Python function returns None after checking if user input is valid

Question

I've just started learning programming some time ago and been playing with Python a bit. I wrote a small program that rolls dice based on user input (number of sides on the die and number of dice). Here's the code:

from random import randrange


def number_of_sides():
    n = input("How many sides? ")  # Get input from the user
    if n.isdigit():  # Check if the input is a digit
        n = int(n)  # If it is, turn it into an integer
        return n  # And return the value of n
    else:
        print("Invalid input. ")  # If test returns false, rerun function
        number_of_sides()


def number_of_dice():
    m = input("How many dice? ")  # Get input from the user
    if m.isdigit():  # Check if the input is a digit
        m = int(m)  # If it is, turn it into an integer
        return m  # And return the value of m
    else:
        print("Invalid input. ")  # If test returns false, rerun function
        number_of_dice()


def play_again() -> object:  # Checks if user answered yes or no, then reruns everything or exits with exit code 0
    answ = input("Do you want to play again?(yes/no) ")
    if answ == "yes":
        dice_roll()
    elif answ == "no":
        print("Ok then")
        return True
    else:
        print("Input invalid, trying again")
        play_again()


def dice_roll():  # sides - for number of sides, dice - for number of dice.
    sides = number_of_sides()  # Whatever number the function returns
    dice = number_of_dice()  # Whatever number the function returns
    results = [] # empty list, here is where results will be appended
    for i in range(1, dice + 1):  # That returns values for each dice specified in the dice variable
        throw = randrange(1, sides + 1)
        results.append(throw)
        results_str = " | ".join(str(i) for i in results)  # Turn the list into string, separate results with a pipe
    print(results_str)  # Print the results of throws
    play_again()  # Ask the user to play again


dice_roll()

Everything works ok when I provide valid input (so digits like 6 and 6 for sides and dice), it crashes, however, when I try providing invalid input first. For some reason the number_of_sides() and number_of_dice() functions return None when they're first fed invalid input and then crash the code in the for loop in the dice_roll() function:

How many sides? a
Invalid input. 
How many sides? 6
How many dice? 6
Traceback (most recent call last):
  File "<directory>/dice_roll.py", line 48, in <module>
    dice_roll()
  File "<directory>/dice_roll.py", line 41, in dice_roll
    throw = randrange(1, sides + 1)
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'

Process finished with exit code 1

I tested these functions separately and they seem to be working fine. This snippet:

def number_of_sides():
    n = input("How many sides? ")
    if n.isdigit():
        n = int(n)
        print(n, ", ", type(n))
        return n
    else:
        print(n, ", ", type(n))
        number_of_sides()

Returns str if you provide a string and int if you provide it with digits. I would like to know what may be causing this weird behaviour.

You need to return something from your else blocks in number_of_sides and number_of_dice. If there is no explicit return statement, Python functions implicitly return None. I would advise against using recursion/mutual recursion here. It is only going to make it hard to reason about. — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Jun 20, 2017 at 7:39

Błotosmętek · Accepted Answer · 2017-06-20 07:37:34Z

3

The problem is that you are calling number_of_dice recursively, but you ignore its result. You should replace the recursive call with:

return number_of_dice()

or even better, get rid of recursion by using a simple while loop:

def number_of_dice():
    while True:
        m = input("How many dice? ")  # Get input from the user
        if m.isdigit():  # Check if the input is a digit
            m = int(m)  # If it is, turn it into an integer
            return m  # And return the value of m
        else:
            print("Invalid input. ")

The same, obviously, holds for the other function.

answered Jun 20, 2017 at 7:37

Błotosmętek

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2 Comments

Azat Ibrakov Over a year ago

how about EAFP?

Błotosmętek Over a year ago

@AzatIbrakov yes, replacing if with try came to my mind too, but that's not directly related to the question.

Nono RAM · Accepted Answer · 2017-06-20 07:58:25Z

1

I seem that the functions number_of_sides() and number_of_dice() only return a valid value when the first input is correct otherwise they loop themselves but the new input will not be returned to the main function. I think recursivity is not good in that case.

You could transform number_of_dices like this :

def number_of_sides():
    nb_sides = input("How many sides? ")

    while (nb_sides.isdigit())==False:
        print("Invalid input. ")
        nb_sides=input("How many sides? ")

    return int(nb_sides)

For your information, you can also use :

import pdb
pdb.set_trace()

to pause the code in a specific line

answered Jun 20, 2017 at 7:58

Nono RAM

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3 Comments

Mroov Over a year ago

I can't seem to grasp why they do not return new input after they're looped.

Nono RAM Over a year ago

In fact, after futher investigations, you are missing the return statement in your functions :

Nono RAM Over a year ago

def number_of_sides(): n = input("How many sides? ") # Get input from the user if n.isdigit(): # Check if the input is a digit n = int(n) # If it is, turn it into an integer return n # And return the value of n else: print("Invalid input. ") # If test returns false, rerun function return number_of_sides()

Nono RAM · Accepted Answer · 2017-06-22 12:37:06Z

"return" statement from your fail case is missing in the 2 statements

from random import randrange


def number_of_sides():
    n = input("How many sides? ")  # Get input from the user
    if n.isdigit():  # Check if the input is a digit
        n = int(n)  # If it is, turn it into an integer
        return n  # And return the value of n
    else:
        print("Invalid input. ")  # If test returns false, rerun function
        return number_of_sides()


def number_of_dice():
    m = input("How many dice? ")  # Get input from the user
    if m.isdigit():  # Check if the input is a digit
        m = int(m)  # If it is, turn it into an integer
        return m  # And return the value of m
    else:
        print("Invalid input. ")  # If test returns false, rerun function
        return number_of_dice()


def play_again() -> object:  # Checks if user answered yes or no, then reruns everything or exits with exit code 0
    answ = input("Do you want to play again?(yes/no) ")
    if answ == "yes":
        dice_roll()
    elif answ == "no":
        print("Ok then")
        return True
    else:
        print("Input invalid, trying again")
        play_again()


def dice_roll():  # sides - for number of sides, dice - for number of dice.
    sides = number_of_sides()  # Whatever number the function returns
    dice = number_of_dice()  # Whatever number the function returns
    results = [] # empty list, here is where results will be appended
    for i in range(1, dice + 1):  # That returns values for each dice specified in the dice variable
        throw = randrange(1, sides + 1)
        results.append(throw)
        results_str = " | ".join(str(i) for i in results)  # Turn the list into string, separate results with a pipe
    print(results_str)  # Print the results of throws
    play_again()  # Ask the user to play again


dice_roll()

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