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

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Borislav Hadzhiev

Last updated: Apr 20, 2022

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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.

attributeerror nonetype object has no attribute append

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

main.py
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.

If you print the variable you are calling append() on, it will be None, so you have to track down where the variable gets assigned a None value and correct or remove the assignment.

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

main.py
# 👇️ 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.

We assigned the result of calling the function to a variable and tried to call append which caused the error.

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 a 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.

main.py
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.

Avoid assignment with methods that mutate the original list in place, as most of them don't return a value and implicitly return None.

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().

main.py
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.

main.py
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.

In all other cases, the function doesn't return anything and ends up implicitly returning None.

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.

main.py
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.

Conclusion #

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.

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