 -0.424972 2000-01-06 0.113648 -0.673690 2000-01-07 0.577046 0.404705 2000-01-08 -1.157892 -0.370647 In [10]: df.loc[:, ['B', 'A']] = df[['A', 'B']] In [11]: df[['A', 'B']] Out[11]: A B 2000-01-01 -0.282863 0.469112 2000-01-02 -0.173215 1.212112 2000-01-03 -2.104569 -0.861849 2000-01-04 -0.706771 0.721555 2000-01-05 0.567020 -0.424972 2000-01-06 0.113648 -0.673690 2000-01-07 0.577046 0.404705 2000-01-08 -1.157892 -0.370647

The correct way to swap column values is by using raw values:

In [12]: df.loc[:, ['B', 'A']] = df[['A', 'B']].to_numpy() In [13]: df[['A', 'B']] Out[13]: A B 2000-01-01 0.469112 -0.282863 2000-01-02 1.212112 -0.173215 2000-01-03 -0.861849 -2.104569 2000-01-04 0.721555 -0.706771 2000-01-05 -0.424972 0.567020 2000-01-06 -0.673690 0.113648 2000-01-07 0.404705 0.577046 2000-01-08 -0.370647 -1.157892

You may access an index on a Series or column on a DataFrame directly as an attribute:

In [14]: sa = pd.Series([1, 2, 3], index=list('abc')) In [15]: dfa = df.copy()

In [16]: sa.b Out[16]: 2 In [17]: dfa.A Out[17]: 2000-01-01 0.469112 2000-01-02 1.212112 2000-01-03 -0.861849 2000-01-04 0.721555 2000-01-05 -0.424972 2000-01-06 -0.673690 2000-01-07 0.404705 2000-01-08 -0.370647 Freq: D, Name: A, dtype: float64

In [18]: sa.a = 5 In [19]: sa Out[19]: a 5 b 2 c 3 dtype: int64 In [20]: dfa.A = list(range(len(dfa.index))) # ok if A already exists In [21]: dfa Out[21]: A B C D 2000-01-01 0 -0.282863 -1.509059 -1.135632 2000-01-02 1 -0.173215 0.119209 -1.044236 2000-01-03 2 -2.104569 -0.494929 1.071804 2000-01-04 3 -0.706771 -1.039575 0.271860 2000-01-05 4 0.567020 0.276232 -1.087401 2000-01-06 5 0.113648 -1.478427 0.524988 2000-01-07 6 0.577046 -1.715002 -1.039268 2000-01-08 7 -1.157892 -1.344312 0.844885 In [22]: dfa['A'] = list(range(len(dfa.index))) # use this form to create a new column In [23]: dfa Out[23]: A B C D 2000-01-01 0 -0.282863 -1.509059 -1.135632 2000-01-02 1 -0.173215 0.119209 -1.044236 2000-01-03 2 -2.104569 -0.494929 1.071804 2000-01-04 3 -0.706771 -1.039575 0.271860 2000-01-05 4 0.567020 0.276232 -1.087401 2000-01-06 5 0.113648 -1.478427 0.524988 2000-01-07 6 0.577046 -1.715002 -1.039268 2000-01-08 7 -1.157892 -1.344312 0.844885

Warning

You can use this access only if the index element is a valid Python identifier, e.g. s.1 is not allowed. See here for an explanation of valid identifiers.
The attribute will not be available if it conflicts with an existing method name, e.g. s.min is not allowed, but s['min'] is possible.
Similarly, the attribute will not be available if it conflicts with any of the following list: index, major_axis, minor_axis, items.
In any of these cases, standard indexing will still work, e.g. s['1'], s['min'], and s['index'] will access the corresponding element or column.

If you are using the IPython environment, you may also use tab-completion to see these accessible attributes.

You can also assign a dict to a row of a DataFrame:

In [24]: x = pd.DataFrame({'x': [1, 2, 3], 'y': [3, 4, 5]}) In [25]: x.iloc[1] = {'x': 9, 'y': 99} In [26]: x Out[26]: x y 0 1 3 1 9 99 2 3 5

You can use attribute access to modify an existing element of a Series or column of a DataFrame, but be careful; if you try to use attribute access to create a new column, it creates a new attribute rather than a new column. In 0.21.0 and later, this will raise a UserWarning:

In [1]: df = pd.DataFrame({'one': [1., 2., 3.]}) In [2]: df.two = [4, 5, 6] UserWarning: Pandas doesn't allow Series to be assigned into nonexistent columns - see https://pandas.pydata.org/pandas-docs/stable/indexing.html#attribute_access In [3]: df Out[3]: one 0 1.0 1 2.0 2 3.0

The most robust and consistent way of slicing ranges along arbitrary axes is described in the Selection by Position section detailing the .iloc method. For now, we explain the semantics of slicing using the [] operator.

With Series, the syntax works exactly as with an ndarray, returning a slice of the values and the corresponding labels:

In [27]: s[:5] Out[27]: 2000-01-01 0.469112 2000-01-02 1.212112 2000-01-03 -0.861849 2000-01-04 0.721555 2000-01-05 -0.424972 Freq: D, Name: A, dtype: float64 In [28]: s[::2] Out[28]: 2000-01-01 0.469112 2000-01-03 -0.861849 2000-01-05 -0.424972 2000-01-07 0.404705 Freq: 2D, Name: A, dtype: float64 In [29]: s[::-1] Out[29]: 2000-01-08 -0.370647 2000-01-07 0.404705 2000-01-06 -0.673690 2000-01-05 -0.424972 2000-01-04 0.721555 2000-01-03 -0.861849 2000-01-02 1.212112 2000-01-01 0.469112 Freq: -1D, Name: A, dtype: float64

Note that setting works as well:

In [30]: s2 = s.copy() In [31]: s2[:5] = 0 In [32]: s2 Out[32]: 2000-01-01 0.000000 2000-01-02 0.000000 2000-01-03 0.000000 2000-01-04 0.000000 2000-01-05 0.000000 2000-01-06 -0.673690 2000-01-07 0.404705 2000-01-08 -0.370647 Freq: D, Name: A, dtype: float64

With DataFrame, slicing inside of [] slices the rows. This is provided largely as a convenience since it is such a common operation.

In [33]: df[:3] Out[33]: A B C D 2000-01-01 0.469112 -0.282863 -1.509059 -1.135632 2000-01-02 1.212112 -0.173215 0.119209 -1.044236 2000-01-03 -0.861849 -2.104569 -0.494929 1.071804 In [34]: df[::-1] Out[34]: A B C D 2000-01-08 -0.370647 -1.157892 -1.344312 0.844885 2000-01-07 0.404705 0.577046 -1.715002 -1.039268 2000-01-06 -0.673690 0.113648 -1.478427 0.524988 2000-01-05 -0.424972 0.567020 0.276232 -1.087401 2000-01-04 0.721555 -0.706771 -1.039575 0.271860 2000-01-03 -0.861849 -2.104569 -0.494929 1.071804 2000-01-02 1.212112 -0.173215 0.119209 -1.044236 2000-01-01 0.469112 -0.282863 -1.509059 -1.135632

Warning

Whether a copy or a reference is returned for a setting operation, may depend on the context. This is sometimes called chained assignment and should be avoided. See Returning a View versus Copy.

Warning

.loc is strict when you present slicers that are not compatible (or convertible) with the index type. For example using integers in a DatetimeIndex. These will raise a TypeError.

In [35]: dfl = pd.DataFrame(np.random.randn(5, 4), ....: columns=list('ABCD'), ....: index=pd.date_range('20130101', periods=5)) ....: In [36]: dfl Out[36]: A B C D 2013-01-01 1.075770 -0.109050 1.643563 -1.469388 2013-01-02 0.357021 -0.674600 -1.776904 -0.968914 2013-01-03 -1.294524 0.413738 0.276662 -0.472035 2013-01-04 -0.013960 -0.362543 -0.006154 -0.923061 2013-01-05 0.895717 0.805244 -1.206412 2.565646

In [4]: dfl.loc[2:3] TypeError: cannot do slice indexing on with these indexers [2] of

String likes in slicing can be convertible to the type of the index and lead to natural slicing.

In [37]: dfl.loc['20130102':'20130104'] Out[37]: A B C D 2013-01-02 0.357021 -0.674600 -1.776904 -0.968914 2013-01-03 -1.294524 0.413738 0.276662 -0.472035 2013-01-04 -0.013960 -0.362543 -0.006154 -0.923061

pandas provides a suite of methods in order to have purely label based indexing. This is a strict inclusion based protocol. Every label asked for must be in the index, or a KeyError will be raised. When slicing, both the start bound AND the stop bound are included, if present in the index. Integers are valid labels, but they refer to the label and not the position.

The .loc attribute is the primary access method. The following are valid inputs:

A single label, e.g. 5 or 'a' (Note that 5 is interpreted as a label of the index. This use is not an integer position along the index.).
A list or array of labels ['a', 'b', 'c'].
A slice object with labels 'a':'f' (Note that contrary to usual Python slices, both the start and the stop are included, when present in the index! See Slicing with labels.
A boolean array.
A callable, see Selection By Callable.

