What is list comprehension and how does it differ from a regular for loop?

List comprehension is a concise, readable way to build a new list by describing what each element should be, rather than imperatively appending in a loop. It runs faster than an equivalent for loop + append because CPython optimises the comprehension into a single opcode sequence without repeated list method lookups.

# Regular loop approach
squares = []
for x in range(1, 6):
    squares.append(x ** 2)

# Equivalent list comprehension
squares = [x ** 2 for x in range(1, 6)]  # [1, 4, 9, 16, 25]

# With a filter condition
even_squares = [x ** 2 for x in range(1, 11) if x % 2 == 0]
# [4, 16, 36, 64, 100]

# Nested comprehension (matrix flattening)
matrix = [[1, 2], [3, 4], [5, 6]]
flat = [num for row in matrix for num in row]
# [1, 2, 3, 4, 5, 6]

The pattern is always [expression for variable in iterable if condition]. The if clause is optional. For multiple nested loops, earlier for clauses are the outer loops — same order as you would write them imperatively.

When the result is not a list but needs to be computed once, use a generator expression ((x**2 for x in range(n))) to avoid building the full list in memory. For dictionaries use dict comprehension {k: v for ...}, for sets use set comprehension {expr for ...}.

Avoid cramming complex logic into a comprehension — if you need more than one condition or a nested if/else, a regular loop is often cleaner and easier to debug.