Asymmetry in globals __getitem__/__setitem__
Peter Otten
__peter__ at web.de
Fri Jun 13 06:53:54 EDT 2014
robert at robertlehmann.de wrote:
> On Friday, June 13, 2014 8:07:45 AM UTC+2, Marko Rauhamaa wrote:
>>
>> The documentation is a bit vague about it:
>>
>> If only globals is provided, it must be a dictionary, which will be
>> used for both the global and the local variables. If globals and
>> locals are given, they are used for the global and local variables,
>> respectively. If provided, locals can be any mapping object.
>
>
> Interesting. This paragraph explicitly states "locals can be any mapping
> object," but that seems to be false:
>
>
> class Namespace(dict):
> def __getitem__(self, key):
> print("getitem", key)
> def __setitem__(self, key, value):
> print("setitem", key, value)
>
> def fun():
> x # should call locals.__getitem__
No, x is a global here.
> y = 1 # should call locals.__setitem__
>
> exec(fun.__code__, {}, Namespace())
>
>
> Neither __getitem__ nor __setitem__ seem to be called on the local
> variables.
Accessing fun.__code__ is clever, but unfortunately the compiler produces
different bytecodes for loading/storing variables inside a function.
Compare:
>>> import dis
>>> def fun(x=2):
... x
... y = 1
...
>>> dis.dis(fun.__code__)
2 0 LOAD_FAST 0 (x)
3 POP_TOP
3 4 LOAD_CONST 1 (1)
7 STORE_FAST 1 (y)
10 LOAD_CONST 0 (None)
13 RETURN_VALUE
>>> dis.dis(compile("x\ny=2", "<nofile>", "exec"))
1 0 LOAD_NAME 0 (x)
3 POP_TOP
2 4 LOAD_CONST 0 (2)
7 STORE_NAME 1 (y)
10 LOAD_CONST 1 (None)
13 RETURN_VALUE
Only the latter works as advertised:
>>> exec("x\ny=1", {}, Namespace())
getitem x
setitem y 1
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