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Sender: robert@robertlehmann.de
Date: Thu, 12 Jun 2014 20:18:29 +0200
Subject: Asymmetry in globals __getitem__/__setitem__
From: Robert Lehmann <mail@robertlehmann.de>
To: python-list@python.org
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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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<div dir=3D"ltr">Hi all,<div><br></div><div>I have noticed there is a sligh=
t asymmetry in the way the interpreter (v3.3.5, reproduced also in v3.5.x) =
loads and stores globals. =C2=A0While loading globals from a custom mapping=
 triggers __getitem__ just fine, writing seems to silently ignore __setitem=
__.</div>
<div><br></div><div><font face=3D"courier new, monospace">class Namespace(d=
ict):</font></div><div><font face=3D"courier new, monospace">=C2=A0 =C2=A0 =
def __getitem__(self, key):</font></div><div><font face=3D"courier new, mon=
ospace">=C2=A0 =C2=A0 =C2=A0 =C2=A0 print(&quot;getitem&quot;, key)</font><=
/div>
<div><font face=3D"courier new, monospace">=C2=A0 =C2=A0 def __setitem__(se=
lf, key, value):</font></div><div><font face=3D"courier new, monospace">=C2=
=A0 =C2=A0 =C2=A0 =C2=A0 print(&quot;setitem&quot;, key, value)</font></div=
><div><font face=3D"courier new, monospace"><br>
</font></div><div><font face=3D"courier new, monospace">def fun():</font></=
div><div><font face=3D"courier new, monospace">=C2=A0 =C2=A0 global x, y</f=
ont></div><div><font face=3D"courier new, monospace">=C2=A0 =C2=A0 x =C2=A0=
# should call globals.__getitem__</font></div>
<div><font face=3D"courier new, monospace">=C2=A0 =C2=A0 y =3D 1 =C2=A0# sh=
ould call globals.__setitem__</font></div><div><font face=3D"courier new, m=
onospace"><br></font></div><div><font face=3D"courier new, monospace">exec(=
fun.__code__, Namespace())</font></div>
<div><font face=3D"courier new, monospace"># =3D&gt; getitem x</font></div>=
<div><br></div><div>I would have expected &quot;setitem y 1&quot; to show u=
p as well, but to no avail. =C2=A0Am I doing something wrong? =C2=A0Is this=
 on purpose?</div>
<div><br></div><div>Cheers,</div><div>Robert</div><div><br></div><div>PS. =
=C2=A0I found a 3.3.x commit (<a href=3D"http://hg.python.org/cpython/rev/e=
3ab8aa0216c">e3ab8aa</a>) which fixed the LOAD_GLOBAL opcode to support oth=
er types than dict, but STORE_GLOBAL seems to use bare PyDict_SetItem inste=
ad of dispatching to PyObject_SetItem.</div>
<div><br></div></div>

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