This AttributeError NoneType append is a classic combination of two common problems: a NoneType error and a List method error.
It means: You are trying to use the .append() method, but the variable you’re using it on is None, not a list.
The Cause
This almost always happens when you call a function that was supposed to return a list, but it failed or had a code path that returned None instead.
Problem Code:
def get_user_scores(user_id):
if user_id > 10:
return [90, 85, 100]
else:
# This user has no scores, so we return nothing
return None
# 1. This works fine
high_scorer_list = get_user_scores(20)
high_scorer_list.append(99) # Appends to [90, 85, 100]
# 2. This crashes
low_scorer_list = get_user_scores(5) # This variable is now None
low_scorer_list.append(40)
# CRASH! AttributeError: 'NoneType' object has no attribute 'append'The Fix: Always Return an Iterable (e.g., [])
The best fix is to make your function “safer.” A function that “gets a list” should always return a list, even if it’s an empty one.
Corrected Function:
def get_user_scores(user_id):
if user_id > 10:
return [90, 85, 100]
else:
# THE FIX: Return an empty list
return []Now, the code will run without crashing:
low_scorer_list = get_user_scores(5) # This variable is now [] low_scorer_list.append(40) print(low_scorer_list) # Output: [40]
