Investigate RenameMethodsBase effect on yaml serialization

[Ilya Semenov]

Django 1.7.10, PyYAML 3.11 (Python 3.4):

>>> import yaml
>>> from django import forms
>>> yaml.dump(forms.ChoiceField)
"!!python/name:django.forms.fields.ChoiceField ''\n"

Django 1.8.14, PyYAML 3.11 (Python 3.5):

>>> import yaml
>>> from django import forms
>>> yaml.dump(forms.ChoiceField)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/semenov/work/test/var/venv/lib/python3.5/site-packages/yaml/__init__.py", line 200, in dump
    return dump_all([data], stream, Dumper=Dumper, **kwds)
  File "/Users/semenov/work/test/var/venv/lib/python3.5/site-packages/yaml/__init__.py", line 188, in dump_all
    dumper.represent(data)
  File "/Users/semenov/work/test/var/venv/lib/python3.5/site-packages/yaml/representer.py", line 26, in represent
    node = self.represent_data(data)
  File "/Users/semenov/work/test/var/venv/lib/python3.5/site-packages/yaml/representer.py", line 51, in represent_data
    node = self.yaml_multi_representers[data_type](self, data)
  File "/Users/semenov/work/test/var/venv/lib/python3.5/site-packages/yaml/representer.py", line 313, in represent_object
    reduce = data.__reduce_ex__(2)
  File "/Users/semenov/work/test/var/venv/lib/python3.5/copyreg.py", line 65, in _reduce_ex
    raise TypeError("can't pickle %s objects" % base.__name__)
TypeError: can't pickle int objects

Anything we can do to have Django class types YAML-compatible again?

[Tim Graham]

Fixed in Django 1.10 by a3fe4addcb9063fc327c8609c7ebba4d531da4ce. Unfortunately, Django 1.8 and 1.9 are only receiving security and data loss fixes by now. Perhaps it's worth investigating the cause of this (RenameMethodsBase, possibly) to try to prevent future regressions.

[Ryan Castner]

Ok did a bit of investigating, RenameMethodsBase inherits from type. I may be crazy but that sounds a lot like ​https://docs.python.org/2/library/pickle.html#pickling-and-unpickling-extension-types. Specifically:

When the Pickler encounters an object of a type it knows nothing about — such as an extension type — it looks in two places for a hint of how to pickle it.

Those hints are the __reduce__ and __reduce_ex__ methods. The __reduce_ex__ method states method will be called with a single integer argument, the protocol version.

And that call seems to fail here ​https://github.com/python/cpython/blob/c30098c8c6014f3340a369a31df9c74bdbacc269/Objects/typeobject.c#L4169