In [38]: s1 = pd.Series(np.random.randn(6), index=list('abcdef')) In [39]: s1 Out[39]: a 1.431256 b 1.340309 c -1.170299 d -0.226169 e 0.410835 f 0.813850 dtype: float64 In [40]: s1.loc['c':] Out[40]: c -1.170299 d -0.226169 e 0.410835 f 0.813850 dtype: float64 In [41]: s1.loc['b'] Out[41]: 1.3403088497993827

Note that setting works as well:

In [42]: s1.loc['c':] = 0 In [43]: s1 Out[43]: a 1.431256 b 1.340309 c 0.000000 d 0.000000 e 0.000000 f 0.000000 dtype: float64

With a DataFrame:

In [44]: df1 = pd.DataFrame(np.random.randn(6, 4), ....: index=list('abcdef'), ....: columns=list('ABCD')) ....: In [45]: df1 Out[45]: A B C D a 0.132003 -0.827317 -0.076467 -1.187678 b 1.130127 -1.436737 -1.413681 1.607920 c 1.024180 0.569605 0.875906 -2.211372 d 0.974466 -2.006747 -0.410001 -0.078638 e 0.545952 -1.219217 -1.226825 0.769804 f -1.281247 -0.727707 -0.121306 -0.097883 In [46]: df1.loc[['a', 'b', 'd'], :] Out[46]: A B C D a 0.132003 -0.827317 -0.076467 -1.187678 b 1.130127 -1.436737 -1.413681 1.607920 d 0.974466 -2.006747 -0.410001 -0.078638

Accessing via label slices:

In [47]: df1.loc['d':, 'A':'C'] Out[47]: A B C d 0.974466 -2.006747 -0.410001 e 0.545952 -1.219217 -1.226825 f -1.281247 -0.727707 -0.121306

For getting a cross section using a label (equivalent to df.xs('a')):

In [48]: df1.loc['a'] Out[48]: A 0.132003 B -0.827317 C -0.076467 D -1.187678 Name: a, dtype: float64

For getting values with a boolean array:

In [49]: df1.loc['a'] > 0 Out[49]: A True B False C False D False Name: a, dtype: bool In [50]: df1.loc[:, df1.loc['a'] > 0] Out[50]: A a 0.132003 b 1.130127 c 1.024180 d 0.974466 e 0.545952 f -1.281247

NA values in a boolean array propagate as False:

Changed in version 1.0.2.

In [51]: mask = pd.array([True, False, True, False, pd.NA, False], dtype="boolean") In [52]: mask Out[52]: [True, False, True, False, , False] Length: 6, dtype: boolean In [53]: df1[mask] Out[53]: A B C D a 0.132003 -0.827317 -0.076467 -1.187678 c 1.024180 0.569605 0.875906 -2.211372

For getting a value explicitly:

# this is also equivalent to ``df1.at['a','A']`` In [54]: df1.loc['a', 'A'] Out[54]: 0.13200317033032932

When using .loc with slices, if both the start and the stop labels are present in the index, then elements located between the two (including them) are returned:

In [55]: s = pd.Series(list('abcde'), index=[0, 3, 2, 5, 4]) In [56]: s.loc[3:5] Out[56]: 3 b 2 c 5 d dtype: object

If at least one of the two is absent, but the index is sorted, and can be compared against start and stop labels, then slicing will still work as expected, by selecting labels which rank between the two:

In [57]: s.sort_index() Out[57]: 0 a 2 c 3 b 4 e 5 d dtype: object In [58]: s.sort_index().loc[1:6] Out[58]: 2 c 3 b 4 e 5 d dtype: object

However, if at least one of the two is absent and the index is not sorted, an error will be raised (since doing otherwise would be computationally expensive, as well as potentially ambiguous for mixed type indexes). For instance, in the above example, s.loc[1:6] would raise KeyError.

For the rationale behind this behavior, see Endpoints are inclusive.

In [59]: s = pd.Series(list('abcdef'), index=[0, 3, 2, 5, 4, 2]) In [60]: s.loc[3:5] Out[60]: 3 b 2 c 5 d dtype: object

Also, if the index has duplicate labels and either the start or the stop label is duplicated, an error will be raised. For instance, in the above example, s.loc[2:5] would raise a KeyError.

For more information about duplicate labels, see Duplicate Labels.

Warning

Whether a copy or a reference is returned for a setting operation, may depend on the context. This is sometimes called chained assignment and should be avoided. See Returning a View versus Copy.

pandas provides a suite of methods in order to get purely integer based indexing. The semantics follow closely Python and NumPy slicing. These are 0-based indexing. When slicing, the start bound is included, while the upper bound is excluded. Trying to use a non-integer, even a valid label will raise an IndexError.

The .iloc attribute is the primary access method. The following are valid inputs:

An integer e.g. 5.
A list or array of integers [4, 3, 0].
A slice object with ints 1:7.
A boolean array.
A callable, see Selection By Callable.

In [61]: s1 = pd.Series(np.random.randn(5), index=list(range(0, 10, 2))) In [62]: s1 Out[62]: 0 0.695775 2 0.341734 4 0.959726 6 -1.110336 8 -0.619976 dtype: float64 In [63]: s1.iloc[:3] Out[63]: 0 0.695775 2 0.341734 4 0.959726 dtype: float64 In [64]: s1.iloc[3] Out[64]: -1.110336102891167

Note that setting works as well:

In [65]: s1.iloc[:3] = 0 In [66]: s1 Out[66]: 0 0.000000 2 0.000000 4 0.000000 6 -1.110336 8 -0.619976 dtype: float64

With a DataFrame:

In [67]: df1 = pd.DataFrame(np.random.randn(6, 4), ....: index=list(range(0, 12, 2)), ....: columns=list(range(0, 8, 2))) ....: In [68]: df1 Out[68]: 0 2 4 6 0 0.149748 -0.732339 0.687738 0.176444 2 0.403310 -0.154951 0.301624 -2.179861 4 -1.369849 -0.954208 1.462696 -1.743161 6 -0.826591 -0.345352 1.314232 0.690579 8 0.995761 2.396780 0.014871 3.357427 10 -0.317441 -1.236269 0.896171 -0.487602

Select via integer slicing:

In [69]: df1.iloc[:3] Out[69]: 0 2 4 6 0 0.149748 -0.732339 0.687738 0.176444 2 0.403310 -0.154951 0.301624 -2.179861 4 -1.369849 -0.954208 1.462696 -1.743161 In [70]: df1.iloc[1:5, 2:4] Out[70]: 4 6 2 0.301624 -2.179861 4 1.462696 -1.743161 6 1.314232 0.690579 8 0.014871 3.357427

Select via integer list:

In [71]: df1.iloc[[1, 3, 5], [1, 3]] Out[71]: 2 6 2 -0.154951 -2.179861 6 -0.345352 0.690579 10 -1.236269 -0.487602

In [72]: df1.iloc[1:3, :] Out[72]: 0 2 4 6 2 0.403310 -0.154951 0.301624 -2.179861 4 -1.369849 -0.954208 1.462696 -1.743161

In [73]: df1.iloc[:, 1:3] Out[73]: 2 4 0 -0.732339 0.687738 2 -0.154951 0.301624 4 -0.954208 1.462696 6 -0.345352 1.314232 8 2.396780 0.014871 10 -1.236269 0.896171

# this is also equivalent to ``df1.iat[1,1]`` In [74]: df1.iloc[1, 1] Out[74]: -0.1549507744249032

For getting a cross section using an integer position (equiv to df.xs(1)):

In [75]: df1.iloc[1] Out[75]: 0 0.403310 2 -0.154951 4 0.301624 6 -2.179861 Name: 2, dtype: float64

Out of range slice indexes are handled gracefully just as in Python/NumPy.

# these are allowed in Python/NumPy. In [76]: x = list('abcdef') In [77]: x Out[77]: ['a', 'b', 'c', 'd', 'e', 'f'] In [78]: x[4:10] Out[78]: ['e', 'f'] In [79]: x[8:10] Out[79]: [] In [80]: s = pd.Series(x) In [81]: s Out[81]: 0 a 1 b 2 c 3 d 4 e 5 f dtype: object In [82]: s.iloc[4:10] Out[82]: 4 e 5 f dtype: object In [83]: s.iloc[8:10] Out[83]: Series([], dtype: object)

Note that using slices that go out of bounds can result in an empty axis (e.g. an empty DataFrame being returned).

In [84]: dfl = pd.DataFrame(np.random.randn(5, 2), columns=list('AB')) In [85]: dfl Out[85]: A B 0 -0.082240 -2.182937 1 0.380396 0.084844 2 0.432390 1.519970 3 -0.493662 0.600178 4 0.274230 0.132885 In [86]: dfl.iloc[:, 2:3] Out[86]: Empty DataFrame Columns: [] Index: [0, 1, 2, 3, 4] In [87]: dfl.iloc[:, 1:3] Out[87]: B 0 -2.182937 1 0.084844 2 1.519970 3 0.600178 4 0.132885 In [88]: dfl.iloc[4:6] Out[88]: A B 4 0.27423 0.132885

A single indexer that is out of bounds will raise an IndexError. A list of indexers where any element is out of bounds will raise an IndexError.

>>> dfl.iloc[[4, 5, 6]] IndexError: positional indexers are out-of-bounds >>> dfl.iloc[:, 4] IndexError: single positional indexer is out-of-bounds

.loc, .iloc, and also [] indexing can accept a callable as indexer. The callable must be a function with one argument (the calling Series or DataFrame) that returns valid output for indexing.

