AttributeError: 'NoneType' object has no attribute 'append'

AttributeError: 'NoneType' object has no attribute 'append' #

The Python "AttributeError: 'NoneType' object has no attribute 'append'" occurs when we try to call the append() method on a None value, e.g. assignment from function that doesn't return anything. To solve the error, make sure to only call append() on list objects.

Here is a very simple example of how the error occurs.

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my_list = None

# ⛔️ AttributeError: 'NoneType' object has no attribute 'append'
my_list.append('hello')

# ✅ if you need to check if not None before calling append()
if my_list is not None:
    print('variable is NOT None')
    my_list.append('hello')
else:
    print('variable is None')

Trying to call the append() method on a None value is what causes the error.

The most common source of a None value (other than an explicit assignment) is a function that doesn't return anything.

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


# 👇️ None
my_list = get_list()

# ⛔️ AttributeError: 'NoneType' object has no attribute 'append'
my_list.append('d')

Notice that our get_list function doesn't explicitly return a value, so it implicitly returns None.

The "AttributeError: 'NoneType' object has no attribute 'append'" occurs for multiple reasons:

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.

Some built-in methods (e.g. sort) mutate a data structure in place and don't return a value. In other words, they implicitly return None.

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my_list = ['c', 'b', 'a']

my_sorted_list = my_list.sort()

print(my_sorted_list)  # 👉️ None

# ⛔️ AttributeError: 'NoneType' object has no attribute 'append'
my_sorted_list.append('d')

The sort() method sorts a list in place and doesn't return anything, so when we assign the result of calling the method to a variable, we assign a None value to the variable.

If a variable might sometimes store a list and sometimes store None, you can explicitly check if the variable is not None before you call append().

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my_list = ['a', 'b', 'c']

if my_list is not None:
    print('variable is not None')
    my_list.append('d')
else:
    print('variable is None')


print(my_list)  # 👉️ ['a', 'b', 'c', 'd']

The if block will run only if the my_list variable does not store a None value, otherwise the else block runs.

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


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

print(my_list)  # 👉️ None

The if statement in the get_list function is only ran if the passed in list 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 []


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

print(my_list)  # 👉️ []

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