python - 我如何在列表理解中表示嵌套 for？

Question

我有一个类似于下面的构造，我如何使用 1 个(或多个)列表推导来获得相同的输出？

f2remove = []
for path in (a,b):
    for item in os.listdir(path):
        if os.path.isdir(os.path.join(path,item)):
            x = parse_name(item)
            if x and (ref - x).days >= 0:
                f2remove.append(os.path.join(path,item))

我试过很多东西，比如

files = [parse_name(item)\
         for item in os.listdir(path) \
         for path in (a,b)\
         if os.path.isdir(os.path.join(path,item))] # get name error
f2remove = [] # problem, don't have path...

错误:

Traceback (most recent call last):
  File "C:\Users\karuna\Desktop\test.py", line 33, in <module>
    for item in os.listdir(path) \
NameError: name 'path' is not defined

Answer 1

for 的顺序不变。你的情况变得尴尬:

f2remove = [
    os.path.join(path, item)
    for path in (a,b)
        for item in os.listdir(path)
            if os.path.isdir(os.path.join(path, item))
               for x in (parse_name(item),)
                  if x and (ref - x).days >= 0
   ]

基本上，要将嵌套的 for 转换为列表推导式，您只需将要附加的内容移到前面:

result = []
for a in A:
    for b in B:
      if test(a, b):
         result.append((a, b))

成为

result = [
    (a, b)
    for a in A
    for b in B
    if test(a, b)
]