在Python中预分配或不预分配列表 [英] To preallocate or not to preallocate lists in Python

问题描述

何时和不应该在python中预分配列表列表? 例如，我有一个接受2个列表并从中创建列表列表的函数. 很像，但不完全是矩阵乘法.我应该预分配结果吗?

When should and shouldn't I preallocate a list of lists in python? For example, I have a function that takes 2 lists and creates a lists of lists out of it. Quite like, but not exactly, matrix multiplication. Should I preallocate the result,

X = Len(M)
Y = Len(F)
B = [[None for y in range(Y)] for x in range(X)]
for x in range(X):
    for y in range(Y):
        B[x][y] = foo(M[x], F[y])
return B

还是在创建过程中动态创建它?

or dynamically create it as I go?

B = []
for m in M:
    B.append([])
    for f in F:
        B[-1].append(foo(m, f))
return B

预分配似乎是不必要的，并且可能会更慢，但动态地看起来很模糊.特别是B[-1].append(...)似乎难以辨认.

Preallocating seems unnecessary and perhaps slower, but dynamically looks obfuscated. In particular, B[-1].append(...) seems illegible.

推荐答案

使用列表理解来简单地创建列表:

Simply create the list using list comprehension:

[[foo(m, f) for f in F] for m in M]

与预分配有关:预分配None的列表

Related to pre-allocation: Pre-allocating a list of None

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