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a clarification on the "global" statement sought

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  a clarification on the "global" statement sought "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2016-03-11 08:13 +0000
    Re: a clarification on the "global" statement sought Chris Angelico <rosuav@gmail.com> - 2016-03-11 19:29 +1100
      Re: a clarification on the "global" statement sought "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2016-03-11 08:53 +0000
        Re: a clarification on the "global" statement sought Peter Otten <__peter__@web.de> - 2016-03-11 10:23 +0100
    Re: a clarification on the "global" statement sought Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-11 08:31 +0000
      Re: a clarification on the "global" statement sought "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2016-03-11 09:09 +0000
        Re: a clarification on the "global" statement sought Chris Angelico <rosuav@gmail.com> - 2016-03-11 20:29 +1100
    Re: a clarification on the "global" statement sought eryk sun <eryksun@gmail.com> - 2016-03-11 03:20 -0600

#104589 — a clarification on the "global" statement sought

When might a "global" statement be used in the outermost level of a module?

(whereby, I assume a module is equivalent to a python file, correct?)

TIA for any thoughts.

cts

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#104591

On Fri, Mar 11, 2016 at 7:13 PM, Charles T. Smith
<cts.private.yahoo@gmail.com> wrote:
> When might a "global" statement be used in the outermost level of a module?
>
> (whereby, I assume a module is equivalent to a python file, correct?)
>

Usefully? Never.

Simple question - simple answer :)

ChrisA

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#104594

On Fri, 11 Mar 2016 19:29:20 +1100, Chris Angelico wrote:

> Usefully? Never.
> 
> Simple question - simple answer :)
> 
> ChrisA


Right, that was the expected answer as well.  I just ran into that in
legacy code, checked out the documentation and couldn't really make that
out.  So I figured I better ask here before I end up with egg on my face

:)

Thanks folks.

Actually, it would be nice if there were a -lint or -warnings flag
for such cases, a la perl or gcc.

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#104600

Charles T. Smith wrote:

> On Fri, 11 Mar 2016 19:29:20 +1100, Chris Angelico wrote:
> 
>> Usefully? Never.
>> 
>> Simple question - simple answer :)
>> 
>> ChrisA
> 
> 
> Right, that was the expected answer as well.  I just ran into that in
> legacy code, checked out the documentation and couldn't really make that
> out.  So I figured I better ask here before I end up with egg on my face
> 
> :)
> 
> Thanks folks.
> 
> Actually, it would be nice if there were a -lint or -warnings flag
> for such cases, a la perl or gcc.

There is an external tool called pylint that among other things detects 
superfluous global statements.

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#104592

On 11/03/2016 08:13, Charles T. Smith wrote:
> When might a "global" statement be used in the outermost level of a module?

Never. Hopefully this 
http://www.python-course.eu/python3_global_vs_local_variables.php can 
explain it better than I can :)

>
> (whereby, I assume a module is equivalent to a python file, correct?)

Correct.

>
> TIA for any thoughts.
>
> cts
>

-- 
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.

Mark Lawrence

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#104597

On Fri, 11 Mar 2016 08:31:22 +0000, Mark Lawrence wrote:

> Never. Hopefully this
> http://www.python-course.eu/python3_global_vs_local_variables.php can
> explain it better than I can :)


The article is good, I'm glad to have confirmed what I have so empirical
stumbled over.
... Irrespective of the "python-speak" (or to be fair,
evangelical-speak):

  "Python deals with variables the other way around.
   They are local, if not otherwise declared.
   ...
    def f(): 
        print(s) 
    s = "I love Paris in the summer!"
    f()
    ...
    As there is no local variable s, i.e. no assignment to s, the value
    from the ***global*** variable s will be used."

Indeed "maverick": that a variable can be an undefined global
and then only appears as such when assigned to, has caused me
no end of grief.

Nevertheless, python is still a fun language to program in.

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#104603

On Fri, Mar 11, 2016 at 8:09 PM, Charles T. Smith
<cts.private.yahoo@gmail.com> wrote:
>   "Python deals with variables the other way around.
>    They are local, if not otherwise declared.
>    ...
>     def f():
>         print(s)
>     s = "I love Paris in the summer!"
>     f()
>     ...
>     As there is no local variable s, i.e. no assignment to s, the value
>     from the ***global*** variable s will be used."
>
> Indeed "maverick": that a variable can be an undefined global
> and then only appears as such when assigned to, has caused me
> no end of grief.

Looking purely at the function definition, you can see with 100%
certainty that it references two non-local names: "print" and "s".
Neither is assigned to within the function, so both are looked up
externally. At run time, s is resolved as a module-level name; print
is not, so Python looks further, to the builtins. Had s not been
assigned to, it would still be a global name in the function, but it
would fail to be found in either the module namespace or the builtins,
and would result in NameError. But either way, it's a global name,
whether it's assigned to or not.

ChrisA

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#104599

On Fri, Mar 11, 2016 at 2:13 AM, Charles T. Smith
<cts.private.yahoo@gmail.com> wrote:
> When might a "global" statement be used in the outermost level of a module?

You wouldn't need this in a normal module, because locals and globals
are the same. It may be useful if you're using exec with separate
locals and globals, and need to set a global. For example:

    source = 'global y; x, y = 1, 2'
    gvars, lvars = {}, {}
    exec(source, gvars, lvars)

    >>> lvars
    {'x': 1}
    >>> gvars['y']
    2

Probably you'll never need this.

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