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Use a trailing if to filter items out; use value_if_true if condition else value_if_false before the for to choose a value for each item. The difference is what happens when the condition is false: the item is either omitted or included with an alternate value.
Filtering items: put if after for
A filter decides whether an iteration contributes anything to the result. If its condition is false, Python skips that item; there is no alternate value to provide.
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numbers = [1, 2, 3, 4, 5]
evens = [n for n in numbers if n % 2 == 0]
# [2, 4]This is equivalent to an ordinary loop that appends only when the condition is true:
evens = []
for n in numbers:
if n % 2 == 0:
evens.append(n)The comprehension’s filter clause has no else: its job is to allow or skip an iteration, not to produce a value. The Python Language Reference defines comprehension structure and conditional expressions; the official Functional Programming HOWTO explains that a filtered item is added only when the condition is true.
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Choosing an output value: put if/else before for
When every input should produce an output, put a conditional expression in the leading expression. Its form is value_if_true if condition else value_if_false; Python evaluates the condition and returns the selected branch.
numbers = [1, 2, 3, 4, 5]
labels = ["even" if n % 2 == 0 else "odd" for n in numbers]
# ['odd', 'even', 'odd', 'even', 'odd']The else is required because this expression must provide a value for either outcome. The Python Language Reference describes it as an alternative to the if–else statement, but inside a comprehension it is an expression that supplies the value to add to the list.
Use both when you need to filter and choose
A comprehension can contain both forms: the leading conditional expression chooses the output value, while the trailing filter decides whether the input participates at all.
labels_for_valid = [
"even" if n % 2 == 0 else "odd"
for n in numbers
if n > 1
]
# ['even', 'odd', 'even', 'odd']Read it as: for each number, first keep it only if it is greater than 1; for each kept number, produce either "even" or "odd". The corresponding loop makes the two jobs explicit:
labels_for_valid = []
for n in numbers:
if n > 1:
labels_for_valid.append("even" if n % 2 == 0 else "odd")A quick decision rule
- If a false condition means “leave this item out,” use a trailing filter:
[result for item in items if condition]. - If a false condition means “include this item with another value,” use a conditional expression before
for:[yes_value if condition else no_value for item in items]. - If you need both behaviors, combine the conditional expression and trailing filter.
Common placement mistakes
Adding else to a filter
This is invalid because a comprehension filter does not have an alternate-result branch:
[n for n in numbers if n % 2 == 0 else 0]If every number should remain and odd numbers should become zero, make the condition part of the leading expression instead:
[n if n % 2 == 0 else 0 for n in numbers]Using a conditional expression when you meant to omit items
This keeps every input and inserts None for odd numbers:
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[n for n in numbers if n % 2 == 0]Writing an if statement in the expression
A comprehension’s result expression must be an expression, not a statement block. Use yes_value if condition else no_value, rather than an if condition: / else: statement.
When a generator expression may fit better
A list comprehension, written with square brackets, creates a list. A generator expression, written with parentheses, produces an iterator that computes values as they are consumed. The official Functional Programming HOWTO recommends considering generators for very large data or infinite streams when materializing every value at once is unsuitable. Use a list comprehension when the program needs the list itself.
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