Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #81970

Re: About Modifying Globals

Show key headers only | View raw

On 12/04, LJ 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.

Well. To understand the difference, one first has to know the mechanism of CPython.
When you have a script, CPython will first parse it, and then generate bytecode representation, and then finally execute it.
If you take a look at the bytecode of both your source code, you will notice, that.

In the first snippet, there is something like LOAD_NAME, which loads the global name a.
However, in the second snippet, there would be a LOAD_FAST, which loads the local name b (which is, technically speaking, stored in the PyFrameObj).

The reason Python treats it differently, is, IIUC, for better semantic meaning, and also, for huge performance improvement.


Hope that explains.


Regards.

-- 
Shiyao Ma
http://introo.me

Back to comp.lang.python | Previous | Next — Previous in thread | Find similar | Unroll thread

Thread

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

csiph-web