How should we use global variables correctly?

[Windson Yang]

I also want to know what is the difference between "using 'global
variables' in a py module" and "using a variable in class". For example:

In global.py:

    foo = 1
    def bar():
        global foo
        return foo + 1

In class.py

     class Example:
        def __init__(self):
            self.foo = 1
        def bar()
            return self.foo + 1

Expect the syntax, why using class variable self.foo would be better (or
more common)? I think the 'global' here is relative, foo is global in
global.py and self.foo is global in Example class. If the global.py is
short and clean enough (didn't have a lot of other class), they are pretty
much the same. Or I missed something?




Chris Angelico <rosuav at gmail.com> 于2019年8月23日周五 上午9:34写道：

> On Fri, Aug 23, 2019 at 11:24 AM Windson Yang <wiwindson at gmail.com> wrote:
> >
> > Thank you all for the great explanation, I still trying to find some good
> > example to use 'global', In CPython, I found an example use 'global' in
> > cpython/Lib/zipfile.py
> >
> >     _crctable = None
> >     def _gen_crc(crc):
> >         for j in range(8):
> >             if crc & 1:
> >                 crc = (crc >> 1) ^ 0xEDB88320
> >             else:
> >                 crc >>= 1
> >         return crc
> >
> >     def _ZipDecrypter(pwd):
> >         key0 = 305419896
> >         key1 = 591751049
> >         key2 = 878082192
> >
> >         global _crctable
> >         if _crctable is None:
> >             _crctable = list(map(_gen_crc, range(256)))
> >         crctable = _crctable
> >
> > _crctable only been used in the _ZipDecrypter function. IIUC, the code
> can
> > be refactored to
> >
> >     def _gen_crc(crc):
> >         ...stay the same
> >
> >     def _ZipDecrypter(pwd, _crctable=list(map(_gen_crc, range(256)))):
> >         key0 = 305419896
> >         key1 = 591751049
> >         key2 = 878082192
> >        crctable = _crctable
> >
> > Which avoid using 'global' keyword. Why we are not doing this? I guess
> the
> > reason we use 'global' here because we don't want to  create `_crctable =
> > list(map(_gen_crc, range(256)))`  every time when we run '_ZipDecrypter'
> > function. So we kinda cache _crctable with 'global', am I right?
>
> It's a cache that is made ONLY when it's first needed. If you put it
> in the function header, it has to be created eagerly as soon as the
> module is imported.
>
> ChrisA
> --
> https://mail.python.org/mailman/listinfo/python-list
>