List Comprehensions

Note

Source: Python-specific — no equivalent exists in the C# edition. List comprehensions are a concise Python idiom for building lists from sequences. Every list comprehension has an equivalent for loop; the examples below show both forms so you can see the correspondence.

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

squares = []
for n in range(1, 6):
    squares.append(n ** 2)
print(squares)

Output:

[1, 4, 9, 16, 25]

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:

squares = [n ** 2 for n in range(1, 6)]
print(squares)

Output:

[1, 4, 9, 16, 25]

More examples:

# ASCII codes of each character in a string
codes = [ord(ch) for ch in "hello"]
print(codes)

Output:

[104, 101, 108, 108, 111]

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

Output:

[3, 8, 2]

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:

evens = [n for n in range(20) if n % 2 == 0]
print(evens)

Output:

[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

Example — extract only the vowels from a string:

s = "introduction"
vowels = [ch for ch in s if ch in "aeiou"]
print(vowels)

Output:

['i', 't', 'o', 'd', 'u', 't', 'i', 'o']

Nested Comprehensions

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

pairs = [(r, c) for r in range(3) for c in range(3)]
print(pairs)

Output:

[(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]

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.