List Comprehension

列表推导式

≈ 20 min · python-lists python-loops · Open in WolfCode

The map + filter pattern from Think Python §10.7, compressed into one line.

The shape

[ expression  for  var  in  iterable  if  condition ]

Squares of every even number in 0..n

def squares_of_evens(n):
    return [x * x for x in range(n) if x % 2 == 0]

print(squares_of_evens(6))     # [0, 4, 16]
print(squares_of_evens(10))    # [0, 4, 16, 36, 64]

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Pure filter — leave the expression as the variable

Python · runnable

def words_above_length(words, min_len):
    return [w for w in words if len(w) > min_len]

print(words_above_length(["a", "bee", "cat", "donkey"], 2))
# ['bee', 'cat', 'donkey']

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Equivalent loop version

Python · runnable

# These two produce the same list.
result_a = [x * x for x in range(10) if x % 2 == 0]

result_b = []
for x in range(10):
    if x % 2 == 0:
        result_b.append(x * x)

print(result_a == result_b)    # True

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