Scoping rules for class definitions
Rotwang
sg552 at hotmail.co.uk
Tue Apr 8 18:09:11 EDT 2014
On 04/04/2014 19:55, Ian Kelly wrote:
> On Fri, Apr 4, 2014 at 12:37 PM, Rotwang <sg552 at hotmail.co.uk> wrote:
>> Hi all. I thought I had a pretty good grasp of Python's scoping rules, but
>> today I noticed something that I don't understand. Can anyone explain to me
>> why this happens?
>>
>>>>> x = 'global'
>>>>> def f1():
>> x = 'local'
>> class C:
>> y = x
>> return C.y
>>
>>>>> def f2():
>> x = 'local'
>> class C:
>> x = x
>> return C.x
>>
>>>>> f1()
>> 'local'
>>>>> f2()
>> 'global'
>
> Start by comparing the disassembly of the two class bodies:
>
>>>> dis.dis(f1.__code__.co_consts[2])
> 3 0 LOAD_NAME 0 (__name__)
> 3 STORE_NAME 1 (__module__)
> 6 LOAD_CONST 0 ('f1.<locals>.C')
> 9 STORE_NAME 2 (__qualname__)
>
> 4 12 LOAD_CLASSDEREF 0 (x)
> 15 STORE_NAME 3 (y)
> 18 LOAD_CONST 1 (None)
> 21 RETURN_VALUE
>>>> dis.dis(f2.__code__.co_consts[2])
> 3 0 LOAD_NAME 0 (__name__)
> 3 STORE_NAME 1 (__module__)
> 6 LOAD_CONST 0 ('f2.<locals>.C')
> 9 STORE_NAME 2 (__qualname__)
>
> 4 12 LOAD_NAME 3 (x)
> 15 STORE_NAME 3 (x)
> 18 LOAD_CONST 1 (None)
> 21 RETURN_VALUE
>
> The only significant difference is that the first uses
> LOAD_CLASSDEREF, which I guess is the class version of LOAD_DEREF for
> loading values from closures, at line 4 whereas the second uses
> LOAD_NAME. So the first one knows about the x in the nonlocal scope,
> whereas the second does not and just loads the global (since x doesn't
> yet exist in the locals dict).
>
> Now why doesn't the second version also use LOAD_CLASSDEREF? My guess
> is because it's the name of a local; if it were referenced a second
> time in the class then the second LOAD_CLASSDEREF would again get the
> x from the nonlocal scope, which would be incorrect.
Thanks (sorry for the slow reply, I've had a busy few days).
For anyone who's interested, I also found an interesting discussion of
the above in the following thread:
https://mail.python.org/pipermail/python-dev/2002-April/023427.html
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