How to use nested loops propely

Question

I have this nested loop in my project (it's a lot more complicated of course I just simplify it so you can see what I mean). I know there is no label and goto in python, I just want to show what I want to do.

From line #goto third I want go back to place where you can see #label third.

I tried different setups of my loops but they never do what I want

import time
onoff = "on"
t=0
while onoff == "on":
 #label first
 for x in range (5):
 print("first loop")
 time.sleep(1)
 for y in range (5):
 print("second loop")
 time.sleep(1)
 p = 0 #for testing
 t=0 #for testing
 if p != 5:
 if t == 0:
 print("third loop")
 time.sleep(1)
 p2 = 5 #for testing
 t=0
 if p2 != 5: #label third
 if t == 0:
 print("go back to first loop")
 time.sleep(1)
 #goto first
 else:
 print("lock")
 #lock.acquire()
 else:
 if t == 0:
 print("go back to third loop")
 p2 = 3
 time.sleep(1)
 #goto third
 else:
 print("lock")
 #lock.acquire()
 else:
 print("lock")
 #lock.acquire()

Every path in this nested loops seems to work fine but I want my loop to go back to #label third from #goto third and it goes back to #label first. How can I change my loops to make it possible?

• But I'm Not A Wrapper Class: It'll eventually have to go from goto third to label first when the for y in range(5) loop completes for every x. If you want more loop control, then I suggest using a while instead of a for

Answer 1

Actions like goto first which break 'for' loops are evil in many ways. While loops are more elegant, but maybe a 'state machine' like solution is better for you. Something like:

state = 0
while is_on:
 if state == 0: # do outer loop things
 <do things>
 state = 1 # to do inner loop things
 elif state == 1:
 n = 0
 # do inner loop things 
 n += 1
 if n == 5:
 state = 0
 elif state == 2: # do even more nested things
 p = 0
 if <some condition>:
 state = 0
 p += 1
 if p == 5:
 state = <whatever>

A state machine permits much more flexibility. Also, it won't cause as much indentation as nested loop. If the complexity gets larger, there are some libraries which can help you. Interesting links on Finite State Machines (FSM):

https://python-3-patterns-idioms-test.readthedocs.io/en/latest/StateMachine.html

https://www.python-course.eu/finite_state_machine.php

• ostry0728: Yeah, I think I can make it using this method. Thank you.