List Comprehension

List comprehension builds a new list from an existing one in a single, readable line.

The classic example

Old way:

python
fruits = ["apple", "banana", "cherry"]
newlist = []
for x in fruits:
    if "a" in x:
        newlist.append(x)
print(newlist)

### ['apple', 'banana']

New way:

python
newlist = [x for x in fruits if "a" in x]
print(newlist)

### ['apple', 'banana']

The syntax

python
newlist = [expression for item in iterable if condition]

• expression: what to put in the new list.
• item: the loop variable.
• iterable: the collection to loop over.
• condition (optional): a filter.

Transform as you go

python
newlist = [x.upper() for x in fruits]
print(newlist)

### ['APPLE', 'BANANA', 'CHERRY']

Filter with condition

python
newlist = [x for x in fruits if x != "apple"]
print(newlist)

### ['banana', 'cherry']

Use range

python
squares = [x * x for x in range(6)]
print(squares)

### [0, 1, 4, 9, 16, 25]

Use else in the expression

python
labels = ["big" if x > 10 else "small" for x in numbers]

TL;DR

• [expr for item in iterable if cond] builds a list.
• The condition filters; the expression transforms.
• It replaces many for-loop-plus-append patterns.