Python: Can't pickle type X, attribute lookup failed

Question

I am trying to pickle a namedtuple:

from collections import namedtuple import cPickle  class Foo:      Bar = namedtuple('Bar', ['x', 'y'])      def baz(self):         s = set()         s.add(Foo.Bar(x=2, y=3))         print cPickle.dumps(s)  if __name__ == '__main__':     f = Foo()     f.baz() 

This produces the following output:

Traceback (most recent call last):   File "scratch.py", line 15, in <module>     f.baz()   File "scratch.py", line 11, in baz     print cPickle.dumps(s) cPickle.PicklingError: Can't pickle <class '__main__.Bar'>: attribute lookup __main__.Bar failed 

What am I doing wrong? Is the problem that Bar is a member of Foo? (Moving the definition of Bar to the top level solves the problem, although I'm still curious why this happens.)

Answer 1

Yes, the fact that it's a class member is a problem:

>>> class Foo(): ...     Bar = namedtuple('Bar', ['x','y']) ...     def baz(self): ...         b = Foo.Bar(x=2, y=3) ...         print(type(b)) ... >>> a = Foo() >>> a.baz() <class '__main__.Bar'> 

The problem is that when namedtuple() returns a type object, it isn't aware of the fact that it's being assigned to a class member - and thus, it tells the type object that its type name should be __main__.Bar, even though it should really be __main__.Foo.Bar.