TypeError: can only concatenate str (not "NoneType") to str

TypeError: can only concatenate str (not "NoneType") to str #

The Python "TypeError: can only concatenate str (not "NoneType") to str" occurs when we try to concatenate a string and a None value. To solve the error, correct the assignment or check if the variable doesn't store a None value before concatenating.

Here is an example of how the error occurs.

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example = None

# ⛔️ TypeError: can only concatenate str (not "NoneType") to str
result = 'hello ' + example

We tried to use the addition (+) operator to concatenate a string and a None value which caused the error.

The most common sources of a None value 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.

Here is an example of getting a None value from a function that doesn't return anything (implicitly returns None).

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def get_name():
    print('James Doe')


# ⛔️ TypeError: can only concatenate str (not "NoneType") to str
result = 'hello ' + get_name()

The get_name function doesn't return anything, so it implicitly returns None.

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

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def get_name():
    return 'James Doe'

result = 'hello ' + get_name()

print(result)  # 👉️ "hello James Doe"

You can use an if statement if you only want to concatenate the values if the variable doesn't store None.

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name = None

if name is not None:
    result = 'hello ' + name
    print(result)
else:
    # 👉️ this runs
    print('variable stores a None value')

The if block is only ran if the name variable doesn't store a None value, otherwise the else block is ran.

Alternatively, you can assign a fallback value to the variable if it is None.

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name = None

if name is None:
    name = ''

result = 'hello ' + name

print(result)  # 👉️ 'hello'

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

Note that if you use a formatted string literal with an expression that returns None, the None value will be included in the string.

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name = None

result = f'hello {name}'

print(result)  # 👉️ 'hello None'

Make sure to wrap expressions in curly braces - {expression}.

Note that there are many built-in methods that mutate the original object (e.g. sort()) and therefore don't return anything (implicitly return None), so make sure you aren't storing the result of calling one in a variable.

Another common source of None values is having a function that only returns a value if a certain condition is met.

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def get_name(a):
    if len(a) > 3:
        return a


result = get_name('Bob')

print(result)  # 👉️ None

The if statement in the get_name function is only ran if the passed in argument has a length greater than 3.

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.

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def get_name(a):
    if len(a) > 3:
        return a
    return '' # 👈️ return empty string if condition not met


result = get_name('Bob')

print(result)  # 👉️ ""

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