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Asymmetry in globals __getitem__/__setitem__

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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?

Cheers,
Robert

PS.  I found a 3.3.x commit (e3ab8aa
<http://hg.python.org/cpython/rev/e3ab8aa0216c>) which fixed the
LOAD_GLOBAL opcode to support other types than dict, but STORE_GLOBAL seems
to use bare PyDict_SetItem instead of dispatching to PyObject_SetItem.

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Asymmetry in globals __getitem__/__setitem__ Robert Lehmann <mail@robertlehmann.de> - 2014-06-12 20:18 +0200
  Re: Asymmetry in globals __getitem__/__setitem__ Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-06-13 12:38 +1200
    Re: Asymmetry in globals __getitem__/__setitem__ Marko Rauhamaa <marko@pacujo.net> - 2014-06-13 09:07 +0300
      Re: Asymmetry in globals __getitem__/__setitem__ robert@robertlehmann.de - 2014-06-13 03:13 -0700
        Re: Asymmetry in globals __getitem__/__setitem__ Peter Otten <__peter__@web.de> - 2014-06-13 12:53 +0200
        Re: Asymmetry in globals __getitem__/__setitem__ Paul Sokolovsky <pmiscml@gmail.com> - 2014-06-13 14:28 +0300
          Re: Asymmetry in globals __getitem__/__setitem__ Marko Rauhamaa <marko@pacujo.net> - 2014-06-13 15:32 +0300

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