Asymmetry in globals __getitem__/__setitem__

[Peter Otten]

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