Python 列表理解与求和

Question

我有一些看起来像这样的 python 代码:

mat = [[3], [4], [4], [0], [1, 2]]
nwalls = 5*[1]
for i in range(1,3):
    nwalls = [sum(nwalls[k] for k in mat[j]) for j in range(5)]
# nwalls = [1, 2, 2, 1, 2]

如果不使用列表理解来让我理解，我一辈子都无法将其展开为语法。请协助。

Answer 1

直接翻译就是

mat = [[3], [4], [4], [0], [1, 2]]
nwalls = 5*[1]
for i in range(1, 3):
    _nwalls = []
    for j in range(5):
        tot = 0                # - sum
        for k in mat[j]:       #  /
            tot += nwalls[k]   # /
        _nwalls.append(tot)
    nwalls = _nwalls

(nwalls[k] for k in mat[j]) 它自己是一个生成器，在 python repl 中，你可以检查它:

>>> y = (x for x in range(10))
>>> type(y)
<class 'generator'>

和sum可以采用生成器，如sum( (x for x in range(10)) )，以及PEP289说

if a function call has a single positional argument, it can be a generator expression without extra parentheses, but in all other cases you have to parenthesize it.

所以它看起来像 sum(x for x in range(10))