On Thu, Jun 12, 2014 at 12:18 PM, Robert Lehmann <m...@robertlehmann.de> wrote:
> Hi all,
>
> I have noticed there is a slight asymmetry in the way the interpreter
> (v3.3.5, reproduced also in v3.5.x) loads and stores globals. While loading
> globals from a custom mapping triggers __getitem__ just fine, writing seems
> to silently ignore __setitem__.
>
> class Namespace(dict):
> def __getitem__(self, key):
> print("getitem", key)
> def __setitem__(self, key, value):
> print("setitem", key, value)
>
> def fun():
> global x, y
> x # should call globals.__getitem__
> y = 1 # should call globals.__setitem__
>
> exec(fun.__code__, Namespace())
> # => getitem x
>
> I would have expected "setitem y 1" to show up as well, but to no avail. Am
> I doing something wrong? Is this on purpose?
Seems like a bug to me. I note that the STORE_NAME opcode does call
__setitem__:
>>> code = compile('x = 1', '', 'exec')
>>> dis.dis(code)
1 0 LOAD_CONST 0 (1)
3 STORE_NAME 0 (x)
6 LOAD_CONST 1 (None)
9 RETURN_VALUE
>>> exec(code, Namespace())
setitem x 1
But STORE_GLOBAL does not:
>>> code = compile('global x; x = 1', '', 'exec')
>>> dis.dis(code)
1 0 LOAD_CONST 0 (1)
3 STORE_GLOBAL 0 (x)
6 LOAD_CONST 1 (None)
9 RETURN_VALUE
>>> exec(code, Namespace())
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