#10249: Cannot create a consistent method resolution order (MRO) for bases
FieldFile, File
-------------------------------------------+--------------------------------
Reporter: kmtracey | Owner: nobody
Status: new | Milestone:
Component: File uploads/storage | Version: 1.0
Resolution: | Keywords:
Stage: Unreviewed | Has_patch: 0
Needs_docs: 0 | Needs_tests: 0
Needs_better_patch: 0 |
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Comment (by kmtracey):
The problem is with this code in
source:django/trunk/django/db/models/fields/files.py in the
!FileDescriptor class (around line 130):
{{{
#!python
elif isinstance(file, File) and not isinstance(file, FieldFile):
# Other types of files may be assigned as well, but they need
to
# have the FieldFile interface added to them
file_copy = copy.copy(file)
file_copy.__class__ = type(file.__class__.__name__,
(file.__class__,
self.field.attr_class), {})
}}}
`self.field.attr_class` is going to be !FieldFile in the specific case
noted here (it can also be !ImageFieldFile). But !FieldFile is based on
File, so that code is essentially trying to do:
{{{
>>> class File(object): pass
...
>>> class FieldFile(File): pass
...
>>> class X(File, FieldFile): pass
...
Traceback (most recent call last):
File "<console>", line 1, in <module>
TypeError: Error when calling the metaclass bases
Cannot create a consistent method resolution
order (MRO) for bases FieldFile, File
}}}
Specifying File first contradicts the lookup order specified by the fact
that !FieldFile derives from File, and Python complains. Reversing the
order works:
{{{
>>> class X(FieldFile, File): pass
...
>>>
}}}
But I'm questioning whether this code is really the right approach to 'add
the !FieldFile interface' to something. Based on what's being seen in
#10300 it is also having the effect of causing the !FieldFile methods to
be called for routines that used to get resolved via inheritance in the
class being copied here, with unexpected results. That is, it isn't just
adding some missing methods, but changing how some existing ones are
resolved. Perhaps there is some cleaner way to just add methods, which
seems to be what the comment (and possibly the order in which the bases
are currently listed) implies is the intent of the code? Mucking with an
object's `__class__` is messing with a level of Python I'm not really
comfortable with, so feedback from someone with more Python depth would be
useful here.
--
Ticket URL: <http://code.djangoproject.com/ticket/10249#comment:2>
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