Unexpected behaviour of getattr(obj, __dict__)
Steven D'Aprano
Example:
>>> class Parrot(object):
... thing = [1,2,3]
...
>>> getattr(Parrot, "thing") is Parrot.thing
True
>>> getattr(Parrot, "__dict__") is Parrot.__dict__
False
I would have expected that the object returned by getattr would be the
same object as the object returned by standard attribute access. This is
true for some attributes, but not for __dict__. I don't know if there are
other examples.
Why is this? Is there some documentation I can read up about this? I've
tried searching, but can't find anything useful.
--
Steven.
bruno at modulix
> I came across this unexpected behaviour of getattr for new style classes.
> Example:
>
>
>>>>class Parrot(object):
>
> ... thing = [1,2,3]
> ...
>
>>>>getattr(Parrot, "thing") is Parrot.thing
>
> True
>
>>>>getattr(Parrot, "__dict__") is Parrot.__dict__
>
> False
hint:
>>> getattr(object, '__dict__')
<dictproxy object at 0x2aaaaab2ff30>
--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'on...@xiludom.gro'.split('@')])"
Raymond Hettinger
> I came across this unexpected behaviour of getattr for new style classes.
> Example:
>
> >>> class Parrot(object):
> ... thing = [1,2,3]
> ...
> >>> getattr(Parrot, "thing") is Parrot.thing
> True
> >>> getattr(Parrot, "__dict__") is Parrot.__dict__
> False
>
> I would have expected that the object returned by getattr would be the
> same object as the object returned by standard attribute access.
The returned object is a wrapper created on-the-fly as needed. You've
requested two of them and each wrapper has a different object id but
wraps an identical source.
Contemplate this for a bit:
>>> class Parrot(object):
thing = [1,2,3]
def f(self): pass
>>> getattr(Parrot, 'f') is getattr(Parrot, 'f')
False
>>> getattr(Parrot, '__dict__') is getattr(Parrot, '__dict__')
False
The reason why is evident when you check the object representation. It
shows that your lookup returned a wrapper:
>>> getattr(Parrot, 'f') # creates a function wrapper
<unbound method Parrot.f>
>>> getattr(Parrot, '__dict__') # creates a dictionary wrapper
<dictproxy object at 0x00C41770>
IOW, attribute lookup can do more than just return the result of a
straight-lookup. The underlying mechanism is a deep and interesting
subject. If you want to know more, try this link:
http://users.rcn.com/python/download/Descriptor.htm
Raymond
Steven D'Aprano
> Steven D'Aprano wrote:
>> I came across this unexpected behaviour of getattr for new style classes.
>> Example:
>>
>>
>>>>>class Parrot(object):
>>
>> ... thing = [1,2,3]
>> ...
>>
>>>>>getattr(Parrot, "thing") is Parrot.thing
>>
>> True
>>
>>>>>getattr(Parrot, "__dict__") is Parrot.__dict__
>>
>> False
>
>
> hint:
>>>> getattr(object, '__dict__')
> <dictproxy object at 0x2aaaaab2ff30>
That doesn't answer the question, it just re-words it. Why is the
dictproxy returned by getattr a different instance from the dictproxy that
you get when you say object.__dict__?
--
Steven.
Steven D'Aprano
> Steven D'Aprano wrote:
>> I came across this unexpected behaviour of getattr for new style classes.
>> Example:
>>
>> >>> class Parrot(object):
>> ... thing = [1,2,3]
>> ...
>> >>> getattr(Parrot, "thing") is Parrot.thing
>> True
>> >>> getattr(Parrot, "__dict__") is Parrot.__dict__
>> False
>>
>> I would have expected that the object returned by getattr would be the
>> same object as the object returned by standard attribute access.
>
> The returned object is a wrapper created on-the-fly as needed. You've
> requested two of them and each wrapper has a different object id but
> wraps an identical source.
[penny drops]
That would certainly explain it.
Is there a canonical list of attributes which are wrapped in this way? I
suppose not... it is probably the sort of thing which is subject to change
as Python evolves.
I knew methods were wrapped, but I didn't know they were wrapped on the
fly, nor did I connect the two phenomena. Thank you for the concise and
simple answer.
--
Steven.
Carl Banks
> Steven D'Aprano wrote:
> > I came across this unexpected behaviour of getattr for new style classes.
> > Example:
> >
> > >>> class Parrot(object):
> > ... thing = [1,2,3]
> > ...
> > >>> getattr(Parrot, "thing") is Parrot.thing
> > True
> > >>> getattr(Parrot, "__dict__") is Parrot.__dict__
> > False
> >
> > I would have expected that the object returned by getattr would be the
> > same object as the object returned by standard attribute access.
