How to fix AttributeError: 'NoneType' object has no attribute 'append'

1. calling the append() method on a NoneType object

To show you how this error happens, suppose you try to call the append() method on a NoneType object as follows:

fruit_list = None
fruit_list.append("Apple")

In the above example, the fruit_list variable is assigned the None value, which is an instance of the NoneType object.

When you call the append() method on the object, the error gets raised:

Traceback (most recent call last):
  File "main.py", line 3, in <module>
    fruit_list.append("Apple")
AttributeError: 'NoneType' object has no attribute 'append'

To fix this error, you need to call the append() method from a list object.

Assign an empty list [] to the fruit_list variable as follows:

fruit_list = []
fruit_list.append("Apple")
print(fruit_list)  # ['Apple']

This time, the code is valid and the error has been fixed.

2. Assigning append() to the list variable

But suppose you’re not calling append() on a NoneType object. Let’s say you’re extracting a list of first names from a dictionary object as follows:

customers = [
    {"first_name": "Lisa", "last_name": "Smith"},
    {"first_name": "John", "last_name": "Doe"},
    {"first_name": "Andy", "last_name": "Rock"},
first_names = []
for item in customers:
    first_names = first_names.append(item['first_name'])

At first glance, this example looks valid because the append() method is called on a list.

But instead, an error happens as follows:

Traceback (most recent call last):
  File "main.py", line 11, in <module>
    fruit_list = fruit_list.append(item['first_name'])
AttributeError: 'NoneType' object has no attribute 'append'

This is because the append() method returns None , so when you do an assignment inside the for loop like this:

first_names = first_names.append(item['first_name'])

The first_names variable becomes a None object, causing the error on the next iteration of the for loop.

You can verify this by printing the first_names variable as follows:

for item in customers:
    first_names = first_names.append(item['first_name'])
    print(first_names)

The for loop will run once before raising the error as follows:

None
Traceback (most recent call last):

As you can see, first_names returns None and then raises the error on the second iteration.

To resolve this error, you need to remove the assignment line and just call the append() method:

customers = [
    {"first_name": "Lisa", "last_name": "Smith"},
    {"first_name": "John", "last_name": "Doe"},
    {"first_name": "Andy", "last_name": "Rock"},
first_names = []
for item in customers:
    first_names.append(item["first_name"])
print(first_names)

Output:

['Lisa', 'John', 'Andy']