KeyError: 'key' in Python

KeyError: 'b' means the code read d["b"] and the dictionary d has no key 'b'. The text after KeyError: is the repr() of the missing key. If a missing key is a normal case, use d.get("b", default). If it is a bug, compare the key you look up with d.keys().

Reproduce it

cat > app.py <<'EOF'
d = {"a": 1}
print(d["b"])
EOF
python app.py

Traceback (most recent call last):
  File "/tmp/work/app.py", line 2, in <module>
    print(d["b"])
          ~^^^^^
KeyError: 'b'

Fixes

cat > fix.py <<'EOF'
d = {"a": 1, 1: "int key"}
print(d.get("b", 0))        # default instead of an error
print("b" in d)             # membership test
print(d["1"] if "1" in d else "'1' is not 1")   # str and int keys differ
try:
    d["b"]
except KeyError as e:
    print("KeyError for", repr(e.args[0]))
EOF
python fix.py

0
False
'1' is not 1
KeyError for 'b'

Count or group without KeyError

collections.defaultdict creates the missing entry on first access.

cat > count.py <<'EOF'
from collections import defaultdict
counts = defaultdict(int)
for word in "a b a".split():
    counts[word] += 1
print(dict(counts))
EOF
python count.py

{'a': 2, 'b': 1}

Keys from JSON and the environment

JSON object keys are always strings, so a dictionary loaded with json.loads has "1", not 1, and environment variables live in os.environ, which raises KeyError for an unset name:

cat > sources.py <<'EOF'
import json, os
d = json.loads('{"1": "one"}')
print(list(d))                      # keys are strings
print(d.get(1), d.get("1"))         # int lookup misses, str lookup hits
print(os.environ.get("APP_MODE_NOT_SET", "development"))
EOF
python sources.py

['1']
None one
development

How to recognise the cause

On Python 3.11 and later the carets under the failing line point at the exact subscript, which helps with nested lookups such as data["user"]["email"].

Prevent it

Validate the shape of external data once, where it enters the program, instead of indexing deep into it everywhere. Use d.get() only for keys that are truly optional, setdefault() or defaultdict when you build up a dictionary, and keep d[key] for keys that must exist so a missing one fails loudly at the right place.

FAQ