How does passing an instance of one class to another work?-CodePudding

For a couple of days I've been attempting to pass an instance of a variable from one of my classes to another, and until recently I have not been able to get it to work. I've read this document regarding classes and instances, I've searched online and stackoverflow and I couldn't get an answer until I tried a solution from a previous asked question and it solved my problem. However, I'm not exactly sure on the why and the how it works.

My code goes as follows, :

from tkinter import *

def main():
    main_window = Tk()
    app = First(main_window)
    main_window.mainloop()

class First:
    def __init__(self, root):
        self.root = root
        self.root.title('First Window')

        self.entry1 = Entry(self.root)
        self.entry1.pack()

        btn = Button(self.root, text="click", command=self.clicked)
        btn.pack()
    def clicked(self):
        writeDoc(First=self)

class writeDoc:
    def __init__(self, First):
        self.First = First
        txt = self.First.entry1.get()
        print(txt)

if __name__ == '__main__'
    main()

Under the class writeDoc whenever I hover over the parameter First it is not a reference to the Class First, so I am unsure how it has access to entry1. The same goes for the First within writeDoc(First=self). Addtionally, why does writeDoc(First=self) work but not writeDoc(self.First)?

I assumed when I want to reference to another class and want to pass its instances over I would need to do

class First:
....
def clicked(self):
writeDoc(First)
....

class writeDoc(First):
    def __init__(self, First):
    super().__init__(First):
...

But, this didn't work for me at all.

I hope this isn't a loaded/jumbled question. I want to understand how this works to create better code and avoid misunderstanding how python classes works.

CodePudding user response：

Inside the clicked() method, self refers to an instance of the First class. It then does

writeDoc(First=self)

which passes that instance to the writeDoc class's __init__() method, as the value of its First parameter. This is no different from passing any other object as a method parameter.

writeDoc(self.First) doesn't work because there's no First attribute in the self instance. First is the name of the class, not an attribute.