python - 使用 os.walk 的嵌套列表理解

Question

尝试枚举特定目录中的所有文件(如 Linux 中的“find.”，或 Windows 中的“dir/s/b”)。

我提出了以下嵌套列表理解:

from os import walk
from os.path import join
root = r'c:\windows'      #choose any folder here
allfiles = [join(root,f) for f in files for root,dirs,files in walk(root)]

不幸的是，对于最后一个表达式，我得到:

NameError: 未定义名称"file"

与this相关问题，哪个(虽然有效)我无法理解嵌套列表理解的语法。

Answer 1

需要反转嵌套；

allfiles = [join(root,f) for root,dirs,files in walk(root) for f in files]

参见 list comprehension documentation :

When a list comprehension is supplied, it consists of a single expression followed by at least one for clause and zero or more for or if clauses. In this case, the elements of the new list are those that would be produced by considering each of the for or if clauses a block, nesting from left to right, and evaluating the expression to produce a list element each time the innermost block is reached.

换句话说，因为你基本上想要的道德等价物是:

allfiles = []
for root, dirs, files in walk(root):
    for f in files:
        allfiles.append(f)

您的列表理解应遵循相同的顺序。