Python: Change bound method to another method

Question

I am trying to learn about bound methods in python and have implemented the code below:

class Point:
    def __init__(self, x,y):
        self.__x=x
        self.__y=y

    def draw(self):
        print(self.__x, self.__y)

def draw2(self):
    print("x",self.__x, "y", self.__y)

p1=Point(1,2)
p2=Point(3,4)
p1.draw()
p2.draw()
p1.draw=draw2
p1.draw(p1)

When I run this code, the following output is produced:

1 2
3 4
Traceback (most recent call last):
  File "main.py", line 17, in <module>
    p1.draw(p1)
  File "main.py", line 10, in draw2
    print("x",self.__x, "y", self.__y)
AttributeError: 'Point' object has no attribute '__x'

Why is it not possible for me to change p1.draw() after changing it so that it points at draw2?

Answer 1

Well, that's what you get for trying to enforce privacy in Python. ;)

Outside the body of the class, you must refer to the attributes __x and __y as _Point__x and _Point__y because of name mangling.

If you change the two attributes to non-mangled names (e.g. _x and _y) or use the names _Point__x and _Point__y in draw2, your code won't throw an error.

In my opinion you should think thrice before using mangled names. Write proper docstrings, but don't restrict the user of your class in such an annoying manner. Using single underscore names is well understood as "don't touch this" in the community.

As you already seem to have noticed, p1.draw behaves differently after your monkey patch because draw2 is not a bound method of the instance p1, so you need to pass p1 explicitly as the argument. I suggest that you bind the instance p1 to draw2 by leveraging the function's descriptor protocol before you reassign the name draw.

Putting everything together, the code

class Point:
    def __init__(self, x,y):
        self._x=x
        self._y=y

    def draw(self):
        print(self._x, self._y)

def draw2(self):
    print("x",self._x, "y", self._y)

p1 = Point(1,2)
p2 = Point(3,4)
p1.draw()
p2.draw()
p1.draw = draw2.__get__(p1)
p1.draw()

produces the output

1 2
3 4
x 1 y 2

where draw2.__get__(p1) produces a callable that behaves like draw2, but automatically passes p1 as the first argument.

Answer 2

Double underscores cause Python to 'mangle' the name of the attribute. It will actually be stored as _Point__x instead of __x. If you change your function like this, it will work:

def draw2(self):
    print("x",self._Point__x, "y", self._Point__y)

The double underscores are supposed to indicate a variable that is private to the class and should not be accessed outside it. Name mangling makes it more difficult to do so accidentally.