Calling a Method of another class with a lambda expression in python - python-3.x

Question

I am trying to create a little GUI with multiple pages. The First page has a button which raises the second page and changes the label text on the second page. However, I fail to call the method of the second page which is supposed to change the text. Can somebody tell me, why I get the following error when calling the method: SecondPage.changeLabel()?
TypeError: changeLabel() missing 1 required positional argument: 'self'
UPDATE:
import tkinter as tk
class ExampleApp(tk.Tk):
def __init__(self, *args, **kwargs):
tk.Tk.__init__(self, *args, **kwargs)
container = tk.Frame(self)
container.pack()
self.frames = {}
for F in (FirstPage, SecondPage):
frame = F(container, self)
self.frames[F] = frame
frame.grid(row=0, column=0, sticky="nsew")
self.show_frame(FirstPage)
def show_frame(self, cont):
frame = self.frames[cont]
frame.tkraise()
def get_page(self, frame_class):
return self.frames[frame_class]
class FirstPage(tk.Frame):
def __init__(self, parent, controller):
tk.Frame.__init__(self, parent)
page = controller.get_page(SecondPage)
self.buttonFP = tk.Button(self, text="Next Page",
command=lambda : [f for f in [page.changeLabel(),
controller.show_frame(SecondPage)]])
self.buttonFP.pack()
class SecondPage(tk.Frame):
def __init__(self, parent, controller):
tk.Frame.__init__(self, parent)
self.label = tk.Label(self, text="Test")
self.label.pack()
def changeLabel(self):
"""change text of label"""
newLabel = "changed"
self.label.configure(text=newLabel)
if __name__ == "__main__":
app = ExampleApp()
app.mainloop()

Answer 1

You get the error because you are trying to call the method on the class rather than on the instance of the class.
Since your architecture has a controller, you can modify the controller to have a function that returns the instance of a page. You can then use the instance to call methods on that page.
Example:
class ExampleApp(tk.Tk):
...
def get_page(self, frame_class):
return self.frames[frame_class]
Later, you can use this method to get the instance of the page, and use the instance to call the method:
page = self.controller.get_page(SecondPage)
page.changeLabel()