In [89]: df1 = pd.DataFrame(np.random.randn(6, 4), ....: index=list('abcdef'), ....: columns=list('ABCD')) ....: In [90]: df1 Out[90]: A B C D a -0.023688 2.410179 1.450520 0.206053 b -0.251905 -2.213588 1.063327 1.266143 c 0.299368 -0.863838 0.408204 -1.048089 d -0.025747 -0.988387 0.094055 1.262731 e 1.289997 0.082423 -0.055758 0.536580 f -0.489682 0.369374 -0.034571 -2.484478 In [91]: df1.loc[lambda df: df['A'] > 0, :] Out[91]: A B C D c 0.299368 -0.863838 0.408204 -1.048089 e 1.289997 0.082423 -0.055758 0.536580 In [92]: df1.loc[:, lambda df: ['A', 'B']] Out[92]: A B a -0.023688 2.410179 b -0.251905 -2.213588 c 0.299368 -0.863838 d -0.025747 -0.988387 e 1.289997 0.082423 f -0.489682 0.369374 In [93]: df1.iloc[:, lambda df: [0, 1]] Out[93]: A B a -0.023688 2.410179 b -0.251905 -2.213588 c 0.299368 -0.863838 d -0.025747 -0.988387 e 1.289997 0.082423 f -0.489682 0.369374 In [94]: df1[lambda df: df.columns[0]] Out[94]: a -0.023688 b -0.251905 c 0.299368 d -0.025747 e 1.289997 f -0.489682 Name: A, dtype: float64

You can use callable indexing in Series.

In [95]: df1['A'].loc[lambda s: s > 0] Out[95]: c 0.299368 e 1.289997 Name: A, dtype: float64

Using these methods / indexers, you can chain data selection operations without using a temporary variable.

In [96]: bb = pd.read_csv('data/baseball.csv', index_col='id') In [97]: (bb.groupby(['year', 'team']).sum() ....: .loc[lambda df: df['r'] > 100]) ....: Out[97]: stint g ab r h X2b X3b hr rbi sb cs bb so ibb hbp sh sf gidp year team 2007 CIN 6 379 745 101 203 35 2 36 125.0 10.0 1.0 105 127.0 14.0 1.0 1.0 15.0 18.0 DET 5 301 1062 162 283 54 4 37 144.0 24.0 7.0 97 176.0 3.0 10.0 4.0 8.0 28.0 HOU 4 311 926 109 218 47 6 14 77.0 10.0 4.0 60 212.0 3.0 9.0 16.0 6.0 17.0 LAN 11 413 1021 153 293 61 3 36 154.0 7.0 5.0 114 141.0 8.0 9.0 3.0 8.0 29.0 NYN 13 622 1854 240 509 101 3 61 243.0 22.0 4.0 174 310.0 24.0 23.0 18.0 15.0 48.0 SFN 5 482 1305 198 337 67 6 40 171.0 26.0 7.0 235 188.0 51.0 8.0 16.0 6.0 41.0 TEX 2 198 729 115 200 40 4 28 115.0 21.0 4.0 73 140.0 4.0 5.0 2.0 8.0 16.0 TOR 4 459 1408 187 378 96 2 58 223.0 4.0 2.0 190 265.0 16.0 12.0 4.0 16.0 38.0

If you wish to get the 0th and the 2nd elements from the index in the ‘A’ column, you can do:

In [98]: dfd = pd.DataFrame({'A': [1, 2, 3], ....: 'B': [4, 5, 6]}, ....: index=list('abc')) ....: In [99]: dfd Out[99]: A B a 1 4 b 2 5 c 3 6 In [100]: dfd.loc[dfd.index[[0, 2]], 'A'] Out[100]: a 1 c 3 Name: A, dtype: int64

This can also be expressed using .iloc, by explicitly getting locations on the indexers, and using positional indexing to select things.

In [101]: dfd.iloc[[0, 2], dfd.columns.get_loc('A')] Out[101]: a 1 c 3 Name: A, dtype: int64

For getting multiple indexers, using .get_indexer:

In [102]: dfd.iloc[[0, 2], dfd.columns.get_indexer(['A', 'B'])] Out[102]: A B a 1 4 c 3 6

Warning

Changed in version 1.0.0.

Using .loc or [] with a list with one or more missing labels will no longer reindex, in favor of .reindex.

In prior versions, using .loc[list-of-labels] would work as long as at least 1 of the keys was found (otherwise it would raise a KeyError). This behavior was changed and will now raise a KeyError if at least one label is missing. The recommended alternative is to use .reindex().

For example.

In [103]: s = pd.Series([1, 2, 3]) In [104]: s Out[104]: 0 1 1 2 2 3 dtype: int64

Selection with all keys found is unchanged.

In [105]: s.loc[[1, 2]] Out[105]: 1 2 2 3 dtype: int64

Previous behavior

In [4]: s.loc[[1, 2, 3]] Out[4]: 1 2.0 2 3.0 3 NaN dtype: float64

Current behavior

In [4]: s.loc[[1, 2, 3]] Passing list-likes to .loc with any non-matching elements will raise KeyError in the future, you can use .reindex() as an alternative. See the documentation here: https://pandas.pydata.org/pandas-docs/stable/indexing.html#deprecate-loc-reindex-listlike Out[4]: 1 2.0 2 3.0 3 NaN dtype: float64

The idiomatic way to achieve selecting potentially not-found elements is via .reindex(). See also the section on reindexing.

In [106]: s.reindex([1, 2, 3]) Out[106]: 1 2.0 2 3.0 3 NaN dtype: float64

Alternatively, if you want to select only valid keys, the following is idiomatic and efficient; it is guaranteed to preserve the dtype of the selection.

In [107]: labels = [1, 2, 3] In [108]: s.loc[s.index.intersection(labels)] Out[108]: 1 2 2 3 dtype: int64

Having a duplicated index will raise for a .reindex():

In [109]: s = pd.Series(np.arange(4), index=['a', 'a', 'b', 'c']) In [110]: labels = ['c', 'd']

In [17]: s.reindex(labels) ValueError: cannot reindex on an axis with duplicate labels

Generally, you can intersect the desired labels with the current axis, and then reindex.

In [111]: s.loc[s.index.intersection(labels)].reindex(labels) Out[111]: c 3.0 d NaN dtype: float64

However, this would still raise if your resulting index is duplicated.

In [41]: labels = ['a', 'd'] In [42]: s.loc[s.index.intersection(labels)].reindex(labels) ValueError: cannot reindex on an axis with duplicate labels

A random selection of rows or columns from a Series or DataFrame with the sample() method. The method will sample rows by default, and accepts a specific number of rows/columns to return, or a fraction of rows.

In [112]: s = pd.Series([0, 1, 2, 3, 4, 5]) # When no arguments are passed, returns 1 row. In [113]: s.sample() Out[113]: 4 4 dtype: int64 # One may specify either a number of rows: In [114]: s.sample(n=3) Out[114]: 0 0 4 4 1 1 dtype: int64 # Or a fraction of the rows: In [115]: s.sample(frac=0.5) Out[115]: 5 5 3 3 1 1 dtype: int64

By default, sample will return each row at most once, but one can also sample with replacement using the replace option:

In [116]: s = pd.Series([0, 1, 2, 3, 4, 5]) # Without replacement (default): In [117]: s.sample(n=6, replace=False) Out[117]: 0 0 1 1 5 5 3 3 2 2 4 4 dtype: int64 # With replacement: In [118]: s.sample(n=6, replace=True) Out[118]: 0 0 4 4 3 3 2 2 4 4 4 4 dtype: int64

By default, each row has an equal probability of being selected, but if you want rows to have different probabilities, you can pass the sample function sampling weights as weights. These weights can be a list, a NumPy array, or a Series, but they must be of the same length as the object you are sampling. Missing values will be treated as a weight of zero, and inf values are not allowed. If weights do not sum to 1, they will be re-normalized by dividing all weights by the sum of the weights. For example:

In [119]: s = pd.Series([0, 1, 2, 3, 4, 5]) In [120]: example_weights = [0, 0, 0.2, 0.2, 0.2, 0.4] In [121]: s.sample(n=3, weights=example_weights) Out[121]: 5 5 4 4 3 3 dtype: int64 # Weights will be re-normalized automatically In [122]: example_weights2 = [0.5, 0, 0, 0, 0, 0] In [123]: s.sample(n=1, weights=example_weights2) Out[123]: 0 0 dtype: int64

When applied to a DataFrame, you can use a column of the DataFrame as sampling weights (provided you are sampling rows and not columns) by simply passing the name of the column as a string.

In [124]: df2 = pd.DataFrame({'col1': [9, 8, 7, 6], .....: 'weight_column': [0.5, 0.4, 0.1, 0]}) .....: In [125]: df2.sample(n=3, weights='weight_column') Out[125]: col1 weight_column 1 8 0.4 0 9 0.5 2 7 0.1

sample also allows users to sample columns instead of rows using the axis argument.

In [126]: df3 = pd.DataFrame({'col1': [1, 2, 3], 'col2': [2, 3, 4]}) In [127]: df3.sample(n=1, axis=1) Out[127]: col1 0 1 1 2 2 3

Finally, one can also set a seed for sample’s random number generator using the random_state argument, which will accept either an integer (as a seed) or a NumPy RandomState object.

In [128]: df4 = pd.DataFrame({'col1': [1, 2, 3], 'col2': [2, 3, 4]}) # With a given seed, the sample will always draw the same rows. In [129]: df4.sample(n=2, random_state=2) Out[129]: col1 col2 2 3 4 1 2 3 In [130]: df4.sample(n=2, random_state=2) Out[130]: col1 col2 2 3 4 1 2 3

The .loc/[] operations can perform enlargement when setting a non-existent key for that axis.

In the Series case this is effectively an appending operation.

In [131]: se = pd.Series([1, 2, 3]) In [132]: se Out[132]: 0 1 1 2 2 3 dtype: int64 In [133]: se[5] = 5. In [134]: se Out[134]: 0 1.0 1 2.0 2 3.0 5 5.0 dtype: float64

A DataFrame can be enlarged on either axis via .loc.

