Switching Frames using a function

Question

I have a program that performs frame switching like in this example: Switch between two frames in tkinter

I am trying to make part of my program switch frames through a function, where the command to switch frames is inside another function, rather than the button.

This is the switch frame function:

class MathsApp(tk.Tk):
    def __init__(self):
        tk.Tk.__init__(self)
        self._frame=None
        self.switch_frame(StartPage)
        self.title("Maths Revision App")
        self.geometry("800x500")
        self.configure(bg="white")


    def switch_frame(self, frame_class):
        #Destroys current frame and replaces it with a new one.
        new_frame=frame_class(self)
        if self._frame is not None:
            self._frame.destroy()
        self._frame=new_frame
        self._frame.pack(fill="both",expand=True)

This is the function that I am trying to get to switch frames:

def validateAns(ans):
    global questionpages,currentQ
    if ans=="":
        self.switch_frame(errorPage)
    else:
        if currentQ==9:
            checkAns(ans, currentQ)
            getTimes()
            self.switch_frame(ResultPage)
        else:
            checkAns(ans, currentQ)
            getTimes()
            currentQ=currentQ+1
            self.switch_frame(questionpages[currentQ])

Everything else with this function works except the switch_frame lines.

This is one of the frames that uses this function:

class q1(tk.Frame):
    def __init__(self, master):
        tk.Frame.__init__(self, master, bg="white")
        lbl=tk.Label(self, text="Question 1", font=title_font, bg="white", fg="#004d99")
        lbl.place(x=30, y=20)
        txt=tk.Text(self, height=7, width=70)#the text box that displays the question
        txt.config(state="normal")
        txt.insert(tk.INSERT,qlist[0][0])
        txt.config(state="disabled")
        txt.place(x=35,y=125)
        ans=tk.Entry(self)
        ans.place(x=650, y=350,height=25)
        btn=tk.Button(self, text="Next", height=3, width=15, fg="white", bg="#004d99", command=lambda:validateAns(ans.get()))
        btn.place(x=650, y=400) 

When I click the button on frame q1 that uses "validateAns" I get the error message "name 'self' not defined". I tried replacing self.switch_frame with "master.switch_frame" and got a similar error, but with 'master' not being defined.

Depending on the value passed into the validateAns function, the page/frame should switch to one of three different ones.