List Comprehensions

A common pattern with for loops is building a new list by transforming or filtering an existing sequence. Run it live:

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>>> squares = []
>>> for n in range(1, 6):
...     squares.append(n ** 2)
...
>>> print(squares)
[1, 4, 9, 16, 25]

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Python provides a compact notation for exactly this pattern called a list comprehension:

[expression  for  item  in  iterable]

The result is a new list containing expression evaluated once for each item in iterable.

Basic Form

The squares example above becomes a single line. Run it live:

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>>> squares = [n ** 2 for n in range(1, 6)]
>>> print(squares)
[1, 4, 9, 16, 25]

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More examples:

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>>> # ASCII codes of each character in a string
>>> codes = [ord(ch) for ch in "hello"]
>>> print(codes)
[104, 101, 108, 108, 111]

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Try it live ▶

>>> # Lengths of each word in a list
>>> words = ["cat", "elephant", "ox"]
>>> lengths = [len(w) for w in words]
>>> print(lengths)
[3, 8, 2]

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Filtered Form

Add an if clause to keep only items that satisfy a condition:

[expression  for  item  in  iterable  if  condition]

The equivalent for loop is:

result = []
for item in iterable:
    if condition:
        result.append(expression)

Example — keep only the even numbers from 0 to 19. Run it and change the condition:

Try it live ▶

>>> evens = [n for n in range(20) if n % 2 == 0]
>>> print(evens)
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

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Example — extract only the vowels from a string:

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>>> s = "introduction"
>>> vowels = [ch for ch in s if ch in "aeiou"]
>>> print(vowels)
['i', 'o', 'u', 'i', 'o']

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Comprehensions reward experimentation. Change the expression, the iterable, or the if condition and run again:

Try it live ▶

>>> squares = [n ** 2 for n in range(1, 6)]
>>> print(squares)
[1, 4, 9, 16, 25]
>>> evens = [n for n in range(20) if n % 2 == 0]
>>> print(evens)
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
>>> s = "introduction"
>>> vowels = [ch for ch in s if ch in "aeiou"]
>>> print(vowels)
['i', 'o', 'u', 'i', 'o']

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Nested Comprehensions

You can nest for clauses to iterate over two sequences simultaneously. This produces a flat list — not a list of lists. Run it live:

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>>> pairs = [(r, c) for r in range(3) for c in range(3)]
>>> print(pairs)
[(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]

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The leftmost for is the outer loop; the rightmost is the inner loop — the same order as nested for statements written out explicitly.

When to Use List Comprehensions

Use a list comprehension when the goal is to produce a list from a sequence. Prefer a regular for loop when the body has side effects (printing, writing a file, mutating external state) or when the logic is complex enough that a one-liner hurts readability.

A good rule of thumb: if you can read the comprehension aloud as a single English phrase — “the square of n for n in range 1 to 5” — it is probably clear enough to use.