In [135]: dfi = pd.DataFrame(np.arange(6).reshape(3, 2), .....: columns=['A', 'B']) .....: In [136]: dfi Out[136]: A B 0 0 1 1 2 3 2 4 5 In [137]: dfi.loc[:, 'C'] = dfi.loc[:, 'A'] In [138]: dfi Out[138]: A B C 0 0 1 0 1 2 3 2 2 4 5 4

This is like an append operation on the DataFrame.

In [139]: dfi.loc[3] = 5 In [140]: dfi Out[140]: A B C 0 0 1 0 1 2 3 2 2 4 5 4 3 5 5 5

Since indexing with [] must handle a lot of cases (single-label access, slicing, boolean indexing, etc.), it has a bit of overhead in order to figure out what you’re asking for. If you only want to access a scalar value, the fastest way is to use the at and iat methods, which are implemented on all of the data structures.

Similarly to loc, at provides label based scalar lookups, while, iat provides integer based lookups analogously to iloc

In [141]: s.iat[5] Out[141]: 5 In [142]: df.at[dates[5], 'A'] Out[142]: -0.6736897080883706 In [143]: df.iat[3, 0] Out[143]: 0.7215551622443669

You can also set using these same indexers.

In [144]: df.at[dates[5], 'E'] = 7 In [145]: df.iat[3, 0] = 7

at may enlarge the object in-place as above if the indexer is missing.

In [146]: df.at[dates[-1] + pd.Timedelta('1 day'), 0] = 7 In [147]: df Out[147]: A B C D E 0 2000-01-01 0.469112 -0.282863 -1.509059 -1.135632 NaN NaN 2000-01-02 1.212112 -0.173215 0.119209 -1.044236 NaN NaN 2000-01-03 -0.861849 -2.104569 -0.494929 1.071804 NaN NaN 2000-01-04 7.000000 -0.706771 -1.039575 0.271860 NaN NaN 2000-01-05 -0.424972 0.567020 0.276232 -1.087401 NaN NaN 2000-01-06 -0.673690 0.113648 -1.478427 0.524988 7.0 NaN 2000-01-07 0.404705 0.577046 -1.715002 -1.039268 NaN NaN 2000-01-08 -0.370647 -1.157892 -1.344312 0.844885 NaN NaN 2000-01-09 NaN NaN NaN NaN NaN 7.0

Another common operation is the use of boolean vectors to filter the data. The operators are: | for or, & for and, and ~ for not. These must be grouped by using parentheses, since by default Python will evaluate an expression such as df['A'] > 2 & df['B'] < 3 as df['A'] > (2 & df['B']) < 3, while the desired evaluation order is (df['A'] > 2) & (df['B'] < 3).

Using a boolean vector to index a Series works exactly as in a NumPy ndarray:

In [148]: s = pd.Series(range(-3, 4)) In [149]: s Out[149]: 0 -3 1 -2 2 -1 3 0 4 1 5 2 6 3 dtype: int64 In [150]: s[s > 0] Out[150]: 4 1 5 2 6 3 dtype: int64 In [151]: s[(s < -1) | (s > 0.5)] Out[151]: 0 -3 1 -2 4 1 5 2 6 3 dtype: int64 In [152]: s[~(s < 0)] Out[152]: 3 0 4 1 5 2 6 3 dtype: int64

You may select rows from a DataFrame using a boolean vector the same length as the DataFrame’s index (for example, something derived from one of the columns of the DataFrame):

In [153]: df[df['A'] > 0] Out[153]: A B C D E 0 2000-01-01 0.469112 -0.282863 -1.509059 -1.135632 NaN NaN 2000-01-02 1.212112 -0.173215 0.119209 -1.044236 NaN NaN 2000-01-04 7.000000 -0.706771 -1.039575 0.271860 NaN NaN 2000-01-07 0.404705 0.577046 -1.715002 -1.039268 NaN NaN

List comprehensions and the map method of Series can also be used to produce more complex criteria:

In [154]: df2 = pd.DataFrame({'a': ['one', 'one', 'two', 'three', 'two', 'one', 'six'], .....: 'b': ['x', 'y', 'y', 'x', 'y', 'x', 'x'], .....: 'c': np.random.randn(7)}) .....: # only want 'two' or 'three' In [155]: criterion = df2['a'].map(lambda x: x.startswith('t')) In [156]: df2[criterion] Out[156]: a b c 2 two y 0.041290 3 three x 0.361719 4 two y -0.238075 # equivalent but slower In [157]: df2[[x.startswith('t') for x in df2['a']]] Out[157]: a b c 2 two y 0.041290 3 three x 0.361719 4 two y -0.238075 # Multiple criteria In [158]: df2[criterion & (df2['b'] == 'x')] Out[158]: a b c 3 three x 0.361719

With the choice methods Selection by Label, Selection by Position, and Advanced Indexing you may select along more than one axis using boolean vectors combined with other indexing expressions.

In [159]: df2.loc[criterion & (df2['b'] == 'x'), 'b':'c'] Out[159]: b c 3 x 0.361719

Warning

iloc supports two kinds of boolean indexing. If the indexer is a boolean Series, an error will be raised. For instance, in the following example, df.iloc[s.values, 1] is ok. The boolean indexer is an array. But df.iloc[s, 1] would raise ValueError.

In [160]: df = pd.DataFrame([[1, 2], [3, 4], [5, 6]], .....: index=list('abc'), .....: columns=['A', 'B']) .....: In [161]: s = (df['A'] > 2) In [162]: s Out[162]: a False b True c True Name: A, dtype: bool In [163]: df.loc[s, 'B'] Out[163]: b 4 c 6 Name: B, dtype: int64 In [164]: df.iloc[s.values, 1] Out[164]: b 4 c 6 Name: B, dtype: int64

Consider the isin() method of Series, which returns a boolean vector that is true wherever the Series elements exist in the passed list. This allows you to select rows where one or more columns have values you want:

In [165]: s = pd.Series(np.arange(5), index=np.arange(5)[::-1], dtype='int64') In [166]: s Out[166]: 4 0 3 1 2 2 1 3 0 4 dtype: int64 In [167]: s.isin([2, 4, 6]) Out[167]: 4 False 3 False 2 True 1 False 0 True dtype: bool In [168]: s[s.isin([2, 4, 6])] Out[168]: 2 2 0 4 dtype: int64

The same method is available for Index objects and is useful for the cases when you don’t know which of the sought labels are in fact present:

In [169]: s[s.index.isin([2, 4, 6])] Out[169]: 4 0 2 2 dtype: int64 # compare it to the following In [170]: s.reindex([2, 4, 6]) Out[170]: 2 2.0 4 0.0 6 NaN dtype: float64

In addition to that, MultiIndex allows selecting a separate level to use in the membership check:

In [171]: s_mi = pd.Series(np.arange(6), .....: index=pd.MultiIndex.from_product([[0, 1], ['a', 'b', 'c']])) .....: In [172]: s_mi Out[172]: 0 a 0 b 1 c 2 1 a 3 b 4 c 5 dtype: int64 In [173]: s_mi.iloc[s_mi.index.isin([(1, 'a'), (2, 'b'), (0, 'c')])] Out[173]: 0 c 2 1 a 3 dtype: int64 In [174]: s_mi.iloc[s_mi.index.isin(['a', 'c', 'e'], level=1)] Out[174]: 0 a 0 c 2 1 a 3 c 5 dtype: int64

DataFrame also has an isin() method. When calling isin, pass a set of values as either an array or dict. If values is an array, isin returns a DataFrame of booleans that is the same shape as the original DataFrame, with True wherever the element is in the sequence of values.

In [175]: df = pd.DataFrame({'vals': [1, 2, 3, 4], 'ids': ['a', 'b', 'f', 'n'], .....: 'ids2': ['a', 'n', 'c', 'n']}) .....: In [176]: values = ['a', 'b', 1, 3] In [177]: df.isin(values) Out[177]: vals ids ids2 0 True True True 1 False True False 2 True False False 3 False False False

Oftentimes you’ll want to match certain values with certain columns. Just make values a dict where the key is the column, and the value is a list of items you want to check for.

In [178]: values = {'ids': ['a', 'b'], 'vals': [1, 3]} In [179]: df.isin(values) Out[179]: vals ids ids2 0 True True False 1 False True False 2 True False False 3 False False False

To return the DataFrame of booleans where the values are not in the original DataFrame, use the ~ operator:

In [180]: values = {'ids': ['a', 'b'], 'vals': [1, 3]} In [181]: ~df.isin(values) Out[181]: vals ids ids2 0 False False True 1 True False True 2 False True True 3 True True True

Combine DataFrame’s isin with the any() and all() methods to quickly select subsets of your data that meet a given criteria. To select a row where each column meets its own criterion:

In [182]: values = {'ids': ['a', 'b'], 'ids2': ['a', 'c'], 'vals': [1, 3]} In [183]: row_mask = df.isin(values).all(1) In [184]: df[row_mask] Out[184]: vals ids ids2 0 1 a a

Selecting values from a Series with a boolean vector generally returns a subset of the data. To guarantee that selection output has the same shape as the original data, you can use the where method in Series and DataFrame.

To return only the selected rows:

In [185]: s[s > 0] Out[185]: 3 1 2 2 1 3 0 4 dtype: int64

To return a Series of the same shape as the original:

In [186]: s.where(s > 0) Out[186]: 4 NaN 3 1.0 2 2.0 1 3.0 0 4.0 dtype: float64

Selecting values from a DataFrame with a boolean criterion now also preserves input data shape. where is used under the hood as the implementation. The code below is equivalent to df.where(df < 0).

