TypeError: 'NoneType' object is not subscriptable (tested)

TypeError: 'NoneType' object is not subscriptable (and its siblings — not iterable, not callable, no len()) all mean the same root problem as 'NoneType' object has no attribute: a variable you expected to hold a real value is None. The difference is which operation you tried on it — indexing with [], looping with for, or calling with () each produce their own distinct TypeError wording instead of AttributeError.

1. Eight operations, eight different messages

Python's error message depends on exactly which operation failed on None, not just that it failed:

All eight share one cause: something assigned None to a variable that the rest of the code assumes is a list, dict, string, number, or function.

2. "not subscriptable" means someone used [...] on a plain value

The most common single case: indexing the result of a function that forgot a return, or a dict lookup via .get() that didn't find a key:

>>> def no_return(): pass
>>> r = no_return()
>>> r[0]
TypeError: 'NoneType' object is not subscriptable

>>> x = None
>>> x[0]
TypeError: 'NoneType' object is not subscriptable

This is distinct from IndexError: list index out of range, which means you do have a real list, but asked for a position past its length. "not subscriptable" means you don't have a list (or dict, or anything supporting []) at all — you have None (or occasionally an int or other non-container type).

3. "not iterable" and "not callable" follow the same pattern

>>> x = None
>>> for i in x: pass
TypeError: 'NoneType' object is not iterable

>>> x()
TypeError: 'NoneType' object is not callable

"not callable" is a distinctive one worth knowing by sight: it almost always means a variable name shadowed a function. A classic example is accidentally reassigning a built-in or your own function name, like writing list = get_items() and later calling list() — except here list is now whatever get_items() returned, not the built-in.

4. Arithmetic and len() report the type pairing, not just "NoneType"

>>> 5 + None
TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'
>>> len(None)
TypeError: object of type 'NoneType' has no len()
>>> int(None)
TypeError: int() argument must be a string, a bytes-like object or a real number, not 'NoneType'

These three are more verbose because they come from different code paths internally (operator overloading, the len() protocol, and type constructors), but the diagnosis is identical: a None ended up where a number, sized container, or convertible value was expected.

5. Where the None almost always comes from

• A function with a missing or conditional return (see section 4 of the AttributeError guide linked above for the exact pattern).
• dict.get(key) on a missing key, then immediately indexing or iterating the result.
• A database query or API call that returns None for "not found" instead of raising, then used without a check.
• A default function argument of None (def f(items=None):) used directly instead of being replaced with a real empty container first.

6. Fix checklist

• Check if value is not None: before indexing, iterating, or calling it.
• For the common def f(items=None): pattern, replace the default early: if items is None: items = [].
• Read the traceback's line number, then check the line above for where the value was assigned — same debugging approach as any None-related error.
• If the message says "not callable", specifically check for a variable that shadows a function name of the same spelling earlier in the same scope.

For the attribute-access sibling of this error family (.append(), .strip(), etc. instead of [], for, or ()), see AttributeError: 'NoneType' object has no attribute.

Written with AI assistance. Measured on 2026-10-03 with Python 3.13.5 on Linux using plain python3 one-liners. Every error below is copy-pasted output, not paraphrased. PlainHeaders is our extension; Python is free.