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 a 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('bobbyhadz.com')

# ✅ 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')

If you need to check if the variable is not None before calling append(), use an if statement.

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my_list = None

if my_list is not None:
    print('variable is NOT None')
    my_list.append('hello')
else:
    # 👇️ This runs
    print('variable is None')

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

# Functions that don't return anything return None

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(['bobby', 'hadz', '.'])


# 👇️ None
my_list = get_list()

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

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

The error 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 a certain condition is met.

# Many built-in methods return None

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.

# Checking if a variable is not None before calling append()

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 = ['bobby', 'hadz', '.']

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


print(my_list)  # 👉️ ['bobby', 'hadz', '.', 'com']

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

Alternatively, you can check if the variable stores a list before calling list.append().

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my_list = ['bobby', 'hadz', '.']

if isinstance(my_list, list):
    print('variable is not None')
    my_list.append('com')
else:
    print('variable is None')


print(my_list)  # 👉️ ['bobby', 'hadz', '.', 'com']

The isinstance function returns True if the passed-in object is an instance or a subclass of the passed-in class.

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my_list = ['bobby', 'hadz', '.']

print(isinstance(my_list, list)) # 👉️ True
print(isinstance(my_list, str)) # 👉️ False

# 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.

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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 run 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 of whether the condition is met.

# The list.append() method should only be called on a list

The list.append() method adds an item to the end of the list.

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my_list = ['bobby', 'hadz']

my_list.append('com')

print(my_list)  # 👉️ ['bobby', 'hadz', 'com']

The method returns None as it mutates the original list.

If the error persists, follow the instructions in my AttributeError: 'NoneType' object has no attribute 'X' article.

# Additional Resources

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

• AttributeError: 'NoneType' object has no attribute 'group'
• AttributeError: 'NoneType' object has no attribute 'shape'