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.

Copied!
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.

# Checking if the variable doesn't store None

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

Copied!
my_list = None

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

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

# Setting a fallback value if the variable stores None

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

Copied!
my_dict = None

if my_dict is None:
    my_dict = {}

my_dict['name'] = 'Bobby Hadz'

print(my_dict)  # 👉️ {'name': 'Bobby Hadz'}

If the variable stores a None value, we set it to an empty dictionary.

# Track down where the variable got assigned a None value

You have to figure out where the variable got assigned a None value in your code and correct the assignment to a list or a dictionary.

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 a certain condition is met.

# Functions that don't return a value return None

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

Copied!
# 👇️ 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.

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


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

my_list[0] = 'z'

print(my_list)  # 👉️ ['z', 'b', 'c']

The function now returns a list, so we can safely change the value of a list element using square brackets.

# Many built-in functions return None

Note that there are many built-in functions (e.g. sort()) that mutate the original object in place and return None.

Copied!
a_list = ['c', 'b', 'a']

result = a_list.sort()
print(result)  # 👉️ None

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

The sort() method mutates the list in place and returns None, so we shouldn't store the result of calling it into a variable.

To solve the error, remove the assignment.

Copied!
a_list = ['c', 'b', 'a']

a_list.sort()

a_list[0] = 'Z'

print(a_list) # 👉️ ['Z', 'b', 'c']

# A function that returns a value only if a condition is met

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

Copied!
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 run if the passed-in argument has a length greater than 3.

To solve the error, 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_list(a):
    if len(a) > 3:
        return a

    return [] # 👈️ return an 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.

# Additional Resources

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

• How to Return a default value if None in Python
• Why does my function print None in Python [Solved]
• Check if a Variable is or is not None in Python
• Convert None to Empty string or an Integer in Python
• How to Convert JSON NULL values to None using Python
• Join multiple Strings with possibly None values in Python
• Why does list.reverse() return None in Python