March 6th, 2014

Suddenly, Python Magic

Looking at a review by Solly today, I saw something deeply disturbing. A simplified version that I tested follows:

import unittest

class Context(object):
    def __init__(self):
        self.func = None
    def kill(self):

class TextGuruMeditationMock(object):

    # The .run() normally is implemented in the report.Text.
    def run(self):
        return "Guru Meditation Example"

    def setup_autorun(cls, ctx, dump_with=None):
        ctx.func = lambda *args: cls.handle_signal(dump_with,

    def handle_signal(cls, dump_func, *args):
            res = cls().run()
        except Exception:
            dump_func("Unable to run")

class TestSomething(unittest.TestCase):

    def test_dump_with(self):
        ctx = Context()

        class Writr(object):
            def __init__(self):
                self.res = ''

            def go(self, out):
                self.res += out

        target = Writr()
        self.assertIn('Guru Meditation', target.res)

Okay, obviously we're setting a signal handler, which is a little lambda, which invokes the dump_with, which ... is a class method? How does it receive its self?!

I guess that the deep Python magic occurs in how the method target.go is prepared to become an argument. The only explanation I see is that Python creates some kind of activation record for this, which includes the instance (target) and the method, and that record is the object being passed down as dump_with. I knew that Python did it for scoped functions, where we have global dict, local dict, and all that good stuff. But this is different, isn't it? How does it even know that belongs to target? In what part of Python spec is it described?

UPDATE: Commenters provided hints with the key idea being a "bound method" (a kind of user-defined method).

A user-defined method object combines a class, a class instance (or None) and any callable object (normally a user-defined function).

When a user-defined method object is created by retrieving a user-defined function object from a class, its im_self attribute is None and the method object is said to be unbound. When one is created by retrieving a user-defined function object from a class via one of its instances, its im_self attribute is the instance, and the method object is said to be bound.

Thanks, Josh et al.!

UPDATE: See also Chris' explanation and Peter Donis' comment re. unbound methods gone from py3.