>
> The returned object is a wrapper created on-the-fly as needed. You've
> requested two of them and each wrapper has a different object id but
> wraps an identical source.
>
> Contemplate this for a bit:
>
> >>> class Parrot(object):
> thing = [1,2,3]
> def f(self): pass
>
> >>> getattr(Parrot, 'f') is getattr(Parrot, 'f')
> False
> >>> getattr(Parrot, '__dict__') is getattr(Parrot, '__dict__')
> False
Yes, in fact getattr has nothing to do with it. To wit:
>>> Parrot.f is Parrot.f
False
>>> Parrot.__dict__ is Parrot.__dict__
False
But wait, it gets weirder.
>>> id(Parrot.f) == id(Parrot.f)
True
>>> id(Parrot.__dict__) == id(Parrot.__dict__)
True
But that's not all. Redefine Parrot as:
class Parrot(object):
def f(self): pass
def g(self): pass
Then,
>>> id(Parrot.f) == id(Parrot.g)
True
your-milage-may-vary-ly yr's,
Carl Banks
Fredrik Lundh
> >>>>>class Parrot(object):
> >>
> >> ... thing = [1,2,3]
> >> ...
> >>
> >>>>>getattr(Parrot, "thing") is Parrot.thing
> >>
> >> True
> >>
> >>>>>getattr(Parrot, "__dict__") is Parrot.__dict__
> >>
> >> False
> >
> >
> > hint:
> >>>> getattr(object, '__dict__')
> > <dictproxy object at 0x2aaaaab2ff30>
>
> That doesn't answer the question, it just re-words it. Why is the
> dictproxy returned by getattr a different instance from the dictproxy
> that you get when you say object.__dict__?
because it's created on the fly:
>>> Parrot.__dict__
<dictproxy object at 0x009818B0>
>>> Parrot.__dict__
<dictproxy object at 0x00981A10>
>>> Parrot.__dict__
<dictproxy object at 0x009818B0>
>>> Parrot.__dict__
<dictproxy object at 0x00981A10>
>>> Parrot.__dict__
<dictproxy object at 0x009818B0>
>>> Parrot.__dict__
<dictproxy object at 0x00981A10>
>>> Parrot.__dict__
<dictproxy object at 0x009818B0>
>>> Parrot.__dict__
<dictproxy object at 0x00981A10>
>>> Parrot.__dict__
<dictproxy object at 0x009818B0>
>>> Parrot.__dict__
<dictproxy object at 0x00981A10>
>>> Parrot.__dict__
<dictproxy object at 0x009818B0>
the object itself contains a dictionary.
</F>
Terry Reedy
"Carl Banks" <invali...@aerojockey.com> wrote in message
news:1139920307....@g44g2000cwa.googlegroups.com...
> But wait, it gets weirder.
Not weird at all. Just an artifact of CPython's storage recycling
algorithm.
>>>> id(Parrot.f) == id(Parrot.f)
> True
>>>> id(Parrot.__dict__) == id(Parrot.__dict__)
> True
A wrapper is created and passed to id() which returns an int object while
releasing the wrapper back to the free list. Then another wrapper is
created in the same chunk of memory and passed to id, which returns an int
of the same value for comparison. The language ref only guarantees
uniqueness of ids at any particular instant and allows reuse of ids of
deallocated objects.
I half seriously think the lib ref entry for id() should have a warning
that naive use can mislead.
> But that's not all. Redefine Parrot as:
>
> class Parrot(object):
> def f(self): pass
> def g(self): pass
>
> Then,
>
>>>> id(Parrot.f) == id(Parrot.g)
> True
Same thing. The wrapper content is not relevant as long as it uses the
same memory block.
Terry Jan Reedy
Christos Georgiou
<tjr...@udel.edu> might have written:
>>>>> id(Parrot.f) == id(Parrot.f)
>> True
>>>>> id(Parrot.__dict__) == id(Parrot.__dict__)
>> True
>
>A wrapper is created and passed to id() which returns an int object while
>releasing the wrapper back to the free list. Then another wrapper is
>created in the same chunk of memory and passed to id, which returns an int
>of the same value for comparison. The language ref only guarantees
>uniqueness of ids at any particular instant and allows reuse of ids of
>deallocated objects.
>
>I half seriously think the lib ref entry for id() should have a warning
>that naive use can mislead.
Actually, you more-or-less just wrote what could (I also think should) be
included in the docs.
--
TZOTZIOY, I speak England very best.
"Dear Paul,
please stop spamming us."
The Corinthians