Fix: TypeError: ‘NoneType’ object is not subscriptable

The error looks like this:

Traceback (most recent call last):
  File "app.py", line 14, in <module>
    print(user["email"])
TypeError: 'NoneType' object is not subscriptable

“Subscriptable” means “supports square brackets”, like items[0] or data["key"]. None does not. So the variable on that line holds None instead of the list, dict or string you expected. The line in the traceback shows where it crashed; the real bug is wherever that None was produced.

Cause 1: a method that changes things in place

List methods such as sort(), append(), reverse() and dict update() modify the object and return None.

names = ["cy", "ana", "ben"]
sorted_names = names.sort()   # sorted_names is None
print(sorted_names[0])        # TypeError

# fix: use the function that returns a new list
sorted_names = sorted(names)
print(sorted_names[0])        # 'ana'

Cause 2: a function that sometimes returns nothing

A Python function without a return on some path returns None on that path.

def find_user(users, name):
    for user in users:
        if user["name"] == name:
            return user
    # falls through: returns None when nobody matches


user = find_user(users, "bob")
if user is None:
    print("No user named bob")
else:
    print(user["email"])

Decide what “not found” should mean. Either check for None at the call site as above, or make the function raise a clear exception (raise LookupError(f"no user {name!r}")) so the failure happens where the cause is.

Cause 3: dict.get() on a missing key

port = config.get("server")["port"]          # TypeError if "server" is missing

port = config.get("server", {}).get("port", 5432)  # default at each level

Cause 4: a regex that did not match

re.match and re.search return None when nothing matches.

import re

if (m := re.match(r"(\d+)-(\d+)", text)) is not None:
    start, end = m[1], m[2]
else:
    print(f"unexpected format: {text!r}")

Cause 5: data that contains null

JSON null becomes Python None. An API response like {"address": null} makes data["address"]["city"] fail. Check the field before going deeper.

How to find the source quickly

1. Look at the failing line and list every [ ] on it; one of those values is None.
2. Print or log them just before that line: print(f"{user=}").
3. Follow the variable back to where it was assigned. Was it the result of an in-place method, a lookup, or a function with a missing return?
4. Fix the source, or handle the None case explicitly with if x is None:.

Type hints help prevent this class of bug. Annotating def find_user(...) -> dict | None lets a type checker such as mypy or pyright warn you wherever the result is used without a None check. The same reasoning explains the sibling errors 'NoneType' object is not iterable and 'NoneType' object has no attribute ....