Python Language List Comprehensions Conditional List Comprehensions
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]
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]
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]
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.