In [187]: df[df < 0] Out[187]: A B C D 2000-01-01 -2.104139 -1.309525 NaN NaN 2000-01-02 -0.352480 NaN -1.192319 NaN 2000-01-03 -0.864883 NaN -0.227870 NaN 2000-01-04 NaN -1.222082 NaN -1.233203 2000-01-05 NaN -0.605656 -1.169184 NaN 2000-01-06 NaN -0.948458 NaN -0.684718 2000-01-07 -2.670153 -0.114722 NaN -0.048048 2000-01-08 NaN NaN -0.048788 -0.808838

In addition, where takes an optional other argument for replacement of values where the condition is False, in the returned copy.

In [188]: df.where(df < 0, -df) Out[188]: A B C D 2000-01-01 -2.104139 -1.309525 -0.485855 -0.245166 2000-01-02 -0.352480 -0.390389 -1.192319 -1.655824 2000-01-03 -0.864883 -0.299674 -0.227870 -0.281059 2000-01-04 -0.846958 -1.222082 -0.600705 -1.233203 2000-01-05 -0.669692 -0.605656 -1.169184 -0.342416 2000-01-06 -0.868584 -0.948458 -2.297780 -0.684718 2000-01-07 -2.670153 -0.114722 -0.168904 -0.048048 2000-01-08 -0.801196 -1.392071 -0.048788 -0.808838

You may wish to set values based on some boolean criteria. This can be done intuitively like so:

In [189]: s2 = s.copy() In [190]: s2[s2 < 0] = 0 In [191]: s2 Out[191]: 4 0 3 1 2 2 1 3 0 4 dtype: int64 In [192]: df2 = df.copy() In [193]: df2[df2 < 0] = 0 In [194]: df2 Out[194]: A B C D 2000-01-01 0.000000 0.000000 0.485855 0.245166 2000-01-02 0.000000 0.390389 0.000000 1.655824 2000-01-03 0.000000 0.299674 0.000000 0.281059 2000-01-04 0.846958 0.000000 0.600705 0.000000 2000-01-05 0.669692 0.000000 0.000000 0.342416 2000-01-06 0.868584 0.000000 2.297780 0.000000 2000-01-07 0.000000 0.000000 0.168904 0.000000 2000-01-08 0.801196 1.392071 0.000000 0.000000

By default, where returns a modified copy of the data. There is an optional parameter inplace so that the original data can be modified without creating a copy:

In [195]: df_orig = df.copy() In [196]: df_orig.where(df > 0, -df, inplace=True) In [197]: df_orig Out[197]: A B C D 2000-01-01 2.104139 1.309525 0.485855 0.245166 2000-01-02 0.352480 0.390389 1.192319 1.655824 2000-01-03 0.864883 0.299674 0.227870 0.281059 2000-01-04 0.846958 1.222082 0.600705 1.233203 2000-01-05 0.669692 0.605656 1.169184 0.342416 2000-01-06 0.868584 0.948458 2.297780 0.684718 2000-01-07 2.670153 0.114722 0.168904 0.048048 2000-01-08 0.801196 1.392071 0.048788 0.808838

Note

The signature for DataFrame.where() differs from numpy.where(). Roughly df1.where(m, df2) is equivalent to np.where(m, df1, df2).

In [198]: df.where(df < 0, -df) == np.where(df < 0, df, -df) Out[198]: A B C D 2000-01-01 True True True True 2000-01-02 True True True True 2000-01-03 True True True True 2000-01-04 True True True True 2000-01-05 True True True True 2000-01-06 True True True True 2000-01-07 True True True True 2000-01-08 True True True True

Alignment

Furthermore, where aligns the input boolean condition (ndarray or DataFrame), such that partial selection with setting is possible. This is analogous to partial setting via .loc (but on the contents rather than the axis labels).

In [199]: df2 = df.copy() In [200]: df2[df2[1:4] > 0] = 3 In [201]: df2 Out[201]: A B C D 2000-01-01 -2.104139 -1.309525 0.485855 0.245166 2000-01-02 -0.352480 3.000000 -1.192319 3.000000 2000-01-03 -0.864883 3.000000 -0.227870 3.000000 2000-01-04 3.000000 -1.222082 3.000000 -1.233203 2000-01-05 0.669692 -0.605656 -1.169184 0.342416 2000-01-06 0.868584 -0.948458 2.297780 -0.684718 2000-01-07 -2.670153 -0.114722 0.168904 -0.048048 2000-01-08 0.801196 1.392071 -0.048788 -0.808838

Where can also accept axis and level parameters to align the input when performing the where.

In [202]: df2 = df.copy() In [203]: df2.where(df2 > 0, df2['A'], axis='index') Out[203]: A B C D 2000-01-01 -2.104139 -2.104139 0.485855 0.245166 2000-01-02 -0.352480 0.390389 -0.352480 1.655824 2000-01-03 -0.864883 0.299674 -0.864883 0.281059 2000-01-04 0.846958 0.846958 0.600705 0.846958 2000-01-05 0.669692 0.669692 0.669692 0.342416 2000-01-06 0.868584 0.868584 2.297780 0.868584 2000-01-07 -2.670153 -2.670153 0.168904 -2.670153 2000-01-08 0.801196 1.392071 0.801196 0.801196

This is equivalent to (but faster than) the following.

In [204]: df2 = df.copy() In [205]: df.apply(lambda x, y: x.where(x > 0, y), y=df['A']) Out[205]: A B C D 2000-01-01 -2.104139 -2.104139 0.485855 0.245166 2000-01-02 -0.352480 0.390389 -0.352480 1.655824 2000-01-03 -0.864883 0.299674 -0.864883 0.281059 2000-01-04 0.846958 0.846958 0.600705 0.846958 2000-01-05 0.669692 0.669692 0.669692 0.342416 2000-01-06 0.868584 0.868584 2.297780 0.868584 2000-01-07 -2.670153 -2.670153 0.168904 -2.670153 2000-01-08 0.801196 1.392071 0.801196 0.801196

where can accept a callable as condition and other arguments. The function must be with one argument (the calling Series or DataFrame) and that returns valid output as condition and other argument.

In [206]: df3 = pd.DataFrame({'A': [1, 2, 3], .....: 'B': [4, 5, 6], .....: 'C': [7, 8, 9]}) .....: In [207]: df3.where(lambda x: x > 4, lambda x: x + 10) Out[207]: A B C 0 11 14 7 1 12 5 8 2 13 6 9

mask() is the inverse boolean operation of where.

In [208]: s.mask(s >= 0) Out[208]: 4 NaN 3 NaN 2 NaN 1 NaN 0 NaN dtype: float64 In [209]: df.mask(df >= 0) Out[209]: A B C D 2000-01-01 -2.104139 -1.309525 NaN NaN 2000-01-02 -0.352480 NaN -1.192319 NaN 2000-01-03 -0.864883 NaN -0.227870 NaN 2000-01-04 NaN -1.222082 NaN -1.233203 2000-01-05 NaN -0.605656 -1.169184 NaN 2000-01-06 NaN -0.948458 NaN -0.684718 2000-01-07 -2.670153 -0.114722 NaN -0.048048 2000-01-08 NaN NaN -0.048788 -0.808838

An alternative to where() is to use numpy.where(). Combined with setting a new column, you can use it to enlarge a DataFrame where the values are determined conditionally.

Consider you have two choices to choose from in the following DataFrame. And you want to set a new column color to ‘green’ when the second column has ‘Z’. You can do the following:

In [210]: df = pd.DataFrame({'col1': list('ABBC'), 'col2': list('ZZXY')}) In [211]: df['color'] = np.where(df['col2'] == 'Z', 'green', 'red') In [212]: df Out[212]: col1 col2 color 0 A Z green 1 B Z green 2 B X red 3 C Y red

If you have multiple conditions, you can use numpy.select() to achieve that. Say corresponding to three conditions there are three choice of colors, with a fourth color as a fallback, you can do the following.

In [213]: conditions = [ .....: (df['col2'] == 'Z') & (df['col1'] == 'A'), .....: (df['col2'] == 'Z') & (df['col1'] == 'B'), .....: (df['col1'] == 'B') .....: ] .....: In [214]: choices = ['yellow', 'blue', 'purple'] In [215]: df['color'] = np.select(conditions, choices, default='black') In [216]: df Out[216]: col1 col2 color 0 A Z yellow 1 B Z blue 2 B X purple 3 C Y black

DataFrame objects have a query() method that allows selection using an expression.

You can get the value of the frame where column b has values between the values of columns a and c. For example:

In [217]: n = 10 In [218]: df = pd.DataFrame(np.random.rand(n, 3), columns=list('abc')) In [219]: df Out[219]: a b c 0 0.438921 0.118680 0.863670 1 0.138138 0.577363 0.686602 2 0.595307 0.564592 0.520630 3 0.913052 0.926075 0.616184 4 0.078718 0.854477 0.898725 5 0.076404 0.523211 0.591538 6 0.792342 0.216974 0.564056 7 0.397890 0.454131 0.915716 8 0.074315 0.437913 0.019794 9 0.559209 0.502065 0.026437 # pure python In [220]: df[(df['a'] < df['b']) & (df['b'] < df['c'])] Out[220]: a b c 1 0.138138 0.577363 0.686602 4 0.078718 0.854477 0.898725 5 0.076404 0.523211 0.591538 7 0.397890 0.454131 0.915716 # query In [221]: df.query('(a < b) & (b < c)') Out[221]: a b c 1 0.138138 0.577363 0.686602 4 0.078718 0.854477 0.898725 5 0.076404 0.523211 0.591538 7 0.397890 0.454131 0.915716

Do the same thing but fall back on a named index if there is no column with the name a.

