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

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.