python # None

None
The sole value of types.NoneType.
None is frequently used to represent the absence of a value, as when default arguments are not passed to a function. (python 2.7.15)
Changed in version 2.4: Assignments to None are illegal and raise a SyntaxError.

如果没有传递参数给函数，则默认参数为None.

The equivalent of the null keyword in Python is None. It was designed this way for two reasons:
Many would argue that the word "null" is somewhat esoteric.
It's not exactly the most friendliest word to programming novices. Also, "None" refers exactly to the intended functionality - it is nothing, and has no behaviour.
In most object-oriented languages, the naming of objects tend to use camel-case syntax. eg. ThisIsMyObject.
As you'll see soon, Python's None type is an object, and behaves as one.

assign the None type to a variable

The syntax to assign the None type to a variable, is very simple. As follows:

my_none_variable = None

Checking if a Variable is None

There are two ways to check if a variable is None. One way can be performed by using the is keyword. Another is using the == syntax. Both comparison methods are different, and you'll see why later:

null_variable = None
not_null_variable = 'Hello There!'
 
# The is keyword
if null_variable is None:
    print('null_variable is None')
else:
    print('null_variable is not None')
 
if not_null_variable is None:
    print('not_null_variable is None')
else:
    print('not_null_variable is not None')
 

# The == operator
if null_variable == None:
    print('null_variable is None')
else:
    print('null_variable is not None')
 
if not_null_variable == None:
    print('not_null_variable is None')
else:
    print('not_null_variable is not None')

运行结果:

null_variable is None
not_null_variable is not None
null_variable is None
not_null_variable is not None

References:

https://docs.python.org/2/library/constants.html
https://www.pythoncentral.io/python-null-equivalent-none/