How does Python distinguish callback function which is a member of a class?

Question

Please look at the simple example:

class A:
  def __init__(self, flag):
    self.flag = flag

  def func(self):
    print self.flag

a = A(1)
b = A(2)
callback_a = a.func
callback_b = b.func

callback_a()
callback_b()

The result is:

1
2

It runs as expected. But I have a question. In C, the callback function is passed as a pointer. In Python, it should have a similar way to do this, so the caller knows the address of the function. But in my example, not only the function pointer is passed, but also the parameter (self) is passed, because the same method of the same class prints different results. So my questions are:

Does such a method in Python only has one copy in memory? My meaning is that the code of any method only has one copy, and in my example the method won’t be cloned itself. I think it should have only one copy, but here I still make this question in order to get more inputs.
I remember everything in Python is an object. So in my example, are there two function instances with different parameters but only one copy of code?

Asked By: flypen

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Answer 1

In Python, the callback is not simply a reference to a member function. Instead, it is “bound” to the object that it refers to when it was created. So a.func creates a callable that is bound to a, and b.func creates a callable that is bound to b.

Python only needs one implementation of func() in memory, but it will probably create one or more “trampoline” functions at runtime to accomplish the binding (I’m not certain of the internal details on this, and it would differ between Python implementations anyway).

If you print id(callback_a) and id(callback_b) you will get different results, showing that they are indeed different callable objects.

Answered By: Greg Hewgill

Answer 2

Specific to CPython, there is only one copy of the function object. During instance creation, the class wraps the unbound functions in its namespace as bound methods. But they all wrap the same function.

Here’s your example expanded to show what’s going on.

class A(object):
  def __init__(self, flag):
    self.flag = flag

  def func(self):
    print self.flag

a = A(1)
b = A(2)

callback_a = a.func
callback_b = b.func

print "typeof(callback_a) = {0}".format(type(callback_a))
print "typeof(callback_b) = {0}".format(type(callback_b))

print "typeof(callback_a.__func__) = {0}".format(type(callback_a.__func__))
print "typeof(callback_b.__func__) = {0}".format(type(callback_b.__func__))

print "'callback_a.__func__ is callback_b.__func__'  is {0}".format(callback_a.__func__ is callback_b.__func__)

callback_a()
callback_b()

This code outputs

typeof(callback_a) = <type 'instancemethod'>
typeof(callback_b) = <type 'instancemethod'>
typeof(callback_a.__func__) = <type 'function'>
typeof(callback_b.__func__) = <type 'function'>
'callback_a.__func__ is callback_b.__func__'  is True

You can clearly see, using the is operator, that both instancemethod classes are sharing the same function object.

Answered By: aaronasterling

How does Python distinguish callback function which is a member of a class?

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