How to bind partial function as method on Python class instances

Question

I want to bind this function as a method on some class:

def foo_method(msg, self):
    print self.prefix + msg

This works:

class A(object):
    prefix = 'msg: '

A.foo = lambda *args: foo_method('hello', *args)

A().foo()
# => msg: hello

This, however, does not:

from functools import partial

class B(object):
    prefix = 'msg: '

B.foo = partial(foo_method, 'hello')

B().foo()
# => TypeError: foo_method() takes exactly 2 arguments (1 given)

The first example appears to be bound correctly, whereas the second does not.

Why?

Interesting. Investigating this now. I hope this is just an experiment you're playing around with and you're not actually trying to dynamically add partially applied methods to classes. — Bailey Parker
– Bailey Parker, Commented Jan 19, 2018 at 9:04
You don't need the setattr here. A.foo = lambda # ... also exhibits this behavior. — Bailey Parker
– Bailey Parker, Commented Jan 19, 2018 at 9:09
I’d venture this is some strange interaction due to lambda being a proper function and partial being a Callable (it’s actually implemented as a class). — Bailey Parker
– Bailey Parker, Commented Jan 19, 2018 at 9:14
@BaileyParker Yeah, I was guessing it had something to do with partial not being a function, I just haven't been able to find an explanation why that matters. Maybe it's something in the object model that I don't understand? — nfelger
– nfelger, Commented Jan 19, 2018 at 9:21
So it's somewhere in the self binding in __getattribute__. A().foo produces <bound method <lambda> of ...>> and B().foo produces functools.partial(<function foo_method at ...>, 'hello'). So it looks like in B __getattribute__ doesn't self bind. To get an answer you have to delve into the world of metaclasses. — Bailey Parker
– Bailey Parker, Commented Jan 19, 2018 at 9:23

nfelger · Accepted Answer · 2018-01-19 09:58:54Z

The answer is: descriptors! Self-binding only happens during attribute access. There is no special language support for methods - they work using just function objects and descriptors.

Quoting https://docs.python.org/2/howto/descriptor.html#functions-and-methods:

Class dictionaries store methods as functions. [...] To support method calls, functions include the __get__() method for binding methods during attribute access. This means that all functions are non-data descriptors which return bound or unbound methods depending whether they are invoked from an object or a class. In pure python, it works like this:
class Function(object):
    def __get__(self, obj, objtype=None):
        "Simulate func_descr_get() in Objects/funcobject.c"
        return types.MethodType(self, obj, objtype)

Fascinating!

When accessing A().foo, the descriptor mechanism is invoked, calling foo_method.__get__, which returns a bound method:

A().foo
# => <bound method A.<lambda> of <__main__.A object at 0x10ca9c0d0>>

Since functools.partial does not implement __get__, it does not get bound:

B().foo
# => <functools.partial at 0x10caf3aa0>

hasattr(partial(foo_method, 'hello'), '__get__')
# => False

The question remains: if it's so simple, why doesn't functools.partial implement this as well?

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