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Re: At a loss on python scoping.

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  Re: At a loss on python scoping. Shiyao Ma <i@introo.me> - 2013-03-26 14:19 +0800
    Re: At a loss on python scoping. Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-03-26 12:43 +0000

#41856 — Re: At a loss on python scoping.

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PS, I now python's scoping rule is lexical rule (aka static rule). How does
LEGB apply to class?

On Tue, Mar 26, 2013 at 2:17 PM, Shiyao Ma <i@introo.me> wrote:

> Hi,
> suppose I have a file like this:
> class A:
>     r = 5
>     def func(self, s):
>         self.s = s
> a = A()
> print(a.r)    # this should print 5, but where does py store the name of r
>
> a.func(3)
> print(a.s)    # this should print 3, also where does py store this name.
> what's the underlying difference between the above example?
>
>
> --
> My gpg pubring is available via: gpg --keyserver subkeys.pgp.net--recv-keys 307CF736
>
> More on: http://about.me/introom
>
>


-- 
My gpg pubring is available via: gpg --keyserver
subkeys.pgp.net--recv-keys 307CF736

More on: http://about.me/introom

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#41895

On Tue, 26 Mar 2013 14:19:21 +0800, Shiyao Ma wrote:

> PS, I now python's scoping rule is lexical rule (aka static rule). How
> does LEGB apply to class?

It doesn't. Python does not use the same lookup rules for attributes and 
unqualified names.

Attribute lookups follow inheritance rules. `instance.name` searches in 
this order, from first to last:

if it exists, call instance.__class__.__getattribute__(name);
look in the instance __dict__;
look in the class __dict__; 
for each superclass in the inheritance chain:
    look in the superclass __dict__;
if it exists, call instance.__class__.__getattr__(name)

(the above is a little simplified, but is close enough for ordinary work).


Unqualified `name` follow this lookup rule:

if name is recognised by the compiler as a local name:
    look in the local function namespace;
otherwise:
    look in any enclosing function scopes;
    look in the global scope;
    look in the builtins.



-- 
Steven

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