TypeError: NoneType object does not support item assignment

TypeError: NoneType object does not support item assignment #

The Python "TypeError: NoneType object does not support item assignment" occurs when we try to perform an item assignment on 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.

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my_list = None

# ⛔️ TypeError: 'NoneType' object does not support item assignment
my_list[0] = 'a'

We tried to assign a value to a variable that stores None.

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.

Functions that don't explicitly return a value return None.

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# 👇️ this function returns None
def get_list():
    print(['a', 'b', 'c'])


# 👇️ None
my_list = get_list()

# ⛔️ TypeError: 'NoneType' object does not support item assignment
my_list[0] = 'z'

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

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def get_list():
    return ['a', 'b', 'c']


# 👇️ ['a', 'b', 'c']
my_list = get_list()

my_list[0] = 'z'

print(my_list)  # 👉️ ['z', 'b', 'c']

Use an if statement if you need to check whether a variable doesn't store a None value before the assignment.

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my_list = None

if my_list is not None:
    my_list[0] = 'z'
else:
    # 👇️ this runs
    print('variable stores a None value')

Alternatively, you can set a fallback value if the variable stores None.

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my_dict = None

if my_dict is None:
    my_dict = {}

my_dict['name'] = 'Alice'

print(my_dict)  # 👉️ {'name': 'Alice'}

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.

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


# 👇️ None
my_list = get_list(['a', 'b'])

The if statement in the get_list 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_list(a):
    if len(a) > 3:
        return a

    return [] # 👈️ return empty list if condition not met


# 👇️ []
my_list = get_list(['a', 'b'])

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