How to wrap a function as a class?

Question

I have a function, say:

def func(arg1, arg2=5): blah blah

Unfortunately, a library that I'm using asks for an input to be instance of a class, so I cannot just pass my custom made function above.

I need a class wrapper around it, so that I pass: MyClass(arg2=4) and the result is the function shown above, ready to be executed with its second argument set to 4.

How can I do that?

My attempt: lambda x: func(x, arg2=4)

However, this is not an instance of a class. I want to mimic that library where their functions are instances of classes.

Please check the following link: stackoverflow.com/questions/1466676/… — b107
– b107, Commented Aug 7, 2019 at 6:14

Laurens Koppenol · Accepted Answer · 2019-08-07 06:23:11Z

you can use the __call__ function of a class.

class CallableObject(object): def __init__(self, f): self.f = f def __call__(self, *args, **kwargs): return self.f(*args, **kwargs)

then you can "call" the class like this:

def func(arg1, arg2=5): return arg1 + arg2 callable_object = CallableObject(func) callable_object(1, 3)

returns 4

args in __init__

To give arguments when instantiating the class you can add them to init.

class CallableObject(object): def __init__(self, f, *args, **kwargs): self.f = f self.args = args self.kwargs = kwargs def __call__(self): return self.f(*self.args, **self.kwargs)

Usage then becomes:

def func(arg1, arg2=5): return arg1 + arg2 callable_object = CallableObject(func, 1, arg2=3) callable_object()

returning 4

and you can still call

def func(arg1, arg2=5): return arg1 + arg2 callable_object = CallableObject(func, 1) callable_object()

returning 6 (arg2 = 5)

I think this is what I'm looking for. Except that it is not exactly what I imagined. I thought I wouldn't need to redefine the function inside the class.
What about one of the arguments being set while instantiating the class? which is arg2 in my example

Laurens Koppenol · Accepted Answer · 2019-08-07 07:28:38Z

Using metaclass, you can solve this

class MetaClass(type): @staticmethod def wrap(run): """Return a wrapped instance method""" def outer(self): print "PRE", return_value = run(self) print "POST" return return_value return outer def __new__(cls, name, bases, attrs): """If the class has a 'run' method, wrap it""" if 'run' in attrs: attrs['run'] = cls.wrap(attrs['run']) return super(MetaClass, cls).__new__(cls, name, bases, attrs) class MyClass(object): """Use MetaClass to make this class""" __metaclass__ = MetaClass def run(self): print 'RUN', myinstance = MyClass() myinstance.run()

I think this is too complicated for me. But I will start looking into this metaclass beast. Thanks.

Collectives™ on Stack Overflow

How to wrap a function as a class?

2 Answers 2

3 Comments

1 Comment

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

1 Comment

Linked

Related