How to intercept a method call which doesn't exist?

Question

I want to create a class that doesn't gives an Attribute Error on call of any method that may or may not exists:

My class:

class magic_class:
    ...
    # How to over-ride method calls
    ...

Expected Output:

ob = magic_class()
ob.unknown_method()
# Prints 'unknown_method' was called

ob.unknown_method2()
# Prints 'unknown_method2' was called

Now, unknown_method and unknown_method2 doesn't actually exists in the class, but how can we intercept the method call in python ?

possible duplicate of [Intercept method calls in Python](http://stackoverflow.com/questions/2704434/intercept-method-calls-in-python) — jamylak, Jun 04 '12 at 10:29
Side note FYI: PEP8 says it should be named like `class MagicClass:` and new style should at least inherit object probably: `class MagicClass(object):` — mVChr, Jun 04 '12 at 10:29

Sven Marnach · Accepted Answer · 2012-06-04T10:36:43.590

36

Overwrite the __getattr__() magic method:

class MagicClass(object):
    def __getattr__(self, name):
        def wrapper(*args, **kwargs):
            print "'%s' was called" % name
        return wrapper

ob = MagicClass()
ob.unknown_method()
ob.unknown_method2()

prints

'unknown_method' was called
'unknown_method2' was called

edited Jun 04 '12 at 10:36

answered Jun 04 '12 at 10:31

Sven Marnach

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How will this deal with a method call like : `ob.calc(1000, 2000)`. Means how do you preserve the args to the call ? – Yugal Jindle Jun 04 '12 at 10:32
1

@YugalJindle: Added `*args, **kwargs` to accept arbitrary parameters in the wrapper. – Sven Marnach Jun 04 '12 at 10:37

onofricamila · Answer 2 · 2020-08-21T12:45:45.273

Just in case someone is trying to delegate the unknown method to an object, here's the code:

class MagicClass():
    def __init__(self, obj):
        self.an_obj = obj

    def __getattr__(self, method_name):
        def method(*args, **kwargs):
            print("Handling unknown method: '{}'".format(method_name))
            if kwargs:
                print("It had the following key word arguments: " + str(kwargs))
            if args:
                print("It had the following positional arguments: " + str(args))
            return getattr(self.an_obj, method_name)(*args, **kwargs)
        return method

This is super useful when you need to apply the Proxy pattern.

Moreover, considering both args and kwargs, allows you to generate an interface totally user friendly, as the ones that use MagicClass treat it as it was the real object.

I'm sorry, it's a typo. Should be `self.an_obj ` - 'optimizer' was the name of the object I was wrapping in my own program. I edited the code :) — onofricamila, Aug 21 '20 at 12:45

score 2 · Answer 3 · answered Jun 04 '12 at 10:29

2

Override __getattr__; see http://docs.python.org/reference/datamodel.html

answered Jun 04 '12 at 10:29

LaC

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How to intercept a method call which doesn't exist?

3 Answers3