How to intercept instance method calls?

Question

I am looking for a way to intercept instance method calls in class MyWrapper below:

class SomeClass1:
    def a1(self):
        self.internal_z()
        return "a1"
    def a2(self):
        return "a2"
    def internal_z(self):
        return "z"

class SomeClass2(SomeClass1):
    pass

class MyWrapper(SomeClass2):

    # def INTERCEPT_ALL_FUNCTION_CALLS():
    #      result = Call_Original_Function()
    #      self.str += result  
    #      return result  


    def __init__(self):
        self.str = ''
    def getFinalResult(self):
        return self.str

x = MyWrapper()
x.a1()
x.a2()

I want to intercept all function calls make through my wrapper class. In my wrapper class I want to keep track of all the result strings.

result = x.getFinalResult()
print result == 'a1a2'

Jens

Answer 1

Some quick and dirty code:

class Wrapper:
    def __init__(self, obj):
        self.obj = obj
        self.callable_results = []

    def __getattr__(self, attr):
        print("Getting {0}.{1}".format(type(self.obj).__name__, attr))
        ret = getattr(self.obj, attr)
        if hasattr(ret, "__call__"):
            return self.FunctionWrapper(self, ret)
        return ret

    class FunctionWrapper:
        def __init__(self, parent, callable):
            self.parent = parent
            self.callable = callable

        def __call__(self, *args, **kwargs):
            print("Calling {0}.{1}".format(
                  type(self.parent.obj).__name__, self.callable.__name__))
            ret = self.callable(*args, **kwargs)
            self.parent.callable_results.append(ret)
            return ret

class A:
    def __init__(self, val): self.val = val
    def getval(self): return self.val

w = Wrapper(A(10))
print(w.val)
w.getval()
print(w.callable_results)

Might not be thorough, but could be a decent starting point, I guess.

Mestee

Answer 2

You could wrap your methods with decorators a instanciation time:

#!/usr/bin/env python

import inspect

def log(func):
    def _logged(*args, **kw):
        print "[LD] func", func.__name__, "called with:", args, kw
        result = func(*args, **kw)
        print "[LD] func", func.__name__, "returned:", result
        return result
    return _logged

class A(object):
    def __init__(self):
        for x in inspect.getmembers(self, (inspect.ismethod)):
            if not x[0].startswith('__'):
                setattr(self, x[0], log(getattr(self, x[0])))

    def hello(self):
        print "Hello"

    def bye(self):
        print "Bye"
        return 0

Now if you call hello or bye, the call goes through log first:

a = A()
a.hello()
a.bye()

# [LD] func hello called with: () {}
# Hello
# [LD] func hello returned: None
# [LD] func bye called with: () {}
# Bye
# [LD] func bye returned: 0

Volnay

Answer 3

What you want to do is quite similar to this question. You should take your example code in the reverse order, i mean creating a class to record return values of method calls, and make the classes you want to watch inherit from it. Which would give something like this

class RetValWatcher(object):
    def __init__(self):
        self.retvals = []

    def __getattribute__(self, name):
        attr = super(RetValWatcher, self).__getattribute__(name)
        if callable(attr):
            def wrapped(*args, **kwargs):
                retval = attr(*args, **kwargs)
                self.retvals.append(retval)
                return retval
            return wrapped
        else:
            return attr

    def getFinalResult(self):
        return ''.join(self.retvals)

class MyClass(RetValWatcher):
    def a(self):
        self.internal_z()
        return 'a1'

    def b(self):
        return 'b1'

    def internal_z(self):
        return 'z'

x = MyClass()
x.a()
x.b()
print x.getFinalResult()
#'za1b1'

With some minor changes, this method would also allow you to record return values across all RetValWatcher instances.

Edit: added changes suggested by singularity's comment

Edit2: forgot to handle the case where attr is not a method (thx singularity again)

Edit3: fixed typo

Yost

+1, i like this method more , but i have some remarks : 1) replace retvals = [] with self.retvals = [] , 2) in the case of the OP x.getFinalResult() will return za1a2 not a1a2 , 3) i think it's better to use inspect.ismethod or callable(attr) instead of hasattr(attr, '__call__') . – Jarry

the missing 'self' was just an oversight, but you are right for the other 2 points. edited ;) – Yost

Ahh sorry again ; you forget else: return attr for the if callable(attr): because you don't want to obscure attribute call :) – Jarry

Typo: def getFinalResTTTult(self): – Alarum