Unsupported operand type(s) for +: 'NoneType' and 'str'

Unsupported operand type(s) for +: 'NoneType' and 'str' #

The Python "TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'" occurs when we try to use the addition (+) operator with a None value. To solve the error, figure out where the variable got assigned a None value and correct the assignment.

Here is an example of how the error occurs.

Copied!
my_str_1 = None
my_str_2 = 'world'

# ⛔️ TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'
result = my_str_1 + my_str_2

The variable on the left-hand side of the addition operator stores a None value which caused the error.

The most common sources of None values are:

1. Having a function that doesn't return anything (returns None implicitly).
2. Explicitly setting a variable to None.
3. Assigning a variable to the result of calling a built-in function that doesn't return anything.
4. Having a function that only returns a value if certain condition is met.

Make sure you aren't calling a function that doesn't return anything and expecting the return value to be a string.

Copied!
# 👇️ this function returns None
def get_str():
    print('hello ')


my_str_2 = 'world'

# ⛔️ TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'
result = get_str() + my_str_2

The get_str function doesn't return anything, therefore it implicitly returns None.

You can use a return statement to return a value from a function.

Copied!
def get_str():
    return 'hello '


my_str_2 = 'world'

result = get_str() + my_str_2

print(result)  # 👉️ "hello world"

Use an if statement if you need to check whether a variable doesn't store a None value before using the addition (+) operator.

Copied!
my_str_1 = None
my_str_2 = 'world'

if my_str_1 is not None:
    result = my_str_1 + my_str_2
    print(result)
else:
    # 👇️ this runs
    print('variable stores a None value')

Alternatively, you can provide a default value if the variable stores None.

Copied!
my_str_1 = None
my_str_2 = 'world'

if my_str_1 is None:
    my_str_1 = ''

result = my_str_1 + my_str_2
print(result)  # 👉️ 'world'

We check if the my_str_1 variable stores a None value and if it does, we set it to an empty string.

Make sure you aren't storing the result of calling one in a variable.

Another common cause of the error is having a function that returns a value only if a condition is met.

Copied!
def get_str(a):
    if len(a) > 5:
        return a


my_str_1 = get_str('hello')
print(my_str_1)  # 👉️ None

my_str_2 = 'world'

# ⛔️ TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'
result = my_str_1 + my_str_2

The if block in the get_str function is only ran if the passed in string has a length greater than 5.

To solve the error in this scenario, you either have to check if the function didn't return None or return a default value if the condition is not met.

Copied!
def get_str(a):
    if len(a) > 5:
        return a

    return '' # 👈️ return empty string if condition not met


my_str_1 = get_str('hello')
print(my_str_1)  # 👉️ ""

my_str_2 = 'world'

result = my_str_1 + my_str_2

print(result)  # 👉️ 'world'

Now the function is guaranteed to return a value regardless if the condition is met.