Can some one explain how x pass into function f-CodePudding

def f(obj):
    print('attr =', obj.attr)
class Foo:
    attr =100
    attr_val=f
x =Foo()
print(x.attr)
x.attr_val()

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

CodePudding user response：

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).

CodePudding user response：

In python doc:

Any function object that is a class attribute defines a method for instances of that class.

https://docs.python.org/3/tutorial/classes.html#random-remarks

That is, when you call the class attribute attr_val (which is f; note that attr_val is not an instance attribute), it is designed to supply the instance x (not the class).

CodePudding user response：

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