NameError：全局名称未在python的genexpr中定义

Question

This:

import os

class A(object):

    os_sep = os.sep
    _silentSkipsStart = {u'a dir%s' % os_sep}

    def _refreshBasic(self,os_sep=os_sep, skips_start=tuple(
                x.replace(os_sep, u'') for x in _silentSkipsStart)):
        pass

Fails with:

Traceback (most recent call last):
  File "C:/Users/MrD/.PyCharm50/config/scratches/scratch", line 3, in <module>
    class A(object):
  File "C:/Users/MrD/.PyCharm50/config/scratches/scratch", line 9, in A
    x.replace(os_sep, u'') for x in _silentSkipsStart)):
  File "C:/Users/MrD/.PyCharm50/config/scratches/scratch", line 9, in <genexpr>
    x.replace(os_sep, u'') for x in _silentSkipsStart)):
NameError: global name 'os_sep' is not defined

I guess that bringing os_sep = os.sep to global scope should cure that (and I probably should from a design point of view) - but here I am not getting python scoping rules: why is os_sep resolved alright in the other cases and not in genexpr ?

Answer 1

Well as explained to the answer linked in the comments by @PadraicCunningham:

The scope of names defined in a class block is limited to the class block; it does not extend to the code blocks of methods – this includes comprehensions and generator expressions since they are implemented using a function scope

That is for python 3 - for python 2 list comprehensions would work - but the comprehension variable would leak into class scope - so this would leak x:

    def _refreshBasic(self,os_sep=os_sep, skips_start=tuple(
                [x.replace(os_sep, u'') for x in _silentSkipsStart])):
        pass

So I went with:

import os

os_sep = os.sep
class A(object):
    _silentSkipsStart = {u'a dir%s' % os_sep}

    def _refreshBasic(self,os_sep=os_sep, skips_start=tuple(
                x.replace(os_sep, u'') for x in _silentSkipsStart)):
        pass