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Re: About Modifying Globals

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On Thu, Dec 4, 2014 at 1:09 PM, LJ <luisjosenovoa@gmail.com> wrote:
>
> Hi All,
>
> I have a quick question regarding the modification of global variables
within functions. To illustrate, consider the following toy example:
>
> a={"1": set()}
> b=9
>
> def gt(l):
>    a["1"] = a["1"] | set([l])
>
> When calling this last function and checking the a dictionary, I get:
>
> >>> gt(5)
> >>> a
> {"1": set([5])}
>
>
> The set in the dictionary was modified. The question is, why isn't it
necessary to declare a as global within the gt function, as apposed to a
case like
>
> def gt2(l):
>    b=b+l
>
> where I need to declare b as global within the function to avoid:
>
> UnboundLocalError: local variable 'b' referenced before assignment.

https://docs.python.org/3/faq/programming.html#what-are-the-rules-for-local-and-global-variables-in-python

In the first case, you never assign to a; you only modify it. Since it's
never assigned locally the compiler knows it can't be a local variable, so
it automatically makes it a global.

In the second case, the assignment to b causes the compiler to treat the
variable as local by default, so it needs to be explicitly marked as global.

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About Modifying Globals LJ <luisjosenovoa@gmail.com> - 2014-12-04 12:09 -0800
  Re: About Modifying Globals Chris Angelico <rosuav@gmail.com> - 2014-12-05 07:22 +1100
  Re: About Modifying Globals Ian Kelly <ian.g.kelly@gmail.com> - 2014-12-04 13:21 -0700
  Re: About Modifying Globals Dave Angel <davea@davea.name> - 2014-12-04 18:54 -0500
  Re: About Modifying Globals Chris Angelico <rosuav@gmail.com> - 2014-12-05 10:58 +1100
  Re: About Modifying Globals Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-12-05 13:09 +1100
    The binding operator, and what gets bound to what (was: About Modifying Globals) Ben Finney <ben+python@benfinney.id.au> - 2014-12-05 16:36 +1100
      Re: The binding operator, and what gets bound to what (was: About Modifying Globals) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-12-05 20:53 +1100
        Re: The binding operator, and what gets bound to what (was: About Modifying Globals) Chris Angelico <rosuav@gmail.com> - 2014-12-05 21:03 +1100
          Re: The binding operator, and what gets bound to what (was: About Modifying Globals) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-12-05 23:26 +1100
            Re: The binding operator, and what gets bound to what (was: About Modifying Globals) Chris Angelico <rosuav@gmail.com> - 2014-12-05 23:42 +1100
        Re: The binding operator, and what gets bound to what Ned Batchelder <ned@nedbatchelder.com> - 2014-12-05 07:06 -0500
          Re: The binding operator, and what gets bound to what Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-12-05 23:34 +1100
            Re: The binding operator, and what gets bound to what Chris Angelico <rosuav@gmail.com> - 2014-12-05 23:48 +1100
            Re: The binding operator, and what gets bound to what Ned Batchelder <ned@nedbatchelder.com> - 2014-12-05 07:56 -0500
  Re: About Modifying Globals "shiyao.ma" <i@introo.me> - 2014-12-05 10:53 +0800

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