What Causes My Function to Return None At the End

Why does my recursive function return None?

It is returning None because when you recursively call it:

if my_var != "a" and my_var != "b":
    print('You didn\'t type "a" or "b". Try again.')
    get_input()

..you don't return the value.

So while the recursion does happen, the return value gets discarded, and then you fall off the end of the function. Falling off the end of the function means that python implicitly returns None, just like this:

>>> def f(x):
...     pass
>>> print(f(20))
None

So, instead of just calling get_input() in your if statement, you need to return it:

if my_var != "a" and my_var != "b":
    print('You didn\'t type "a" or "b". Try again.')
    return get_input()

return, return None, and no return at all?

On the actual behavior, there is no difference. They all return None and that's it. However, there is a time and place for all of these.
The following instructions are basically how the different methods should be used (or at least how I was taught they should be used), but they are not absolute rules so you can mix them up if you feel necessary to.

Using return None

This tells that the function is indeed meant to return a value for later use, and in this case it returns None. This value None can then be used elsewhere. return None is never used if there are no other possible return values from the function.

In the following example, we return person's mother if the person given is a human. If it's not a human, we return None since the person doesn't have a mother (let's suppose it's not an animal or something).

def get_mother(person):
    if is_human(person):
        return person.mother
    else:
        return None

Using return

This is used for the same reason as break in loops. The return value doesn't matter and you only want to exit the whole function. It's extremely useful in some places, even though you don't need it that often.

We've got 15 prisoners and we know one of them has a knife. We loop through each prisoner one by one to check if they have a knife. If we hit the person with a knife, we can just exit the function because we know there's only one knife and no reason the check rest of the prisoners. If we don't find the prisoner with a knife, we raise an alert. This could be done in many different ways and using return is probably not even the best way, but it's just an example to show how to use return for exiting a function.

def find_prisoner_with_knife(prisoners):
    for prisoner in prisoners:
        if "knife" in prisoner.items:
            prisoner.move_to_inquisition()
            return # no need to check rest of the prisoners nor raise an alert
    raise_alert()

Note: You should never do var = find_prisoner_with_knife(), since the return value is not meant to be caught.

Using no return at all

This will also return None, but that value is not meant to be used or caught. It simply means that the function ended successfully. It's basically the same as return in void functions in languages such as C++ or Java.

In the following example, we set person's mother's name and then the function exits after completing successfully.

def set_mother(person, mother):
    if is_human(person):
        person.mother = mother

Note: You should never do var = set_mother(my_person, my_mother), since the return value is not meant to be caught.

Why is this printing 'None' in the output?

Because there are two print statements. First is inside function and second is outside function. When a function doesn't return anything, it implicitly returns None.

Use return statement at end of function to return value.

e.g.:

Return None.

>>> def test1():
...    print "In function."
... 
>>> a = test1()
In function.
>>> print a
None
>>> 
>>> print test1()
In function.
None
>>> 
>>> test1()
In function.
>>> 

Use return statement

>>> def test():
...   return "ACV"
... 
>>> print test()
ACV
>>> 
>>> a = test()
>>> print a
ACV
>>> 

Python Script returns unintended None after execution of a function

In python the default return value of a function is None.

>>> def func():pass
>>> print func()     #print or print() prints the return Value
None
>>> func()           #remove print and the returned value is not printed. 
>>>

So, just use:

letter_grade(score) #remove the print

Another alternative is to replace all prints with return:

def letter_grade(score):
    if 90 <= score <= 100:
        return "A"
    elif 80 <= score <= 89:
        return "B"
    elif 70 <= score <= 79:
        return  "C"
    elif 60 <= score <= 69:
        return "D"
    elif score < 60:
        return "F"
    else:
        #This is returned if all other conditions aren't satisfied
        return "Invalid Marks"

Now use print():

>>> print(letter_grade(91))
A
>>> print(letter_grade(45))
F
>>> print(letter_grade(75))
C
>>> print letter_grade(1000)
Invalid Marks

Why is None printed after my function's output?

It's the return value of the function, which you print out. If there is no return statement (or just a return without an argument), an implicit return None is added to the end of a function.

You probably want to return the values in the function instead of printing them:

def jiskya(x, y):
    if x > y:
        return y
    else:
        return x

print(jiskya(2, 3))

Why does this function return None None as well?

Every function always returns a value. If you don't explicitly return a value, and the function just gets all the way to the end, then it automatically returns None. Your function happy doesn't have any return statement, so at the end of the function, it automatically returns None.

why python is returning none after calculation?

multiplicationTable(number) has no return value.

By default, when no return value is given, a python function will return None to signify that it was successfully executed. You have the same behavior in other languages.

To illustrate this, I made your function return something at the end :

def multiplicationTable(number):
    for x in range(1, 11):
        result = number * x
        print(f'{number} X {x} = {result}')
    return 'I have finished'

result = multiplicationTable(5)
print(result)

Output :

5 X 1 = 5
5 X 2 = 10
5 X 3 = 15
5 X 4 = 20
5 X 5 = 25
5 X 6 = 30
5 X 7 = 35
5 X 8 = 40
5 X 9 = 45
5 X 10 = 50
I have finished

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