AttributeError: 'NoneType' object has no attribute — decoded

What this message means

NoneType is the type of exactly one value: None — Python's word for "nothing here". This error says you called a method or read an attribute on a variable that holds None, almost always because a function or method returned None when you expected it to return a value.

The attribute name in the message is a clue, not the crime. 'NoneType' object has no attribute 'append' does not mean append is wrong — it means the thing you called .append on is not a list at all, it is None.

Traceback (most recent call last):
  File "main.py", line 3, in <module>
    result.append(4)
    ^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'append'

A real example

items = [3, 1, 2]

result = items.sort()    # sort() sorts IN PLACE and returns None
result.append(4)         # AttributeError

The bug is on the line *above* the traceback. list.sort() mutates the list and hands back None; storing its return value stores None. The fix is to use the list itself:

items = [3, 1, 2]

items.sort()             # the sorted list is items — always was
items.append(4)

result = sorted(items)   # and when you DO want a returned value: sorted(), not .sort()

The methods that return None (memorise this list)

• list.sort(), list.reverse() — in-place mutators. The *list* changes; the return is None. Their value-returning twins are sorted() and reversed().
• list.append(), extend(), insert(), remove(), clear(), update() on dicts, add() on sets — all mutate in place, all return None.
• print() — returns None. text = print("hi") stores None.
• A function with no return — falls off the end and returns None. So does a function whose return is bare: return.
• A function that only returns on some paths — the branch you took had no return at all.

The fix, step by step

1. Read the variable name before the dot in the traceback — that name currently holds None.
2. Find where it was last assigned. If the assignment is x = something.method(), check that method's return value in the docs — the in-place mutators above return None by design.
3. If it is x = my_function(), open my_function and walk every branch: does each one end in a return with a value?
4. Fix the producer (return something real, or drop the assignment) rather than papering over the consumer.
5. For values that are legitimately sometimes None (a search that found nothing), check before using: if result is not None:.

def find_user(users, name):
    for user in users:
        if user["name"] == name:
            return user        # found
    return None                # not found — say so EXPLICITLY

user = find_user(users, "Aarav")
if user is not None:
    print(user["email"])
else:
    print("No such user")

Why Python is strict about None

A language that silently ignored .append on None would hide the fact that your data pipeline broke three lines earlier. Python makes the nothing loud: None has no attributes at all, so the first use after the break is where you find out. The AttributeError is doing you a favour — the real bug is always at least one line *above* the traceback.

Related messages worth knowing

• AttributeError: the parent guide — reading the type in the message, and every famous wording decoded.
• TypeError: int is not subscriptable — the same producer-lied-to-consumer shape, with an int instead of None.
• AttributeError (Hindi) — the full explanation in Hindi, with browser practice.

Fix it interactively

These free problems practice exactly this — knowing what a function really returns:

• Sorting doesn't return a new list easy
• Function returning nothing easy
• List methods in sequence medium