TypeError: 'NoneType' object is not subscriptable

What this error means

When you see it

Traceback (most recent call last):
  File "report.py", line 4, in <module>
    top = scores[0]
          ~~~~~~^^^
TypeError: 'NoneType' object is not subscriptable

Why it happens

1. A method that changes things in place. list.sort(), list.append(), list.reverse(), dict.update() and random.shuffle() all modify the object and return None. Assigning the result throws your data away.
2. Your own function has a path with no return. A function that reaches its end without a return returns None. Often this happens only for the one input you didn't test.
3. A lookup found nothing. dict.get(key) returns None for a missing key. re.match() and re.search() return None when the pattern doesn't match.
4. The data said null. A JSON field that's null arrives in Python as None.

scores = [72, 95, 88]
scores = scores.sort()     # sorts in place, returns None
print(scores[0])           # TypeError: 'NoneType' object is not subscriptable

How to fix it

Don't assign the result of an in-place method. Either call it on its own line, or use the version that returns a new object:

scores = [72, 95, 88]
scores.sort()              # modifies scores; nothing to assign
ranked = sorted(scores)    # or: make a new sorted list and leave the original as it is
print(ranked[0])           # 72

Check the result of the lookup before you index it. A regex that doesn't match is a normal case, so handle it:

import re

line = "ERROR disk full"
m = re.match(r"(\w+) (.*)", line)
if m is None:
    raise ValueError(f"unrecognised log line: {line!r}")
level, message = m[1], m[2]

Give .get() a default of the right type. If you're going to index the result, the fallback should be indexable too:

user = {"name": "Grace"}
first_tag = user.get("tags", [""])[0]    # "" rather than a crash

Make every path in your function return something. Read it from top to bottom and ask "what does this return when the if is false?" That branch is where the None comes from.

When you'd see this in real code

• rows = rows.sort(key=...) in a report script, and every later rows[i] fails.
• A scraper where soup.find("h1") found nothing, and the next line indexes the result.
• response.json()["data"]["items"] where the API returned "data": null for an empty result.
• os.environ.get("CONFIG")[0:5] on a machine where the variable was never set.

Related errors

• TypeError: 'NoneType' object is not iterable: the same None, used in a for loop instead of with [].
• AttributeError: 'NoneType' object has no attribute 'X': the same None, with a .method() instead of [].
• TypeError: 'int' object is not subscriptable: indexing the wrong type, but this time a number.

See Also

• All Python errors: the full index, by type and by when it happens.
• Functions and scoping: returns, and what a function gives back when it has none.
• Lists and sequences: which list methods return a value and which change the list in place.