Using else in list comprehension
ExampleGiven a list comprehension you can append one or more if conditions to filter values. [ for in if ]For each in ; if evaluates to True, add (usually a function of ) to the returned list. For example, this can be used to extract only even numbers from a sequence of integers: [x for x in range(10) if x % 2 == 0] # Out: [0, 2, 4, 6, 8]Live demo The above code is equivalent to: even_numbers = [] for x in range(10): if x % 2 == 0: even_numbers.append(x) print(even_numbers) # Out: [0, 2, 4, 6, 8]Also, a conditional list comprehension of the form [e for x in y if c] (where e and c are expressions in terms of x) is equivalent to list(filter(lambda x: c, map(lambda x: e, y))). Despite providing the same result, pay attention to the fact that the former example is almost 2x faster than the latter one. For those who are curious, this is a nice explanation of the reason why. Note that this is quite different from the ... if ... else ... conditional expression (sometimes known as a ternary expression) that you can use for the part of the list comprehension. Consider the following example: Live demo Here the conditional expression isn't a filter, but rather an operator determining the value to be used for the list items: if elseThis becomes more obvious if you combine it with other operators: [2 * (x if x % 2 == 0 else -1) + 1 for x in range(10)] # Out: [1, -1, 5, -1, 9, -1, 13, -1, 17, -1]Live demo If you are using Python 2.7, xrange may be better than range for several reasons as described in the xrange documentation. [2 * (x if x % 2 == 0 else -1) + 1 for x in xrange(10)] # Out: [1, -1, 5, -1, 9, -1, 13, -1, 17, -1]The above code is equivalent to: numbers = [] for x in range(10): if x % 2 == 0: temp = x else: temp = -1 numbers.append(2 * temp + 1) print(numbers) # Out: [1, -1, 5, -1, 9, -1, 13, -1, 17, -1]One can combine ternary expressions and if conditions. The ternary operator works on the filtered result: [x if x > 2 else '*' for x in range(10) if x % 2 == 0] # Out: ['*', '*', 4, 6, 8]The same couldn't have been achieved just by ternary operator only: [x if (x > 2 and x % 2 == 0) else '*' for x in range(10)] # Out:['*', '*', '*', '*', 4, '*', 6, '*', 8, '*']See also: Filters, which often provide a sufficient alternative to conditional list comprehensions. |