AttributeError: 'list' object has no attribute 'X'

What this error means

You called a method that lists don't have. There are three common reasons. You remembered the method name wrongly. Or you used a name from another language (JavaScript's .push, Java's .add, C#'s .Length). Or the variable is a list, but you wanted to call the method on one of its elements.

When you see it

Traceback (most recent call last):
  File "main.py", line 3, in <module>
    items.add(42)
AttributeError: 'list' object has no attribute 'add'

Why it happens

There are two kinds of mistake:

1. Wrong method name. Python lists use .append, .extend, .insert, .remove, .pop, .sort, .reverse. They don't have .add, .push, .length, .size, or .contains.
2. Wrong level. You have a list of objects but you're calling the object's method on the list. users.upper() doesn't work, but users[0].upper() does.

How to fix it

Quick translation table for newcomers:

Loop or comprehension when the method belongs to the elements:

names = ["alice", "bob"]
# names.upper()                  # AttributeError
shouty = [n.upper() for n in names]

Use len() not .length:

items = [1, 2, 3]
print(len(items))                # not items.length

When you'd see this in real code

• A JavaScript developer's first day with Python: items.push(x).
• A Java developer: items.size().
• A pandas DataFrame method called on a list: df_list.head() doesn't work if df_list is a Python list, not a DataFrame.
• Calling a string method on a list: result.split(",") when result is already a list of strings.

Related errors

• AttributeError: 'NoneType' object has no attribute 'X': the same kind of error, with a different cause.
• TypeError: list indices must be integers or slices, not str: another error caused by mixing up lists and dicts.
• AttributeError: module 'X' has no attribute 'Y'

See Also

• All Python errors: the full index, by type and by when it happens.
• Lists (tutorial)
• List methods cheat sheet
• List comprehensions