None, Python's null?

There's no null in Python; instead there's None. As stated already, the most accurate way to test that something has been given None as a value is to use the is identity operator, which tests that two variables refer to the same object.

>>> foo is None
True
>>> foo = 'bar'
>>> foo is None
False

The basics

There is and can only be one None

None is the sole instance of the class NoneType and any further attempts at instantiating that class will return the same object, which makes None a singleton. Newcomers to Python often see error messages that mention NoneType and wonder what it is. It's my personal opinion that these messages could simply just mention None by name because, as we'll see shortly, None leaves little room to ambiguity. So if you see some TypeError message that mentions that NoneType can't do this or can't do that, just know that it's simply the one None that was being used in a way that it can't.

Also, None is a built-in constant. As soon as you start Python, it's available to use from everywhere, whether in module, class, or function. NoneType by contrast is not, you'd need to get a reference to it first by querying None for its class.

>>> NoneType
NameError: name 'NoneType' is not defined
>>> type(None)
NoneType

You can check None's uniqueness with Python's identity function id(). It returns the unique number assigned to an object, each object has one. If the id of two variables is the same, then they point in fact to the same object.

>>> NoneType = type(None)
>>> id(None)
10748000
>>> my_none = NoneType()
>>> id(my_none)
10748000
>>> another_none = NoneType()
>>> id(another_none)
10748000
>>> def function_that_does_nothing(): pass
>>> return_value = function_that_does_nothing()
>>> id(return_value)
10748000