None

---

Python has a special value in the language called None to represent the absence of a value. It is common to use None to represent a result of an invalid query without throwing an explicit error.

For example, suppose we had the following increment function that we decided should only increment positive values. You could write code to throw an error if it was given a negative value, but it’s very common in Python to instead return this “missing value” None .

def increment(x):
    """
    Returns the value of incrementing a non-negative value x by 1.
    If x is negative, returns None.
    """
    if x < 0:
        return None
    else:
        return x + 1


print(increment(3))
print(increment(-3))

Warning

Remember: for your take-home assessments, you should always write your code with the main-method pattern. We omit that in most of our lecture readings to keep these snippets shorter.

None should not be confused with the value 0 ! 0 is a valid number in Python (i.e. 1 + 0 is well defined to be 1 ) while None is the absence of a number altogether! If you were to run 1 + None it would cause an error since it doesn’t make sense to add 1 to something that is entirely missing!

A very common error in Python comes from trying to manipulate a None value in such a way that causes an error like described in the last paragraph.

Checking for None

A very common task in Python is to check if a particular value is None or not. This is quite easy to do with the is keyword in Python. is and == are quite similar and in this context, you could use either. We will see in a later lecture the difference between is and == and we’ll understand why is is more common for comparing to None . For now just know that you use is for None checks and == for most other things.

def increment(x):
    """
    Returns the value of incrementing a non-negative value x by 1.
    If x is negative, returns None.
    """
    if x < 0:
        return None
    else:
        return x + 1


x = increment(-1)
if x is None:
    print('Failed')
else:
    print(x)

""" Alternatively you could write
if x is not None:
    print(x)
else:
    print('Failed')
"""