Question

I think my example will make this easier to follow:

class x():
    def a(self):
        return "hello"
    def b(self):
        return self.a() + " world"

test = x()
print test.b()     # prints "hello world" as expected

test.a = lambda(self): "hola"
print test.b()     # throws error:
                   # Traceback (most recent call last):
                   #   File "<stdin>", line 1, in <module>
                   #   File "<stdin>", line 5, in b
                   # TypeError: <lambda>() takes exactly 1 argument (0 given)

Attempting to update to point x().a to another function, but when x().b calls it, it doesn’t appear to pass self as the first argument.

I expected to get “hola world”.

Answer 1

You can see the problem if you do

print type(test.a)  # <type 'function'>
print type(test.b)  # <type 'instancemethod'>

If you really want to patch a only on test (not on all instances of x), you can do:

import types
test.a = types.MethodType((lambda self: "hola"), test, x)

To create an object of type instancemethod.