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How to Use if-elif-else in Python List Comprehensions

List comprehensions in Python provide a concise way to create new lists based on existing iterables. While list comprehensions are often used with a single if condition, it's also possible to incorporate if-elif-else logic for more complex filtering and transformations.

This guide explains how to use if-elif-else (and simple if-else) within list comprehensions, balancing brevity with code readability.

List Comprehensions with if-else​

The basic structure for using an if-else statement in a list comprehension is:

new_list = [expression_if_true if condition else expression_if_false for item in iterable]
  • expression_if_true: The value to include in the new_list if condition is true.
  • condition: The boolean expression to evaluate for each item.
  • expression_if_false: The value to include in the new_list if condition is false.
  • item: The current element being processed from the iterable.
  • iterable: The original list (or other iterable) you're processing.

Example:

a_list = [1, 2, 2, None, 1, None]
new_list = [0 if item is None else item for item in a_list]
print(new_list)  # Output: [1, 2, 2, 0, 1, 0]

In this case, the new_list is created with:

  • Existing items from a_list, if the item is not equal to None
  • 0 if the item is None

List Comprehensions with if-elif-else (Nested Ternary Operators)​

To incorporate if-elif-else logic, you nest ternary operators (condition ? value_if_true : value_if_false). This gets complex quickly, so use it judiciously.

a_list = [1, 2, 2, 5, 1, 9]
new_list = [
    'a' if item == 1
    else 'b' if item == 2
    else 'c'
    for item in a_list
]
print(new_list)  # Output: ['a', 'b', 'b', 'c', 'a', 'c']

The expression is evaluated from left to right. It checks if the item is equal to 1 and returns "a" if it is. Otherwise, it evaluates the next condition.

The structure is:

new_list = [
    expression1 if condition1
    else expression2 if condition2
    else expression3
    for item in iterable
]

Another example:

a_list = [1, 2, 2, None, 1, None]

new_list = [
    0 if item is None
    else item if item % 2 == 0 else item + 11
    for item in a_list
]
print(new_list)  # Output: [12, 2, 2, 0, 12, 0]

Readability and Alternatives​

note

Nested ternary operators within list comprehensions can quickly become hard to read. If you have more than one or two elif conditions, or if the logic is complex, it's almost always better to use a standard for loop with if/elif/else statements for clarity.

Here's the equivalent of the previous example using a standard loop:

a_list = [1, 2, 2, None, 1, None]
new_list = []

for item in a_list:
    if item is None:
        new_list.append(0)
    elif item % 2 == 0:
        new_list.append(item)
    else:
        new_list.append(item + 11)

print(new_list)  # Output: [12, 2, 2, 0, 12, 0]

This for loop version is significantly more readable than the nested ternary version. Prioritize readability, especially for complex logic.

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