In [222]: df = pd.DataFrame(np.random.randint(n / 2, size=(n, 2)), columns=list('bc')) In [223]: df.index.name = 'a' In [224]: df Out[224]: b c a 0 0 4 1 0 1 2 3 4 3 4 3 4 1 4 5 0 3 6 0 1 7 3 4 8 2 3 9 1 1 In [225]: df.query('a < b and b < c') Out[225]: b c a 2 3 4

If instead you don’t want to or cannot name your index, you can use the name index in your query expression:

In [226]: df = pd.DataFrame(np.random.randint(n, size=(n, 2)), columns=list('bc')) In [227]: df Out[227]: b c 0 3 1 1 3 0 2 5 6 3 5 2 4 7 4 5 0 1 6 2 5 7 0 1 8 6 0 9 7 9 In [228]: df.query('index < b < c') Out[228]: b c 2 5 6

Note

If the name of your index overlaps with a column name, the column name is given precedence. For example,

In [229]: df = pd.DataFrame({'a': np.random.randint(5, size=5)}) In [230]: df.index.name = 'a' In [231]: df.query('a > 2') # uses the column 'a', not the index Out[231]: a a 1 3 3 3

You can still use the index in a query expression by using the special identifier ‘index’:

In [232]: df.query('index > 2') Out[232]: a a 3 3 4 2

If for some reason you have a column named index, then you can refer to the index as ilevel_0 as well, but at this point you should consider renaming your columns to something less ambiguous.

You can also use the levels of a DataFrame with a MultiIndex as if they were columns in the frame:

In [233]: n = 10 In [234]: colors = np.random.choice(['red', 'green'], size=n) In [235]: foods = np.random.choice(['eggs', 'ham'], size=n) In [236]: colors Out[236]: array(['red', 'red', 'red', 'green', 'green', 'green', 'green', 'green', 'green', 'green'], dtype=' In [237]: foods Out[237]: array(['ham', 'ham', 'eggs', 'eggs', 'eggs', 'ham', 'ham', 'eggs', 'eggs', 'eggs'], dtype=' In [238]: index = pd.MultiIndex.from_arrays([colors, foods], names=['color', 'food']) In [239]: df = pd.DataFrame(np.random.randn(n, 2), index=index) In [240]: df Out[240]: 0 1 color food red ham 0.194889 -0.381994 ham 0.318587 2.089075 eggs -0.728293 -0.090255 green eggs -0.748199 1.318931 eggs -2.029766 0.792652 ham 0.461007 -0.542749 ham -0.305384 -0.479195 eggs 0.095031 -0.270099 eggs -0.707140 -0.773882 eggs 0.229453 0.304418 In [241]: df.query('color == "red"') Out[241]: 0 1 color food red ham 0.194889 -0.381994 ham 0.318587 2.089075 eggs -0.728293 -0.090255

If the levels of the MultiIndex are unnamed, you can refer to them using special names:

In [242]: df.index.names = [None, None] In [243]: df Out[243]: 0 1 red ham 0.194889 -0.381994 ham 0.318587 2.089075 eggs -0.728293 -0.090255 green eggs -0.748199 1.318931 eggs -2.029766 0.792652 ham 0.461007 -0.542749 ham -0.305384 -0.479195 eggs 0.095031 -0.270099 eggs -0.707140 -0.773882 eggs 0.229453 0.304418 In [244]: df.query('ilevel_0 == "red"') Out[244]: 0 1 red ham 0.194889 -0.381994 ham 0.318587 2.089075 eggs -0.728293 -0.090255

The convention is ilevel_0, which means “index level 0” for the 0th level of the index.

A use case for query() is when you have a collection of DataFrame objects that have a subset of column names (or index levels/names) in common. You can pass the same query to both frames without having to specify which frame you’re interested in querying

In [245]: df = pd.DataFrame(np.random.rand(n, 3), columns=list('abc')) In [246]: df Out[246]: a b c 0 0.224283 0.736107 0.139168 1 0.302827 0.657803 0.713897 2 0.611185 0.136624 0.984960 3 0.195246 0.123436 0.627712 4 0.618673 0.371660 0.047902 5 0.480088 0.062993 0.185760 6 0.568018 0.483467 0.445289 7 0.309040 0.274580 0.587101 8 0.258993 0.477769 0.370255 9 0.550459 0.840870 0.304611 In [247]: df2 = pd.DataFrame(np.random.rand(n + 2, 3), columns=df.columns) In [248]: df2 Out[248]: a b c 0 0.357579 0.229800 0.596001 1 0.309059 0.957923 0.965663 2 0.123102 0.336914 0.318616 3 0.526506 0.323321 0.860813 4 0.518736 0.486514 0.384724 5 0.190804 0.505723 0.614533 6 0.891939 0.623977 0.676639 7 0.480559 0.378528 0.460858 8 0.420223 0.136404 0.141295 9 0.732206 0.419540 0.604675 10 0.604466 0.848974 0.896165 11 0.589168 0.920046 0.732716 In [249]: expr = '0.0 <= a <= c <= 0.5' In [250]: map(lambda frame: frame.query(expr), [df, df2]) Out[250]:

Full numpy-like syntax:

In [251]: df = pd.DataFrame(np.random.randint(n, size=(n, 3)), columns=list('abc')) In [252]: df Out[252]: a b c 0 7 8 9 1 1 0 7 2 2 7 2 3 6 2 2 4 2 6 3 5 3 8 2 6 1 7 2 7 5 1 5 8 9 8 0 9 1 5 0 In [253]: df.query('(a < b) & (b < c)') Out[253]: a b c 0 7 8 9 In [254]: df[(df['a'] < df['b']) & (df['b'] < df['c'])] Out[254]: a b c 0 7 8 9

Slightly nicer by removing the parentheses (comparison operators bind tighter than & and |):

In [255]: df.query('a < b & b < c') Out[255]: a b c 0 7 8 9

Use English instead of symbols:

In [256]: df.query('a < b and b < c') Out[256]: a b c 0 7 8 9

Pretty close to how you might write it on paper:

In [257]: df.query('a < b < c') Out[257]: a b c 0 7 8 9

query() also supports special use of Python’s in and not in comparison operators, providing a succinct syntax for calling the isin method of a Series or DataFrame.

# get all rows where columns "a" and "b" have overlapping values In [258]: df = pd.DataFrame({'a': list('aabbccddeeff'), 'b': list('aaaabbbbcccc'), .....: 'c': np.random.randint(5, size=12), .....: 'd': np.random.randint(9, size=12)}) .....: In [259]: df Out[259]: a b c d 0 a a 2 6 1 a a 4 7 2 b a 1 6 3 b a 2 1 4 c b 3 6 5 c b 0 2 6 d b 3 3 7 d b 2 1 8 e c 4 3 9 e c 2 0 10 f c 0 6 11 f c 1 2 In [260]: df.query('a in b') Out[260]: a b c d 0 a a 2 6 1 a a 4 7 2 b a 1 6 3 b a 2 1 4 c b 3 6 5 c b 0 2 # How you'd do it in pure Python In [261]: df[df['a'].isin(df['b'])] Out[261]: a b c d 0 a a 2 6 1 a a 4 7 2 b a 1 6 3 b a 2 1 4 c b 3 6 5 c b 0 2 In [262]: df.query('a not in b') Out[262]: a b c d 6 d b 3 3 7 d b 2 1 8 e c 4 3 9 e c 2 0 10 f c 0 6 11 f c 1 2 # pure Python In [263]: df[~df['a'].isin(df['b'])] Out[263]: a b c d 6 d b 3 3 7 d b 2 1 8 e c 4 3 9 e c 2 0 10 f c 0 6 11 f c 1 2

You can combine this with other expressions for very succinct queries:

# rows where cols a and b have overlapping values # and col c's values are less than col d's In [264]: df.query('a in b and c < d') Out[264]: a b c d 0 a a 2 6 1 a a 4 7 2 b a 1 6 4 c b 3 6 5 c b 0 2 # pure Python In [265]: df[df['b'].isin(df['a']) & (df['c'] < df['d'])] Out[265]: a b c d 0 a a 2 6 1 a a 4 7 2 b a 1 6 4 c b 3 6 5 c b 0 2 10 f c 0 6 11 f c 1 2

Note

Note that in and not in are evaluated in Python, since numexpr has no equivalent of this operation. However, only the in/not in expression itself is evaluated in vanilla Python. For example, in the expression

df.query('a in b + c + d')

(b + c + d) is evaluated by numexpr and then the in operation is evaluated in plain Python. In general, any operations that can be evaluated using numexpr will be.

Comparing a list of values to a column using ==/!= works similarly to in/not in.

In [266]: df.query('b == ["a", "b", "c"]') Out[266]: a b c d 0 a a 2 6 1 a a 4 7 2 b a 1 6 3 b a 2 1 4 c b 3 6 5 c b 0 2 6 d b 3 3 7 d b 2 1 8 e c 4 3 9 e c 2 0 10 f c 0 6 11 f c 1 2 # pure Python In [267]: df[df['b'].isin(["a", "b", "c"])] Out[267]: a b c d 0 a a 2 6 1 a a 4 7 2 b a 1 6 3 b a 2 1 4 c b 3 6 5 c b 0 2 6 d b 3 3 7 d b 2 1 8 e c 4 3 9 e c 2 0 10 f c 0 6 11 f c 1 2 In [268]: df.query('c == [1, 2]') Out[268]: a b c d 0 a a 2 6 2 b a 1 6 3 b a 2 1 7 d b 2 1 9 e c 2 0 11 f c 1 2 In [269]: df.query('c != [1, 2]') Out[269]: a b c d 1 a a 4 7 4 c b 3 6 5 c b 0 2 6 d b 3 3 8 e c 4 3 10 f c 0 6 # using in/not in In [270]: df.query('[1, 2] in c') Out[270]: a b c d 0 a a 2 6 2 b a 1 6 3 b a 2 1 7 d b 2 1 9 e c 2 0 11 f c 1 2 In [271]: df.query('[1, 2] not in c') Out[271]: a b c d 1 a a 4 7 4 c b 3 6 5 c b 0 2 6 d b 3 3 8 e c 4 3 10 f c 0 6 # pure Python In [272]: df[df['c'].isin([1, 2])] Out[272]: a b c d 0 a a 2 6 2 b a 1 6 3 b a 2 1 7 d b 2 1 9 e c 2 0 11 f c 1 2

You can negate boolean expressions with the word not or the ~ operator.

