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Date: Fri, 3 May 2013 05:34:40 +0600
Subject: Debugging difficulty in python with __getattr__, decorated properties and AttributeError.
From: "Mr. Joe" <titanix88@gmail.com>
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Is there any way to raise the original exception that made the call to
__getattr__? I seem to stumble upon a problem where multi-layered attribute
failure gets obscured due to use of __getattr__. Here's a dummy code to
demonstrate my problems:
"""
import traceback


class BackupAlphabet(object):
    pass


class Alphabet(object):
    @property
    def a(self):
        return backupalphabet.a


    def __getattr__(self, name):
        if name == "b":
            return "banana"

        raise AttributeError(
            "'{0} object has no attribute '{1}'"
            .format(self.__class__.__name__, name))


alphabet = Alphabet()
backupalphabet = BackupAlphabet()

print(alphabet.a)
print(alphabet.b)
"""

Running the above code produces this:
"""
Traceback (most recent call last):
  File "example.py", line 26, in <module>
    print(alphabet.a)
  File "example.py", line 20, in __getattr__
    .format(self.__class__.__name__, name))
AttributeError: 'Alphabet object has no attribute 'a'
"""

While it's easy enough to identify the problem here, the traceback is
rather unhelpful in complex situations. Any comments?

Regards,
TB

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<div dir=3D"ltr">Is there any way to raise the original exception that made=
 the call to __getattr__? I seem to stumble upon a problem where multi-laye=
red attribute failure gets obscured due to use of __getattr__. Here&#39;s a=
 dummy code to demonstrate my problems:<div>
&quot;&quot;&quot;<br><div><div>import traceback</div><div><br></div><div><=
br></div><div>class BackupAlphabet(object):</div><div>=C2=A0 =C2=A0 pass</d=
iv><div><br></div><div><br></div><div>class Alphabet(object):</div><div>=C2=
=A0 =C2=A0 @property</div>
<div>=C2=A0 =C2=A0 def a(self):</div><div>=C2=A0 =C2=A0 =C2=A0 =C2=A0 retur=
n backupalphabet.a</div><div><br></div><div><br></div><div>=C2=A0 =C2=A0 de=
f __getattr__(self, name):</div><div>=C2=A0 =C2=A0 =C2=A0 =C2=A0 if name =
=3D=3D &quot;b&quot;:</div><div>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 r=
eturn &quot;banana&quot;</div>
<div><br></div><div>=C2=A0 =C2=A0 =C2=A0 =C2=A0 raise AttributeError(</div>=
<div>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 &quot;&#39;{0} object has no=
 attribute &#39;{1}&#39;&quot;</div><div>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0=
 =C2=A0 .format(self.__class__.__name__, name))</div><div><br></div><div><b=
r>
</div><div>alphabet =3D Alphabet()</div><div>backupalphabet =3D BackupAlpha=
bet()</div><div><br></div><div>print(alphabet.a)</div><div>print(alphabet.b=
)</div></div><div>&quot;&quot;&quot;</div></div><div><br></div><div style>R=
unning the above code produces this:</div>
<div>&quot;&quot;&quot;<br></div><div><div>Traceback (most recent call last=
):</div><div>=C2=A0 File &quot;example.py&quot;, line 26, in &lt;module&gt;=
</div><div>=C2=A0 =C2=A0 print(alphabet.a)</div><div>=C2=A0 File &quot;exam=
ple.py&quot;, line 20, in __getattr__</div>
<div>=C2=A0 =C2=A0 .format(self.__class__.__name__, name))</div><div>Attrib=
uteError: &#39;Alphabet object has no attribute &#39;a&#39;</div></div><div=
>&quot;&quot;&quot;</div><div><br></div><div style>While it&#39;s easy enou=
gh to identify the problem here, the traceback is rather unhelpful in compl=
ex situations. Any comments?</div>
<div style><br></div><div style>Regards,</div><div style>TB</div></div>

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