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devel / comp.lang.python / Re: Variable scope inside and outside functions - global statement being overridden by assignation unless preceded by reference

Re: Variable scope inside and outside functions - global statement being overridden by assignation unless preceded by reference

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From: mats@wichmann.us (Mats Wichmann)
Newsgroups: comp.lang.python
Subject: Re: Variable scope inside and outside functions - global statement
being overridden by assignation unless preceded by reference
Date: Wed, 6 Mar 2024 06:57:01 -0700
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by: Mats Wichmann - Wed, 6 Mar 2024 13:57 UTC

On 3/6/24 05:55, Jacob Kruger via Python-list wrote:
> Ok, simpler version - all the code in a simpler test file, and working
> with two separate variables to explain exactly what am talking about:

> If you import the contents of that file into the python interpreter,
> dt_expiry will start off as "1970-01-01 00:00", and, if you execute
> do_it function, it will print out the new value assigned to the
> dt_expiry variable inside that function, but if you then again check the
> value of the dt_expiry variable afterwards, it's reverted to the 1970...
> value?
>
>
> If I take out the line that removes values from l_test # l_test.clear()
> # before appending new value to it, then it will also not retain it's
> new/additional child items after the function exits, and will just
> revert back to [1, 2, 3] each and every time.
>
>
> In other words, with some of the variable/object types, if you use a
> function that manipulates the contents of a variable, before then
> re-assigning it a new value, it seems like it might then actually
> update/manipulate the global variable, but, either just calling purely
> content retrieval functions against said objects, or assigning them new
> values from scratch seems to then ignore the global scope specified in
> the first line inside the function?
>
>
> Hope this makes more sense

No, it doesn't. Your code is working as one would expect. For example,
adding prints for the l_test variable, and removing the .clear() which
you claim makes it not work, shows me:

before: l_test=[1, 2, 3], id(l_test)=140153285385856
leaving do_it: l_test=[1, 2, 3, 1, 2, 3, 99], id(l_test)=140153285385856
after: l_test=[1, 2, 3, 1, 2, 3, 99], id(l_test)=140153285385856

It's the same list object, as you can see by the id values. And the list
is updating as expected.

And... you don't need the global statement for l_test. As it's mutable,
you can mutate it in the function; the global only acts on assignment.
Using "global" for that may make your intent more clear to readers
though, although static checkers will grumble at you.

You must be doing something additional that you're not telling us about.

Subject	Author
Re: Variable scope inside and outside functions - global statement being overrid	Mats Wichmann