Editing an object in another class (PyQt5)

Task:
There are two classes. The class «First» creates the main window and button.
The class «Second» changes the label on the button on the main window (created by the class «First»).

What came up:
Below is the code I wrote as an example. The execution of which leads to the creation of many windows (which is wrong), and even a button is not drawn on these windows (which is also wrong).
Obviously, I am referring to the class incorrectly. But google and having tried the options, unfortunately, I did not find an error in the code.

import sys
from time import sleep

from PyQt5.QtWidgets import QMainWindow, QApplication, QPushButton

class First(QMainWindow):
    def __init__(self):
        super(First, self).__init__()
        self.pbutton = QPushButton("First", self)
        self.show()
        self.class_Second = Second()
        self.class_Second.Change()

class Second():
    def __init__(self):
        pass

    def Change(self):
        print("Start Change")
        sleep(3)
        First().pbutton.setText("Second")

app = QApplication(sys.argv)
win = First()
sys.exit(app.exec_())

Also a question:
is the function «def __init __ (self):» called once? Why are there multiple calls to the «def Change (self):» function: if the code has no loops, no conditions, no timers?

If the «self.show()» is set lower, then we get again multiple calls to the function, but this time the main window does not even appear.

import sys
from time import sleep

from PyQt5.QtWidgets import QMainWindow, QApplication, QPushButton

class First(QMainWindow):
    def __init__(self):
        super(First, self).__init__()
        self.pbutton = QPushButton("First", self)
        self.class_Second = Second()
        self.class_Second.Change()
        self.show()

class Second():
    def __init__(self):
        pass

    def Change(self):
        print("Start Change")
        sleep(3)
        First().pbutton.setText("Second")

app = QApplication(sys.argv)
win = First()
sys.exit(app.exec_())

UPD
If you add a «print()» and look at the log in the console, it seems that the function is not fully executed, but it does not end with an error. But the multiple function call continues, despite the error in the class call.

def Change(self):
    print("Start Change")
    sleep(3)
    First().pbutton.setText("Second")
    print("____")

And in the console:

The statement First().pbutton.setText('Second') creates a new instance of First and attempts to set the button text of the new instance. It doesn't update any existing instances of First. However, since First.__init__() calls Second.Change(), and Second.Change() calls First() which calls First.__init__() which calls Second.Change(), etc. you end up in an infinite loop. One way around this is to give the original instance of First as an argument to Second.Change().

The other issue with your code is that sleep in Second.Change will block the execution of your code until after the label has been changed. Instead you could use a singleshot QTimer which will delay the label change without blocking the code, e.g.

class First(QMainWindow):
    def __init__(self):
        super(First, self).__init__()
        self.pbutton = QPushButton("First", self)
        self.class_Second = Second()
        self.class_Second.Change(self)
        self.show()

class Second():
    def __init__(self):
        pass

    def Change(self, first):
        print("Start Change")
        QTimer.singleShot(3000, lambda: first.pbutton.setText("Second"))

With help Heike and also I myself understood how to do without QTimer and without Lambda. + variable first renamed on f so that there is no confusion with the name of my class First.
Work code:

import sys
from time import sleep

from PyQt5.QtWidgets import QMainWindow, QApplication, QPushButton

class First(QMainWindow):
    def __init__(self):
        super(First, self).__init__()
        self.pbutton = QPushButton("First", self)
        self.class_Second = Second()
        self.class_Second.Change(self)
        self.show()

class Second():
    def __init__(self):
        pass

    def Change(self, f):
        print("Start Change")
        sleep(3)
        f.pbutton.setText("Second")

app = QApplication(sys.argv)
win = First()
sys.exit(app.exec_())