How to Return a default value if None in Python

# Table of Contents

1. Return a default value if None in Python
2. Using None as a default argument in Python

# Return a default value if None in Python

Use a conditional expression to return a default value if None in Python.

The conditional expression will return the default value if the variable stores None, otherwise the variable is returned.

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# 👇️ Return a default value if the variable is None
def example():
    my_var = None

    return "default value" if my_var is None else my_var


print(example())  # 👉️ "default value"

# ---------------------------------------------

# 👇️ Return a default value if the variable is None
my_var = None

my_var = "default value" if my_var is None else my_var

print(my_var) # 👉️ default value

Conditional expressions are very similar to an if/else statement.

In the examples, we check if the variable stores None, and if it does, we return the string default value, otherwise, the variable is returned.

Alternatively, you can switch the order of conditions.

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# 👇️ Return a default value if the variable is None
def example():
    my_var = None

    return my_var if my_var is not None else 'default value'


print(example())  # 👉️ "default value"

# ---------------------------------------------

# 👇️ Return a default value if the variable is None
my_var = None

my_var = my_var if my_var is not None else 'default value'

print(my_var)  # 👉️ default value

The code sample first checks if the variable is not None, in which case it returns it.

Otherwise, a default value is returned.

# Return a default value if None using the boolean OR operator

An alternative approach is to use the boolean OR operator.

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# 👇️ Return a default value if the variable is None
def example():
    my_var = None

    return my_var or "default value"


print(example())  # 👉️ "default value"

# ---------------------------------------------

# 👇️ Return a default value if the variable is None
my_var = None

my_var = my_var or "default value"

print(my_var) # 👉️ "default value"

The expression x or y returns the value to the left if it's truthy, otherwise, the value to the right is returned.

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# 👇️ default value
print(None or 'default value')

# 👇️ hello
print('hello' or 'default value')

However, this approach does not explicitly check for None.

All values that are not truthy are considered falsy. The falsy values in Python are:

• constants defined to be falsy: None and False.
• 0 (zero) of any numeric type
• empty sequences and collections: "" (empty string), () (empty tuple), [] (empty list), {} (empty dictionary), set() (empty set), range(0) (empty range).

So if the value to the left is any of the aforementioned falsy values, the value to the right is returned.

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print(0 or 'default value')  # 👉️ default value
print("" or 'default value')  # 👉️ default value

This might or might not suit your use case.

If you only want to check for None, use the conditional expression from the first code snippet.

# Using None as a default argument in Python

None is often used as a default argument value in Python because it allows us to call the function without providing a value for an argument that isn't required on each function invocation.

None is especially useful for list and dict arguments.

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def get_employee(name=None, age=None):
    return {'name': name, 'age': age}


# 👇 {'name': None, 'age': None}
print(get_employee())

# 👇️ {'name': None, 'age': 30}
print(get_employee(age=30))

# 👇️ {'name': 'Bobby Hadz', 'age': 30}
print(get_employee('Bobby Hadz', 30))

We declared a function with 2 default parameters set to None.

Default parameter values have the form parameter = expression.

When we declare a function with one or more default parameter values, the corresponding arguments can be omitted when the function is invoked.

A None value is often used for default parameters that are not essential to the function.

None is used when the function can still run even if a value for the parameter wasn't provided.

# Use None as default arguments, not Objects

Here is an example of how using an empty dictionary as a default value for a parameter can cause issues and how we can fix it with a None value.

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def get_address(address={}):
    return address


addr1 = get_address()
addr2 = get_address()

addr1['country'] = 'Chile'
print(addr1)  # 👉️ {'country': 'Chile'}

print(addr2)  # 👉️ {'country': 'Chile'}

The get_address function has a parameter with a default value of an empty dictionary.

We called the function 2 times and stored the results in variables.

Notice that we only set the country key on one of the dictionaries but both of them got updated.

They are not evaluated each time the function is called.

When a non-primitive default parameter value, such as a dictionary or a list is mutated, it is mutated for all function calls.

To resolve the issue, set the default parameter value to None and conditionally update it in the body of the function.

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def get_address(address=None):
    if address is None:
        address = {}
    return address


addr1 = get_address()
addr2 = get_address()

addr1['country'] = 'Chile'
print(addr1)  # 👉️ {'country': 'Chile'}

print(addr2)  # 👉️ {}

The body of the function is run every time it is invoked, so the issue is resolved.

I've also written an article on how to return multiple values and only use one.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• Why does list.reverse() return None in Python
• Purpose of 'return self' from a class method in Python
• Why does my function print None in Python [Solved]
• How to check if a variable is a Function in Python