Question

I am running an interactive python session which builds big python data-structures (5+ GB) which take a long time to load, and so I want to exploit Python on-the-fly code change abilities at its maximum (though sometimes, without having to plan too much for that).

My current problem is the following: I have an old instance of a class that I have later modified the code and reloaded the module — I would like the old instance to be able to use the new function definitions. How do I do that without just manually copying all the information from the old instance to a new fresh instance?

Here is what I have tried. Suppose I have the module M.py:

class A():
    def f(self):
        print "old class"

Here is an interactive session:

import M
old_a = M.a()

# [suppose now I change the definition of M.A.f in the source file]

reload(M)
# I attempt to use the new class definition with the old instance:
M.A.f(old_a)

at which point I get the following type error from Python:

TypeError: unbound method f() must be called with A instance as first argument (got A instance instead)

Python is obviously not happy to receive an old instance of A even though they are basically functionally equivalent types (in my code) — is there any way I could ‘type cast’ it to the new instance type so that Python wouldn’t complain? Something morally like: M.A.f( (M.A) old_a ) ?

Answer 1

There is no casting in Python but you can change the class of an existing object: It is perfectly legal and does the job:

old_a.__class__=M.A
old_a.f()

As long as you haven’t changed the relation between class methods and instance variables, changed what __init__ does or something like that this is perfectly fine.

EDIT: As jsbueno points out: The __init__ or __new__ methods are not called at the point of changing __class__. Further, the new __del__ will be called at destruction.