KeyError in Python

What you see

config = {"host": "localhost", "port": 8080}
print(config["user"])

Traceback (most recent call last):
  File "/home/me/project/main.py", line 2, in <module>
    print(config["user"])
          ~~~~~~^^^^^^^^
KeyError: 'user'

What it means

You looked up a key the dict does not have. A KeyError message is nothing but that key, 'user' here, so the quoted word on the last line is the answer to "what was missing".

Why it happens

• A spelling or case difference. "User" and "user" are different keys; so are "user" and "user " with a trailing space.
• Data that lacks a field, such as JSON from an API that leaves out empty fields.
• An environment variable that is not set. os.environ["API_KEY"] raises KeyError: 'API_KEY'.
• A pandas column name that does not match, often because of spaces in a CSV header.

How to fix it

Use get() when a missing key is normal, with a default if you want one:

user = config.get("user")            # None when missing
user = config.get("user", "admin")   # "admin" when missing

Check first when the missing case needs its own handling:

if "user" in config:
    print(config["user"])

For counting or grouping, collections.defaultdict creates a key the first time you use it:

from collections import defaultdict
counts = defaultdict(int)
counts["ada"] += 1

For environment variables, os.environ.get("API_KEY") returns None instead of raising.

How each Python version words it

Run on CPython 3.9.6, 3.10.22, 3.11.17, 3.12.15, 3.13.17, 3.14.9 and 3.15.1, on October 25, 2026.

The message is the same in every version here: the key, as repr() shows it.