Python equivalent of Ruby's 'method_missing'

Question

What is Python's equivalent of Ruby's method_missing method? I tried using __getattr__ but this hook applies to fields too. I only want to intercept the method invocations. What is the Python way to do it?

Answer 1

There is no difference in Python between properties and methods. A method is just a property, whose type is just instancemethod, that happens to be callable (supports __call__).

If you want to implement this, your __getattr__ method should return a function (a lambda or a regular def, whatever suite your needs) and maybe check something after the call.

Answer 2

Python doesn't distinguish between methods and attributes (a.k.a. "instance variables") the way Ruby does. Methods and other object attributes are looked up in exactly the same way in Python -- not even Python knows the difference at the look-up stage. Until the attribute is found, it's just a string.

So if you're asking for a way to ensure that __getattr__ is only called for methods, I'm afraid you probably won't find an elegant solution. But it's easy enough to simply return a function (or even a brand-new dynamically bound method) from __getattr__.

Answer 3

You could implement a missing_method like feature in the below way:

https://gist.github.com/gterzian/6400170

import unittest
from functools import partial

class MethodMissing:
    def method_missing(self, name, *args, **kwargs):
        '''please implement'''
        raise NotImplementedError('please implement a "method_missing" method')

    def __getattr__(self, name):
        return partial(self.method_missing, name)


class Wrapper(object, MethodMissing):
    def __init__(self, item):
        self.item = item

    def method_missing(self, name, *args, **kwargs):
        if name in dir(self.item):
            method = getattr(self.item, name)
            if callable(method):
                return method(*args, **kwargs)
            else:
                raise AttributeError(' %s has not method named "%s" ' % (self.item, name))


class Item(object):
    def __init__(self, name):
        self.name = name

    def test(self, string):
        return string + ' was passed on'


class EmptyWrapper(object, MethodMissing):
    '''not implementing a missing_method'''
    pass


class TestWrapper(unittest.TestCase):
    def setUp(self):
        self.item = Item('test')
        self.wrapper = Wrapper(self.item)
        self.empty_wrapper = EmptyWrapper()

    def test_proxy_method_call(self):
        string = self.wrapper.test('message')
        self.assertEqual(string, 'message was passed on')

    def test_normal_attribute_not_proxied(self, ):
        with self.assertRaises(AttributeError):
            self.wrapper.name
            self.wrapper.name()

    def test_empty_wrapper_raises_error(self, ):
        with self.assertRaises(NotImplementedError):
            self.empty_wrapper.test('message')


if __name__ == '__main__':
    unittest.main()

Answer 4

Although I don't recommend it!!!!!!!!!!!!!!!!!!!!!

this sort of comes closer to implementing the behavior of calling the special method for every name that does not correspond to a callable attribute/method. Of course they still don't really have separate namespaces so it may feel a bit weird. It works by overriding __getattribute__ which works at a lower level then __getattr__ it tries to fetch an attribute if it fails it returns a curried special method to call with the name you called it with, if it succeeds it passes it on if its callable otherwise it wraps the result with a proxy object which acts in almost exactly the same way afterwards except it implements call with your special method.

It doesn't allow you to access the calling object because I couldn't think of a good way to do that without sort of leaking memory(the calling object) if it's already a non-callable attribute which you store(the only think I can think of is to start a new thread that deletes it after a minute, by then you have presumably called it unless you are using it in a closure which wouldn't be supported in that case).

Edit: I forgot callable may have some false positives.

depends on the http://pypi.python.org/pypi/ProxyTypes library

from peak.util.proxies import ObjectWrapper
from functools import partial

def m(name, *args, **kwargs):
    print(name,repr(args),repr(kwargs))


class CallProxy(ObjectWrapper):
   def __init__(self, obj, m, method_name):
       ObjectWrapper.__init__(self, obj)
       object.__setattr__(self, "_method_name", method_name)
       object.__setattr__(self, "_m", m)

   def __call__(self, *args, **kwargs):
       return self._m(self._method_name, *args,**kwargs)


class Y(object):
   def __init__(self):
       self.x = [3]
   def __getattribute__(self, name):
       try:
           val = object.__getattribute__(self, name)
           if not callable(val):
               return CallProxy(val, m, name)
           else:
               return val
       except AttributeError:
           return partial(m, name)

---

In [2]: y=Y()

In [3]: y.x
Out[3]: [3]

In [4]: y.z
Out[4]: <functools.partial at 0x2667890>

In [5]: y.zz([12])
('zz', '([12],)', '{}')

In [6]: y.x.append(5)

In [7]: y.x
Out[7]: [3, 5]

In [8]: y.x(1,2,3,key="no")
('x', '(2, 3)', "{'key': 'no'}")