How to Check If a Variable Is None in Python

Introduction

In this lab, you will learn how to check if a variable is None in Python. The lab begins by explaining the None value, a special constant representing the absence of a value. You'll create Python scripts to understand how None is returned by functions when no value is present, how to assign None to variables, and how to use None as a default argument in functions.

The lab then guides you through using the is operator to specifically check for None, differentiating it from other falsy values like 0, False, and empty strings. This ensures you can accurately identify when a variable truly holds the None value in your Python code.

Skills Graph

Understand the None Value

In this step, you will learn about the None value in Python. None is a special constant in Python that represents the absence of a value or a null value. It is often used to indicate that a variable has not been assigned a value or that a function does not return a value.

Let's start by creating a Python file named none_example.py in your ~/project directory using the VS Code editor.

## ~/project/none_example.py
def greet(name):
    if name:
        return "Hello, " + name
    else:
        return None

result = greet("Alice")
print(result)

result = greet("")
print(result)

In this code:

• We define a function greet that takes a name as input.
• If name is not empty, the function returns a greeting string.
• If name is empty, the function returns None.
• We call the greet function with "Alice" and an empty string "" and print the results.

Now, run the Python script:

python ~/project/none_example.py

You should see the following output:

Hello, Alice
None

As you can see, when we call greet with "Alice", it returns "Hello, Alice". When we call it with an empty string, it returns None, which is then printed to the console.

None is not the same as 0, False, or an empty string. It is a distinct value that represents the absence of a value.

You can assign None to a variable:

## ~/project/none_example2.py
x = None
print(x)

Run the script:

python ~/project/none_example2.py

Output:

None

You can also use None as a default argument value in a function:

## ~/project/none_example3.py
def my_function(arg=None):
    if arg is None:
        print("No argument was passed.")
    else:
        print("Argument:", arg)

my_function()
my_function("Hello")

Run the script:

python ~/project/none_example3.py

Output:

No argument was passed.
Argument: Hello

In this example, if no argument is provided when calling my_function, arg will default to None.

Use the is Operator to Check for None

In this step, you will learn how to use the is operator to check if a variable is None. In Python, the is operator is used to test if two variables refer to the same object in memory. This is different from the == operator, which checks if two variables have the same value. When dealing with None, it's best practice to use the is operator because None is a singleton object, meaning there's only one instance of None in memory.

Let's create a Python file named is_none_example.py in your ~/project directory using the VS Code editor.

## ~/project/is_none_example.py
def check_none(value):
    if value is None:
        print("The value is None")
    else:
        print("The value is not None")

x = None
check_none(x)

y = "Hello"
check_none(y)

In this code:

• We define a function check_none that takes a value as input.
• We use the is operator to check if value is None.
• If value is None, we print "The value is None".
• Otherwise, we print "The value is not None".
• We call the check_none function with None and "Hello" to demonstrate the difference.

Now, run the Python script:

python ~/project/is_none_example.py

You should see the following output:

The value is None
The value is not None

As you can see, the is operator correctly identifies when a variable is None.

It's important to use is None instead of == None when checking for None. Although == None might work in some cases, it's not guaranteed to work in all situations, especially when dealing with custom objects that override the == operator. Using is None is the recommended and most reliable way to check for None.

Here's another example to illustrate this:

## ~/project/is_none_example2.py
class MyClass:
    def __eq__(self, other):
        return True  ## Always return True for equality

obj = MyClass()
print(obj == None)  ## Might return True due to the overridden __eq__ method
print(obj is None)  ## Always returns False because obj is not None

Run the script:

python ~/project/is_none_example2.py

Output:

True
False

This example shows that using == None can lead to unexpected results if the object's __eq__ method is overridden. The is None check, however, always correctly identifies whether the object is actually None.

Differentiate None from Other Falsy Values

In this step, you will learn to differentiate None from other falsy values in Python. In Python, several values are considered "falsy," meaning they evaluate to False in a boolean context. These include False, 0, 0.0, empty strings (""), empty lists ([]), empty tuples (()), empty dictionaries ({}), and None.

It's important to understand that while None is falsy, it is not the same as these other values. Using the is operator is crucial when specifically checking for None, as it distinguishes None from other falsy values.

Let's create a Python file named falsy_values.py in your ~/project directory using the VS Code editor.

## ~/project/falsy_values.py
def check_falsy(value):
    if value:
        print(f"{value!r} is considered True")
    else:
        print(f"{value!r} is considered False")

check_falsy(None)
check_falsy(False)
check_falsy(0)
check_falsy("")
check_falsy([])

print("\nChecking with 'is None':")

def check_none(value):
    if value is None:
        print(f"{value!r} is None")
    else:
        print(f"{value!r} is not None")

check_none(None)
check_none(False)
check_none(0)
check_none("")
check_none([])

In this code:

• We define a function check_falsy that takes a value as input and checks its truthiness in a boolean context.
• We then define a function check_none that uses the is None operator to specifically check if the value is None.
• We call both functions with None, False, 0, "", and [] to demonstrate the differences.

Now, run the Python script:

python ~/project/falsy_values.py

You should see the following output:

None is considered False
False is considered False
0 is considered False
'' is considered False
[] is considered False

Checking with 'is None':
None is None
False is not None
0 is not None
'' is not None
[] is not None

This output clearly shows that while all the values are considered falsy in a boolean context, only None is identified as None when using the is None operator.

Using is None ensures that you are specifically checking for the absence of a value, rather than just any value that evaluates to False. This distinction is important for writing robust and accurate Python code.

Summary

In this lab, you learned about the None value in Python, which represents the absence of a value. You saw how a function can return None when a value is not available, and how to assign None to a variable.

Furthermore, you explored using None as a default argument value in a function, allowing the function to behave differently when no argument is provided. The lab demonstrated these concepts through practical examples and code snippets.