Question

I have written a little code so people can input a date. The error check that stops a month being entered which is less than 1 or greater than 12 should return a value only when it is within these bounds. If I enter a few ‘out of bounds’ numbers, it correctly re-asks for a month to be re-entered but returns all the values. What is going on?

# the question asked to get the month input for the xml updater
def month_q():
    try:
        month = int(input('What was the month [MM] which the installers were  updated/created by xxx?:'))
    except:
        print("That was not a valid number. Please re-enter a 2 digit month")
        month_q()
    updatemonth = month_check(month)
    print("Month q returning:", updatemonth)
    return updatemonth


# check the update month is a valid month
def month_check(month):
    if month < 1:
        print("The month must be a number between 01 and 12. Please re-enter")
        month_q()
    elif month > 12:
        print("The month must be a number between 01 and 12. Please re-enter")
        month_q()
    else:
        print("Month_check returning month:", month)
        return month



# this updates the xml file with the date of the last installer    
def xml_updater():
    updatemonth = month_q()
    print("Update month:", updatemonth)


xml_updater()

The result is of entering bad months ’15’, ’14’ and ’13’ before entering a correct one ’12’ is:

What was the month [MM] which the installers were updated/created by xxx?:15
The month must be a number between 01 and 12. Please re-enter
What was the month [MM] which the installers were updated/created by xxx?:14
The month must be a number between 01 and 12. Please re-enter
What was the month [MM] which the installers were updated/created by xxx?:13
The month must be a number between 01 and 12. Please re-enter
What was the month [MM] which the installers were updated/created by xxx?:12
Month_check returning month: 12
Month q returning: 12
Month q returning: None
Month q returning: None
Month q returning: None
Update month: None

What is going on?

Answer 1

Your function is not returning anything; it is calling the question function again and again, but ignores the return value:

def month_check(month):
    if month < 1:
        print("The month must be a number between 01 and 12. Please re-enter")
        month_q()
    elif month > 12:
        print("The month must be a number between 01 and 12. Please re-enter")
        month_q()
    else:
        print("Month_check returning month:", month)
        return month

If month is smaller than 1 or greater than 12 the function will return None (the default). The statement updatemonth = month_check(month) then assigns None to updatemonth and when print("Month q returning:", updatemonth) runs it prints Month q returning: None. But since you called month_q recursively, that is then returned to the previous month_check() call, which returns None again, and so forth.

Here is a nested trace of your functions, as month_q calls month_check calls month_q, etc.:

month_q():
    What was the month [MM] which the installers were updated/created by xxx?:15
    month_check(15):
        The month must be a number between 01 and 12. Please re-enter
        month_q():
            What was the month [MM] which the installers were updated/created by xxx?:14
            month_check(15):
                The month must be a number between 01 and 12. Please re-enter
                month_q():
                    What was the month [MM] which the installers were updated/created by xxx?:13
                    month_check(13):
                        The month must be a number between 01 and 12. Please re-enter
                        month_q():
                            What was the month [MM] which the installers were updated/created by xxx?:12
                            month_check(12):
                                Month_check returning month: 12
                                <- 12
                            Month q returning: 12
                            <- 12
                        <- None
                    Month q returning: None
                    <- None
                <- None
            Month q returning: None
            <- None
        <- None
    Month q returning: None
    <- None
Update month: None

Instead, have the check function return True or False to indicate correct results, and leave the asking for a re-enter to month_q.

Do that with a while loop instead of recursion though; if you have to ask often enough, you’ll end up with a recursion-too-deep exception.

def month_q():
    while True:
        try:
            month = int(input('What was the month [MM] which the installers were  updated/created by xxx?:'))
            if not month_check(month):
                raise ValueError()
        except ValueError:
            print("Please re-enter a 2 digit month")
            continue
        print("Month q returning:", month)
        return month


# check the update month is a valid month
def month_check(month):
    if month < 1 or month > 12:
        print("The month must be a number between 01 and 12")
        return False
    return True

You may as well inline that check:

def month_q():
    while True:
        try:
            month = int(input('What was the month [MM] which the installers were  updated/created by xxx?:'))
            if month < 1 or month > 12:
                raise ValueError()
        except ValueError:
            print("Please re-enter a 2 digit month between 01 and 12.")
            continue
        print("Month q returning:", month)
        return month

Using a blanket except: clause is never a good idea; in the above code I catch the ValueError raised by int() when you enter a non-integer value instead, and raise the same exception if you did enter an integer but it was not a value between 1 and 12 inclusive. That simplifies the ‘not a month’ error handling significantly.