Assigning to an instance's __class__ attribute in Python
metaprogramming, python
Solution
You can only assign to the `__class__` attribute of an instance of a user-defined class (i.e. defined using the `class` keyword), and the new value must also be a user-defined class. Whether the classes are new-style or old-style does not matter. (You can't mix them, though. You can't turn an old-style class instance into a new-style class instance.) See also this issue in the Python bug tracker, which also complains that the error message is somewhat hard to understand.
Just to add what Rafe said in the above comment: Never do this in production.
Problem
Under what circumstances is it possible, and when is it impossible, to assign to an instance's `__class__` attribute in Python? Error messages such as `TypeError: __class__ assignment: only for heap types` don't really do it for me.