Need for nested functions in python

Question

I am familar in c/c++ amd new to python and I am wondering what is the need for nested functions in python. Can anyone give a live example where we use nested functions. Thanks in advance.

Answer 1

Your question is too broad, and is therefore likely to be closed, but two examples are Decorators and Closures.

Answer 2

When you want to use helper functions (and you don't want to supply arguments), especially with threads:

import Tkinter as tk
import thread

def main():
    root = tk.Tk()
    progress_var = tk.StringVar(root)
    progress_var.set('')
    progress = tk.Label(root, textvariable = progress_var)
    progress.pack()
    def thread_helper(): # this can access the local variables in main()
        total = 10000000
        for i in xrange(1, total + 1):
             progress_var.set('Progress: {}%'.format(100 * float(i)/total))
    thread.start_new_thread(thread_helper, ())
    root.mainloop()

What this essentially does is show the progress of the for loop in a window. After the root.mainloop(), no processes will run, so I need to use a thread to actually initiate the for loop, and update the progress display.

The thread requires a function to be called, and it is much easier to just define a helper function (which is nested and only used once) rather than create a new function and pass it various arguments.