What's a List Comprehension in Python?

Introduced in Python 2.0, list comprehensions are a concise way to create a new list by applying an expression to each element of an existing iterable. You can think of it as a shorthand way of writing a for loop that creates a new list based on some condition or transformation.

Syntax

List comprehensions have the following basic syntax:

# Python 2+
new_list = [expression for item in iterable]

Where:

1. expression is the expression that's evaluated and included in the resulting list;
2. item is the variable representing the current element in the iterable;
3. iterable is a collection of elements to iterate over.

Besides the basic syntax (mentioned above), list comprehensions can have:

1. Any kind of expression at the beginning (such as if-else clauses, nested list comprehensions, etc.);
2. Zero or more for clauses and/or if clauses after the main for clause.

A list comprehension is always enclosed within squared brackets ([]), and consists of at least an expression at the beginning followed by the main for clause (after which you can have zero or more for and/or if clauses). It results in a new list created from evaluating the expression in the context of the for and if clauses which follow it.

Examples

For example, consider the following for loop that adds 2 to each number in an existing list:

existing_nums = [0, 1, 2, 3]
new_nums = []

for num in existing_nums:
    new_nums.append(num + 2)

print(new_nums) # [2, 3, 4, 5]

You can rewrite this using list comprehensions, in a shorter, more readable syntax, like so:

existing_nums = [0, 1, 2, 3]
new_nums = [num + 2 for num in existing_nums]

print(new_nums) # [2, 3, 4, 5]

You can also rewrite the same code using map() like so:

existing_nums = [0, 1, 2, 3]
new_nums = list(map(lambda num: num + 2, existing_nums))

print(new_nums) # [2, 3, 4, 5]

They're all equivalent.

Pros and Cons of Using List Comprehensions

Below are some advantages of using list comprehensions in Python:

• Provides a concise syntax for creating new lists based on existing lists or other iterable objects;
• Is more readable than using traditional for loops and conditional statements;
• Has faster execution time compared to traditional loops for simple operations;
• Eliminates the need to create and append to an empty list (as you would in a for loop for the same code);
• Can be used to filter, map, and transform data in a single line of code;
• Can be nested to create complex operations in a readable way;
• Often results in more "Pythonic" code, making it easier for other developers to read and understand.

While list comprehensions have many advantages, there are also some potential disadvantages and limitations to consider, for example, like the following:

• Can be less readable than a traditional for loop for more complex operations or multiple nested conditions;
• May result in long, convoluted lines of code that could be difficult to read and debug;
• Not always the most efficient or memory-friendly solution, especially for large datasets or more complex operations.