In [273]: df = pd.DataFrame(np.random.rand(n, 3), columns=list('abc')) In [274]: df['bools'] = np.random.rand(len(df)) > 0.5 In [275]: df.query('~bools') Out[275]: a b c bools 2 0.697753 0.212799 0.329209 False 7 0.275396 0.691034 0.826619 False 8 0.190649 0.558748 0.262467 False In [276]: df.query('not bools') Out[276]: a b c bools 2 0.697753 0.212799 0.329209 False 7 0.275396 0.691034 0.826619 False 8 0.190649 0.558748 0.262467 False In [277]: df.query('not bools') == df[~df['bools']] Out[277]: a b c bools 2 True True True True 7 True True True True 8 True True True True

Of course, expressions can be arbitrarily complex too:

# short query syntax In [278]: shorter = df.query('a < b < c and (not bools) or bools > 2') # equivalent in pure Python In [279]: longer = df[(df['a'] < df['b']) .....: & (df['b'] < df['c']) .....: & (~df['bools']) .....: | (df['bools'] > 2)] .....: In [280]: shorter Out[280]: a b c bools 7 0.275396 0.691034 0.826619 False In [281]: longer Out[281]: a b c bools 7 0.275396 0.691034 0.826619 False In [282]: shorter == longer Out[282]: a b c bools 7 True True True True

DataFrame.query() using numexpr is slightly faster than Python for large frames.

Note

You will only see the performance benefits of using the numexpr engine with DataFrame.query() if your frame has more than approximately 200,000 rows.

This plot was created using a DataFrame with 3 columns each containing floating point values generated using numpy.random.randn().

If you want to identify and remove duplicate rows in a DataFrame, there are two methods that will help: duplicated and drop_duplicates. Each takes as an argument the columns to use to identify duplicated rows.

duplicated returns a boolean vector whose length is the number of rows, and which indicates whether a row is duplicated.
drop_duplicates removes duplicate rows.

By default, the first observed row of a duplicate set is considered unique, but each method has a keep parameter to specify targets to be kept.

keep='first' (default): mark / drop duplicates except for the first occurrence.
keep='last': mark / drop duplicates except for the last occurrence.
keep=False: mark / drop all duplicates.

In [283]: df2 = pd.DataFrame({'a': ['one', 'one', 'two', 'two', 'two', 'three', 'four'], .....: 'b': ['x', 'y', 'x', 'y', 'x', 'x', 'x'], .....: 'c': np.random.randn(7)}) .....: In [284]: df2 Out[284]: a b c 0 one x -1.067137 1 one y 0.309500 2 two x -0.211056 3 two y -1.842023 4 two x -0.390820 5 three x -1.964475 6 four x 1.298329 In [285]: df2.duplicated('a') Out[285]: 0 False 1 True 2 False 3 True 4 True 5 False 6 False dtype: bool In [286]: df2.duplicated('a', keep='last') Out[286]: 0 True 1 False 2 True 3 True 4 False 5 False 6 False dtype: bool In [287]: df2.duplicated('a', keep=False) Out[287]: 0 True 1 True 2 True 3 True 4 True 5 False 6 False dtype: bool In [288]: df2.drop_duplicates('a') Out[288]: a b c 0 one x -1.067137 2 two x -0.211056 5 three x -1.964475 6 four x 1.298329 In [289]: df2.drop_duplicates('a', keep='last') Out[289]: a b c 1 one y 0.309500 4 two x -0.390820 5 three x -1.964475 6 four x 1.298329 In [290]: df2.drop_duplicates('a', keep=False) Out[290]: a b c 5 three x -1.964475 6 four x 1.298329

Also, you can pass a list of columns to identify duplications.

In [291]: df2.duplicated(['a', 'b']) Out[291]: 0 False 1 False 2 False 3 False 4 True 5 False 6 False dtype: bool In [292]: df2.drop_duplicates(['a', 'b']) Out[292]: a b c 0 one x -1.067137 1 one y 0.309500 2 two x -0.211056 3 two y -1.842023 5 three x -1.964475 6 four x 1.298329

To drop duplicates by index value, use Index.duplicated then perform slicing. The same set of options are available for the keep parameter.

In [293]: df3 = pd.DataFrame({'a': np.arange(6), .....: 'b': np.random.randn(6)}, .....: index=['a', 'a', 'b', 'c', 'b', 'a']) .....: In [294]: df3 Out[294]: a b a 0 1.440455 a 1 2.456086 b 2 1.038402 c 3 -0.894409 b 4 0.683536 a 5 3.082764 In [295]: df3.index.duplicated() Out[295]: array([False, True, False, False, True, True]) In [296]: df3[~df3.index.duplicated()] Out[296]: a b a 0 1.440455 b 2 1.038402 c 3 -0.894409 In [297]: df3[~df3.index.duplicated(keep='last')] Out[297]: a b c 3 -0.894409 b 4 0.683536 a 5 3.082764 In [298]: df3[~df3.index.duplicated(keep=False)] Out[298]: a b c 3 -0.894409

Each of Series or DataFrame have a get method which can return a default value.

In [299]: s = pd.Series([1, 2, 3], index=['a', 'b', 'c']) In [300]: s.get('a') # equivalent to s['a'] Out[300]: 1 In [301]: s.get('x', default=-1) Out[301]: -1

Sometimes you want to extract a set of values given a sequence of row labels and column labels, this can be achieved by pandas.factorize and NumPy indexing. For instance:

In [302]: df = pd.DataFrame({'col': ["A", "A", "B", "B"], .....: 'A': [80, 23, np.nan, 22], .....: 'B': [80, 55, 76, 67]}) .....: In [303]: df Out[303]: col A B 0 A 80.0 80 1 A 23.0 55 2 B NaN 76 3 B 22.0 67 In [304]: idx, cols = pd.factorize(df['col']) In [305]: df.reindex(cols, axis=1).to_numpy()[np.arange(len(df)), idx] Out[305]: array([80., 23., 76., 67.])

Formerly this could be achieved with the dedicated DataFrame.lookup method which was deprecated in version 1.2.0.

The pandas Index class and its subclasses can be viewed as implementing an ordered multiset. Duplicates are allowed. However, if you try to convert an Index object with duplicate entries into a set, an exception will be raised.

Index also provides the infrastructure necessary for lookups, data alignment, and reindexing. The easiest way to create an Index directly is to pass a list or other sequence to Index:

In [306]: index = pd.Index(['e', 'd', 'a', 'b']) In [307]: index Out[307]: Index(['e', 'd', 'a', 'b'], dtype='object') In [308]: 'd' in index Out[308]: True

You can also pass a name to be stored in the index:

In [309]: index = pd.Index(['e', 'd', 'a', 'b'], name='something') In [310]: index.name Out[310]: 'something'

The name, if set, will be shown in the console display:

In [311]: index = pd.Index(list(range(5)), name='rows') In [312]: columns = pd.Index(['A', 'B', 'C'], name='cols') In [313]: df = pd.DataFrame(np.random.randn(5, 3), index=index, columns=columns) In [314]: df Out[314]: cols A B C rows 0 1.295989 -1.051694 1.340429 1 -2.366110 0.428241 0.387275 2 0.433306 0.929548 0.278094 3 2.154730 -0.315628 0.264223 4 1.126818 1.132290 -0.353310 In [315]: df['A'] Out[315]: rows 0 1.295989 1 -2.366110 2 0.433306 3 2.154730 4 1.126818 Name: A, dtype: float64

The two main operations are union and intersection. Difference is provided via the .difference() method.

In [326]: a = pd.Index(['c', 'b', 'a']) In [327]: b = pd.Index(['c', 'e', 'd']) In [328]: a.difference(b) Out[328]: Index(['a', 'b'], dtype='object')

Also available is the symmetric_difference operation, which returns elements that appear in either idx1 or idx2, but not in both. This is equivalent to the Index created by idx1.difference(idx2).union(idx2.difference(idx1)), with duplicates dropped.

In [329]: idx1 = pd.Index([1, 2, 3, 4]) In [330]: idx2 = pd.Index([2, 3, 4, 5]) In [331]: idx1.symmetric_difference(idx2) Out[331]: Int64Index([1, 5], dtype='int64')

Note

The resulting index from a set operation will be sorted in ascending order.

When performing Index.union() between indexes with different dtypes, the indexes must be cast to a common dtype. Typically, though not always, this is object dtype. The exception is when performing a union between integer and float data. In this case, the integer values are converted to float

In [332]: idx1 = pd.Index([0, 1, 2]) In [333]: idx2 = pd.Index([0.5, 1.5]) In [334]: idx1.union(idx2) Out[334]: Float64Index([0.0, 0.5, 1.0, 1.5, 2.0], dtype='float64')

Important

Even though Index can hold missing values (NaN), it should be avoided if you do not want any unexpected results. For example, some operations exclude missing values implicitly.

Index.fillna fills missing values with specified scalar value.

