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:
NoneCode language: Python (python)
If you use the type() function to check the type of the None value, you’ll get NoneType class:
print(type(None))Code language: Python (python)
Output:
<class 'NoneType'>Code language: Python (python)
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)Code language: Python (python)
Output:
True
TrueCode language: Python (python)
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 Trueapple = Apple()
print(apple == None)
Code language: Python (python)
Output:
TrueCode language: Python (python)
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:
Noneis not zero (0, 0.0, …).Noneis not the same asFalse.Noneis not the same as an empty string ('').- Comparing
Noneto any value will returnFalseexceptNoneitself.
The applications of the Python None object
Let’s take some practical examples of using the None object.
1) 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 = NoneCode language: Python (python)
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'Code language: Python (python)
2) 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 colorsCode language: Python (python)
It works as expected if you pass an existing list:
colors = ['red', 'green']
append('blue', colors)print(colors)
Code language: Python (python)
Output:
['red', 'green', 'blue']Code language: Python (python)
However, the problem arises when you use the default value of the second parameter. For example:
hsl = append('hue')
print(hsl)rgb = append('red')
print(rgb)
Code language: Python (python)
Output:
['hue']
['hue', 'red']Code language: Python (python)
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)
Code language: Python (python)
Output:
['hue']
['red']Code language: Python (python)
3) 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)
Code language: Python (python)
The say() function doesn’t return anything; therefore, it returns None.
Summary
Noneis a singleton object of theNoneTypeclass.Noneis not equal to anything except itself.- Use
isoris notoperator to compareNonewith other values.