Mark Dickinson <dicki...@gmail.com> added the comment:
> As you can see from this example, the exec'uted code *does* call the
> instance's overloaded __getitem__ and __missing__ methods when outside a
> function, but doesn't when inside.
Yep; that's because the 's' and 'f' lookups at top level are *local* lookups,
and the 's' lookup from inside the body of 'f' is done as a *global* lookup (as
explained in the docs here: [1]). In the exec statement, the locals can be any
mapping-like object. The behaviour's a bit clearer if you pass separate
globals and locals dictionaries:
>>> source = """\
... print s
... def f():
... print s
... f()
... """
>>> locals = {'s': 1729}
>>> globals = {'s': 31415}
>>> exec source in globals, locals
1729
31415
[1]
http://docs.python.org/reference/executionmodel.html#interaction-with-dynamic-features
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<http://bugs.python.org/issue15099>
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