为什么这个类的方法的实例不使用此参数？

Question

My question is why I can't make a object of the class giving it an integer as an argument. It totally ignores the value that I pass as an argument to it. Yet, if I call the area function from the class and pass the same number into it's parentheses it will output the desired result of 144.

My best guess from what I have read so far is that the way the classes function is coded is incorrect for what I am hoping that it will achieve. Do I need to have the area function take it's argument like: self.length ?

# -*- coding: utf-8 -*-
class square:
    sides = 4
    def __init__(self, length):
        self.length = length
    def area(self, length):
        return length * length

box = square(12)
print(box.area())

Output: TypeError: area() missing 1 required positional argument: 'length'

Shouldn't the object that is created save the value that is given to it for the duration of that objects life? Why does it throw another error when I tell it to print from the area method a second time if the calls looked like this?

print(box.area(12))
output:144
print(box.area())
output: <bound method square_shape.area of <__main__.square_shape object at 0x7f5f88355b70>>

I'm sorry if this question is a little oddly phrased but I'm simply looking for as much information as I can get and attempting to gain a better understanding of what I'm doing here. Other questions on this subject didn't give a good grasp of what I'm trying to figure out.

Answer 1

You are thinking of self.length. length (without a "self." in front of it) is just a local variable. If it's an argument, you need to provide it.

What you probably want is: def area(self): return self.length * self.length

Answer 2

Your area takes the self argument, which is enough to access self.length :

...
def area(self):
  return self.length * self.length

Then you can call square(3).area() and get 9 .

Your current declaration requires an explicit length argument, something like square(3).area(4) which would produce 16 .

One of the key tenets of OOP is that methods have access to object's (or often said instance's ) state, and can operate on that state, instead of passing it explicitly.

In Python, the parameter normally named self is used to pass the object in question to the method. The area method operates on square 's length ; you access it as self.length . That self is an instance of square , the thing that eg square(3) returns.