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Shadowing, was Re: learning python ...

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From: __pete...@web.de (Peter Otten)
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Subject: Shadowing, was Re: learning python ...
Date: Tue, 25 May 2021 11:46:31 +0200
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by: Peter Otten - Tue, 25 May 2021 09:46 UTC

On 25/05/2021 05:20, hw wrote:

>> We're talking about many different things. If it's simply "num = ..."
>> followed by "num = ...", then it's not a new variable or anything,
>> it's simply rebinding the same name. But when you do "int = ...", it's
>> shadowing the builtin name.
>
> Why? And how is that "shadowing"?
>
> What if I wanted to re-define the built-in thing?

When you write

foo

in a module the name "foo" is looked up in the module's global
namespace. If it's not found it is looked up in builtins. If that lookup
fails a NameError exception is raised.

>>> import builtins
>>> builtins.foo = "built-in foo"
>>> foo
'built-in foo'
>>> foo = "module-global-foo" # at this point builtins.foo is shadowed
>>> foo
'module-global-foo'
>>> del foo # delete the global to make the built-in visible again:
>>> foo
'built-in foo'

That mechanism allows newbies who don't know the builtins to write

list = [1, 2, 3]

without affecting other modules they may use. It also allows old scripts
that were written when a builtin name did not yet exist to run without
error.

The problem you ran into, using a name in two roles

float = float("1.2")

could be circumvented by writing

float = builtins.float("1.2")

but most of the time it is more convenient to think of builtins as names
that are predefined in your current module and act accordingly.

As you see redefining a builtin is as easy as importing builtins and
setting the respective attribute

>>> builtins.float = int
>>> float(1.23)
1

Subject	Author
Shadowing, was Re: learning python ...	Peter Otten