Can some one explain how x pass into function f

Question

def f(obj):
    print('attr =', obj.attr)

class Foo:
    attr = 100
    attr_val = f

x = Foo()
print(x.attr)
x.attr_val()

Output:

100
attr = 100

I got this code from real python but I don't understand how x is pass into function f.

Can someone explain that to me, thanks.

Answer 1

x is a class object when you are doing x.attr_val() it automatically takes itself and provides it as a first argument to the function (often arguments like this are named self).

Answer 2

attr_val is what is called an instance method. When your Foo class calls it, it passes the object as first argument automatically, effectively running: f(x)

If you were using a custom __init__ method, the standard practice would be to pass the self variable to indicate this self-reference.

Thus, a more verbose variant would be:

def f(obj):
    print('attr =', obj.attr)

class Foo:
    def __init__(self):
        self.attr = 100
    def attr_val(self):
        f(self) # or "return f(self)"

x = Foo()
x.attr_val()
# attr = 100