Python None

Summary: in this tutorial, you’ll learn about Python None and how to use it properly in your code.

Introduction to the Python None value #

In Python, None is a special object of the NoneType class. To use the None value, you specify the None as follows:

None

If you use the type() function to check the type of the None value, you’ll get NoneType class:

print(type(None))

Try it

Output:

<class 'NoneType'>

The None is a singleton object of the NoneType class. It means that Python creates one and only one None object at runtime.

Therefore, if you use the equality (==) or is operator to compare None with None, you’ll get the result of True:

print(None == None)
print(None is None)

Try it

Output:

True
True

It’s a good practice to use the is or is not operator to compare a value with None.

The reason is that the user-defined objects may change the equality operator’s behavior by overriding the __eq__() method. For example:

class Apple:
    def __eq__(self, other):
        return True


apple = Apple()
print(apple == None)

Try it

Output:

True

Note that you cannot override the is operator behavior like you do with the equality operator (==).

It’s also important to note that the None object has the following features:

None is not zero (0, 0.0, …).
None is not the same as False.
None is not the same as an empty string ('').
Comparing None to any value will return False except None itself.

The applications of the Python None object #

Let’s take some practical examples of using the None object.

Using Python None as an initial value for a variable #

When a variable doesn’t have any meaningful initial value, you can assign None to it, like this:

state = None

Then you can check if the variable is assigned a value or not by checking it with None as follows:

if state is None:
    state = 'start'

Using the Python None object to fix the mutable default argument issue #

The following function appends a color to a list:

def append(color, colors=[]):
    colors.append(color)
    return colors

It works as expected if you pass an existing list:

def append(color, colors=[]):
    colors.append(color)
    return colors

colors = ['red', 'green']
append('blue', colors)

print(colors)

Try it

Output:

['red', 'green', 'blue']

However, the problem arises when you use the default value of the second parameter. For example:

def append(color, colors=[]):
    colors.append(color)
    return colors

hsl = append('hue')
print(hsl)

rgb = append('red')
print(rgb)

Try it

Output:

['hue']
['hue', 'red']

The issue is that the function creates the list once defined and uses the same list in each successive call.

To fix this issue, you can use the None value as a default parameter as follows:

def append(color, colors=None):
    if colors is None:
        colors = []

    colors.append(color)
    return colors


hsl = append('hue')
print(hsl)


rgb = append('red')
print(rgb)

Try it

Output:

['hue']
['red']

Using the Python None object as a return value of a function #

When a function doesn’t have a return value, it returns None by default. For example:

def say(something):
    print(something)


result = say('Hello')
print(result)

Try it

The say() function doesn’t return anything; therefore, it returns None.

Summary #

None is a singleton object of the NoneType class.
None is not equal to anything except itself.
Use is or is not operator to compare None with other values.

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