Conditional List Comprehensions

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Example

Given a list comprehension you can append one or more if conditions to filter values.

[<expression> for <element> in <iterable> if <condition>]

For each <element> in <iterable>; if <condition> evaluates to True, add <expression> (usually a function of <element>) 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 <expression> part of the list comprehension. Consider the following example:

[x if x % 2 == 0 else None for x in range(10)]
# Out: [0, None, 2, None, 4, None, 6, None, 8, None]

Live demo

Here the conditional expression isn't a filter, but rather an operator determining the value to be used for the list items:

<value-if-condition-is-true> if <condition> else <value-if-condition-is-false>

This 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.

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Contributors: 66
2017-06-21
Licensed under: CC-BY-SA

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