What Is the "None" Keyword in Python?

In Python, None is a built-in constant that implies absence of value (or a null value):

• It can be used to assign "null" value to variables, function arguments or function returns;
• It's a singleton object (i.e. it always references to the same object);
• It's the sole instance of the NoneType type;
• It cannot be reassigned;
• It's considered falsy in a boolean context.

For example, you can assign None to a variable in the following way:

foo = None
print(foo) # None

Similarly, you can assign None to a function/method argument (to serve as a default value), like so:

def foo(param = None):
    if param is None:
        return 'empty'

print(foo()) # 'empty'

None can also be returned from a function:

def foo():
    return None

print(foo()) # None

When a function/method does not explicitly return a value, it is implicitly None:

def foo():
    pass

print(foo()) # None