In [335]: idx1 = pd.Index([1, np.nan, 3, 4]) In [336]: idx1 Out[336]: Float64Index([1.0, nan, 3.0, 4.0], dtype='float64') In [337]: idx1.fillna(2) Out[337]: Float64Index([1.0, 2.0, 3.0, 4.0], dtype='float64') In [338]: idx2 = pd.DatetimeIndex([pd.Timestamp('2011-01-01'), .....: pd.NaT, .....: pd.Timestamp('2011-01-03')]) .....: In [339]: idx2 Out[339]: DatetimeIndex(['2011-01-01', 'NaT', '2011-01-03'], dtype='datetime64[ns]', freq=None) In [340]: idx2.fillna(pd.Timestamp('2011-01-02')) Out[340]: DatetimeIndex(['2011-01-01', '2011-01-02', '2011-01-03'], dtype='datetime64[ns]', freq=None)

Occasionally you will load or create a data set into a DataFrame and want to add an index after you’ve already done so. There are a couple of different ways.

DataFrame has a set_index() method which takes a column name (for a regular Index) or a list of column names (for a MultiIndex). To create a new, re-indexed DataFrame:

In [341]: data Out[341]: a b c d 0 bar one z 1.0 1 bar two y 2.0 2 foo one x 3.0 3 foo two w 4.0 In [342]: indexed1 = data.set_index('c') In [343]: indexed1 Out[343]: a b d c z bar one 1.0 y bar two 2.0 x foo one 3.0 w foo two 4.0 In [344]: indexed2 = data.set_index(['a', 'b']) In [345]: indexed2 Out[345]: c d a b bar one z 1.0 two y 2.0 foo one x 3.0 two w 4.0

The append keyword option allow you to keep the existing index and append the given columns to a MultiIndex:

In [346]: frame = data.set_index('c', drop=False) In [347]: frame = frame.set_index(['a', 'b'], append=True) In [348]: frame Out[348]: c d c a b z bar one z 1.0 y bar two y 2.0 x foo one x 3.0 w foo two w 4.0

Other options in set_index allow you not drop the index columns or to add the index in-place (without creating a new object):

In [349]: data.set_index('c', drop=False) Out[349]: a b c d c z bar one z 1.0 y bar two y 2.0 x foo one x 3.0 w foo two w 4.0 In [350]: data.set_index(['a', 'b'], inplace=True) In [351]: data Out[351]: c d a b bar one z 1.0 two y 2.0 foo one x 3.0 two w 4.0

As a convenience, there is a new function on DataFrame called reset_index() which transfers the index values into the DataFrame’s columns and sets a simple integer index. This is the inverse operation of set_index().

In [352]: data Out[352]: c d a b bar one z 1.0 two y 2.0 foo one x 3.0 two w 4.0 In [353]: data.reset_index() Out[353]: a b c d 0 bar one z 1.0 1 bar two y 2.0 2 foo one x 3.0 3 foo two w 4.0

The output is more similar to a SQL table or a record array. The names for the columns derived from the index are the ones stored in the names attribute.

You can use the level keyword to remove only a portion of the index:

In [354]: frame Out[354]: c d c a b z bar one z 1.0 y bar two y 2.0 x foo one x 3.0 w foo two w 4.0 In [355]: frame.reset_index(level=1) Out[355]: a c d c b z one bar z 1.0 y two bar y 2.0 x one foo x 3.0 w two foo w 4.0

reset_index takes an optional parameter drop which if true simply discards the index, instead of putting index values in the DataFrame’s columns.

If you create an index yourself, you can just assign it to the index field:

When setting values in a pandas object, care must be taken to avoid what is called chained indexing. Here is an example.

In [356]: dfmi = pd.DataFrame([list('abcd'), .....: list('efgh'), .....: list('ijkl'), .....: list('mnop')], .....: columns=pd.MultiIndex.from_product([['one', 'two'], .....: ['first', 'second']])) .....: In [357]: dfmi Out[357]: one two first second first second 0 a b c d 1 e f g h 2 i j k l 3 m n o p

Compare these two access methods:

In [358]: dfmi['one']['second'] Out[358]: 0 b 1 f 2 j 3 n Name: second, dtype: object

In [359]: dfmi.loc[:, ('one', 'second')] Out[359]: 0 b 1 f 2 j 3 n Name: (one, second), dtype: object

These both yield the same results, so which should you use? It is instructive to understand the order of operations on these and why method 2 (.loc) is much preferred over method 1 (chained []).

dfmi['one'] selects the first level of the columns and returns a DataFrame that is singly-indexed. Then another Python operation dfmi_with_one['second'] selects the series indexed by 'second'. This is indicated by the variable dfmi_with_one because pandas sees these operations as separate events. e.g. separate calls to __getitem__, so it has to treat them as linear operations, they happen one after another.

Contrast this to df.loc[:,('one','second')] which passes a nested tuple of (slice(None),('one','second')) to a single call to __getitem__. This allows pandas to deal with this as a single entity. Furthermore this order of operations can be significantly faster, and allows one to index both axes if so desired.

The problem in the previous section is just a performance issue. What’s up with the SettingWithCopy warning? We don’t usually throw warnings around when you do something that might cost a few extra milliseconds!

But it turns out that assigning to the product of chained indexing has inherently unpredictable results. To see this, think about how the Python interpreter executes this code:

dfmi.loc[:, ('one', 'second')] = value # becomes dfmi.loc.__setitem__((slice(None), ('one', 'second')), value)

But this code is handled differently:

dfmi['one']['second'] = value # becomes dfmi.__getitem__('one').__setitem__('second', value)

See that __getitem__ in there? Outside of simple cases, it’s very hard to predict whether it will return a view or a copy (it depends on the memory layout of the array, about which pandas makes no guarantees), and therefore whether the __setitem__ will modify dfmi or a temporary object that gets thrown out immediately afterward. That’s what SettingWithCopy is warning you about!

Note

You may be wondering whether we should be concerned about the loc property in the first example. But dfmi.loc is guaranteed to be dfmi itself with modified indexing behavior, so dfmi.loc.__getitem__ / dfmi.loc.__setitem__ operate on dfmi directly. Of course, dfmi.loc.__getitem__(idx) may be a view or a copy of dfmi.

Sometimes a SettingWithCopy warning will arise at times when there’s no obvious chained indexing going on. These are the bugs that SettingWithCopy is designed to catch! pandas is probably trying to warn you that you’ve done this:

def do_something(df): foo = df[['bar', 'baz']] # Is foo a view? A copy? Nobody knows! # ... many lines here ... # We don't know whether this will modify df or not! foo['quux'] = value return foo

Yikes!

When you use chained indexing, the order and type of the indexing operation partially determine whether the result is a slice into the original object, or a copy of the slice.

pandas has the SettingWithCopyWarning because assigning to a copy of a slice is frequently not intentional, but a mistake caused by chained indexing returning a copy where a slice was expected.

If you would like pandas to be more or less trusting about assignment to a chained indexing expression, you can set the option mode.chained_assignment to one of these values:

'warn', the default, means a SettingWithCopyWarning is printed.
'raise' means pandas will raise a SettingWithCopyException you have to deal with.
None will suppress the warnings entirely.

In [360]: dfb = pd.DataFrame({'a': ['one', 'one', 'two', .....: 'three', 'two', 'one', 'six'], .....: 'c': np.arange(7)}) .....: # This will show the SettingWithCopyWarning # but the frame values will be set In [361]: dfb['c'][dfb['a'].str.startswith('o')] = 42

This however is operating on a copy and will not work.

>>> pd.set_option('mode.chained_assignment','warn') >>> dfb[dfb['a'].str.startswith('o')]['c'] = 42 Traceback (most recent call last) ... SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame. Try using .loc[row_index,col_indexer] = value instead

A chained assignment can also crop up in setting in a mixed dtype frame.

Note

These setting rules apply to all of .loc/.iloc.

The following is the recommended access method using .loc for multiple items (using mask) and a single item using a fixed index:

In [362]: dfc = pd.DataFrame({'a': ['one', 'one', 'two', .....: 'three', 'two', 'one', 'six'], .....: 'c': np.arange(7)}) .....: In [363]: dfd = dfc.copy() # Setting multiple items using a mask In [364]: mask = dfd['a'].str.startswith('o') In [365]: dfd.loc[mask, 'c'] = 42 In [366]: dfd Out[366]: a c 0 one 42 1 one 42 2 two 2 3 three 3 4 two 4 5 one 42 6 six 6 # Setting a single item In [367]: dfd = dfc.copy() In [368]: dfd.loc[2, 'a'] = 11 In [369]: dfd Out[369]: a c 0 one 0 1 one 1 2 11 2 3 three 3 4 two 4 5 one 5 6 six 6

The following can work at times, but it is not guaranteed to, and therefore should be avoided:

In [370]: dfd = dfc.copy() In [371]: dfd['a'][2] = 111 In [372]: dfd Out[372]: a c 0 one 0 1 one 1 2 111 2 3 three 3 4 two 4 5 one 5 6 six 6

Last, the subsequent example will not work at all, and so should be avoided:

>>> pd.set_option('mode.chained_assignment','raise') >>> dfd.loc[0]['a'] = 1111 Traceback (most recent call last) ... SettingWithCopyException: A value is trying to be set on a copy of a slice from a DataFrame. Try using .loc[row_index,col_indexer] = value instead

Warning

The chained assignment warnings / exceptions are aiming to inform the user of a possibly invalid assignment. There may be false positives; situations where a chained assignment is inadvertently reported.

Top List List

Can List have multiple columns?

Data Preparation

Select multiple columns in a Python DataFrame

Using the brackets notation:

Using the loc indexer:

Using the iloc indexer

Select Python columns by condition

Drop multiple columns

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