How to Loop Back to Beginning

Is there a way to loop back to the beginning of a certain part of the code in python

Two ways of doing it, using a while True:

while True:
    x=input("input a word: ")
    if len(x) == 8:
        break
    print("word must have 8 characters")

Using recursion:

def get_input():
    x=input("input a word: ")
    if len(x) == 8:
        return x
    print("word must have 8 characters")
    return get_input()

How to make program go back to the top of the code instead of closing

Python, like most modern programming languages, does not support "goto". Instead, you must use control functions. There are essentially two ways to do this.

1. Loops

An example of how you could do exactly what your SmallBasic example does is as follows:

while True :
    print "Poo"

It's that simple.

2. Recursion

def the_func() :
   print "Poo"
   the_func()

the_func()

Note on Recursion: Only do this if you have a specific number of times you want to go back to the beginning (in which case add a case when the recursion should stop). It is a bad idea to do an infinite recursion like I define above, because you will eventually run out of memory!

Edited to Answer Question More Specifically

#Alan's Toolkit for conversions

invalid_input = True
def start() :
    print ("Welcome to the converter toolkit made by Alan.")
    op = input ("Please input what operation you wish to perform. 1 for Fahrenheit to Celsius, 2 for meters to centimetres and 3 for megabytes to gigabytes")
    if op == "1":
        #stuff
        invalid_input = False # Set to False because input was valid


    elif op == "2":
        #stuff
        invalid_input = False # Set to False because input was valid
    elif op == "3": # you still have this as "if"; I would recommend keeping it as elif
        #stuff
        invalid_input = False # Set to False because input was valid
    else:
        print ("Sorry, that was an invalid command!")

while invalid_input: # this will loop until invalid_input is set to be False
    start()

How to loop back to start of question in case of incorrect input in a yes/no game?

A good way to solve this would be error handling - create a custom Exception and throw it whenever a wrong answer is given. Loop until you passed all questions without exception and then break.

I also modified your ask-function , there is no need for recursion in it, simply loop and return when a valid answer is given.

def askYesNoQuestion(question):
    """Asks a question, accepts only Y or N as answer. Repeats until given."""
    while True:
        YesNoAnswer = input(question).strip().upper()
        if YesNoAnswer in ("Y,N"):
            return YesNoAnswer
        else:
            print('Please enter Y or N!')

class WrongAnswer(Exception):
    """Custom exception, thrown (and caught) on bad answers"""
    pass

while True:
    try:
        animal_answer1 = askYesNoQuestion("Is a parrot a bird? Y/N ")
        if animal_answer1 == "Y":
            print("Correct!")
        else:
            raise WrongAnswer()

        animal_answer2 = askYesNoQuestion("Is a whale a mammal? Y/N ")
        if animal_answer2 == "Y":
            print("Correct! Move on")
        else:
            raise WrongAnswer()

        animal_answer3 = askYesNoQuestion("Is a kangaroo a marsupial? Y/N ")
        if animal_answer3 == "Y":
            print("We will see... let us move on")
        else:
            raise WrongAnswer()

        food_answer1 = askYesNoQuestion("Is a carrot a vegetable? Y/N ")
        if food_answer1 == "Y":
            print("Correct!")
        else:
            raise WrongAnswer()

        food_answer2 = askYesNoQuestion("Is a tomato a vegetable? Y/N ")

        if food_answer2 == "N":
            print("Correct! Move on")
        else:
            raise WrongAnswer()

        food_answer3 = askYesNoQuestion("Is a strawberry a berry? Y/N ")
        if food_answer3 == "N":
            print("We will see... let us move on")
        else:
            raise WrongAnswer()

    except WrongAnswer:
        print("Meeeep, wrong - go again.\n")
    else:
        break # executes if all questions passed without raising exception  

print("Cool - all correct!")

Output:

Is a parrot a bird? Y/N y
Correct!
Is a whale a mammal? Y/N n
Meeeep, wrong - go again.

Is a parrot a bird? Y/N y
Correct!
Is a whale a mammal? Y/N y
Correct! Move on
Is a kangaroo a marsupial? Y/N y
We will see... let us move on
Is a carrot a vegetable? Y/N y
Correct!
Is a tomato a vegetable? Y/N n
Correct! Move on
Is a strawberry a berry? Y/N y
Meeeep, wrong - go again.

Is a parrot a bird? Y/N y
Correct!
Is a whale a mammal? Y/N y
Correct! Move on
Is a kangaroo a marsupial? Y/N y
We will see... let us move on
Is a carrot a vegetable? Y/N y
Correct!
Is a tomato a vegetable? Y/N n
Correct! Move on
Is a strawberry a berry? Y/N n
We will see... let us move on
Cool - all correct!

---

Delegating the question-correctness-testing also to askYesNoQuestion leads to much shorter code:

def askYesNoQuestion(question, answer):
    """Asks a 'question', accepts only Y or N, loops until given. 
       Raises WrongAnswer exception if different from 'answer'"""
    while True:
        YesNoAnswer = input(question).strip().upper()
        if YesNoAnswer in ("Y,N"):
            if YesNoAnswer == answer:
                return
            else:
                raise WrongAnswer()
        else:
            print('Please enter Y or N!')

while True:
    try:
        animal_answer1 = askYesNoQuestion("Is a parrot a bird? Y/N ", "Y")
        print("Correct!")

        animal_answer2 = askYesNoQuestion("Is a whale a mammal? Y/N ","Y")
        print("Correct! Move on")

        animal_answer3 = askYesNoQuestion("Is a kangaroo a marsupial? Y/N ","Y")
        print("We will see... let's move on")

        food_answer1 = askYesNoQuestion("Is a carrot a vegetable? Y/N ", "Y")
        print("Correct!")

        food_answer2 = askYesNoQuestion("Is a tomato a vegetable? Y/N ", "N")
        print("Correct! Move on")

        food_answer3 = askYesNoQuestion("Is a strawberry a berry? Y/N ", "N")
        print("We will see... let's move on")

    except WrongAnswer:
        print("Meeeep, wrong - go again.")
    else:
        break

print("Cool - all correct!")

How to loop back to the beginning of a programme - Python

Wrap the whole code into a loop:

while True:

indenting every other line by 4 characters.

Whenever you want to "restart from the beginning", use statement

continue

Whenever you want to terminate the loop and proceed after it, use

break

If you want to terminate the whole program, import sys at the start of your code (before the while True: -- no use repeating the import!-) and whenever you want to terminate the program, use

sys.exit()

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