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Accessing a missing dictionary key
🐛 Fix the bug Topic: dictionaries Difficulty: medium Free
What this problem practises
Dictionaries fail loudly on a missing key with KeyError, quietly through get(), and in genuinely surprising ways when you modify a dict while iterating it or rely on a duplicate key silently replacing the earlier entry.
This is a fix-the-bug problem: a short program is given in its broken state, and the task is to repair the code so it produces the required output. The interactive editor runs your fixed version in the browser and checks it for you — no setup, no local interpreter.
Difficulty: medium — expect to hold two interacting rules in your head at once, such as a boundary combined with a mutation.
The challenge
This should print a user's age, but it crashes because the key doesn't exist yet. Fix it.
The code runs in PyDebug’s in-browser compiler. Open the problem, run the snippet, watch the exact error or output, and keep iterating until it is right. Signed-in solves count toward XP and your daily streak.
The error it raises
Run the broken snippet and Python stops with this exact error: KeyError: 'age'. Match it to the Python errors guide, then repair the line that produced it — knowing the message is half the fix.
The snippet
The interactive editor pre-fills exactly this code, broken and ready to repair:
user = {"name": "Ravi"}
print(user["name"])
print(user["age"])Understand the rule
A KeyError means the dictionary lookup asked for a key that is not present. The important distinction is between data that must exist and data that is optional. Direct indexing with data['age'] is useful when a missing key indicates invalid input; get() is safer when a default value is meaningful.
How to trace it
- Inspect the keys that were actually inserted into the dictionary.
- Compare the spelling and type of the requested key with those stored keys.
- Decide whether missing data should be rejected, defaulted, or collected for validation.
- Use the lookup style that communicates that decision to the next reader.
Common wrong fixes
- Assuming a nearby key such as name automatically creates age.
- Using get() everywhere and silently turning a data-quality problem into None.
- Checking membership after performing the lookup, which is too late.
Rule to remember
Use direct indexing for required fields and get(key, default) for optional fields. The error is often a useful signal that the input contract and the code disagree.
📦 Learning path: Lists and Dictionaries
Problem 13 of 20 in this path. Work them in order — each one builds on the rule before it.
← Wrong function to find list lengthNext in path: Next problem →
More dictionaries problems
Keep going
- Python debugging guide — a systematic method for finding bugs like this one fast.
- Browse every dictionaries problem in this topic.
- Common Python errors and what they actually mean, plus the Python glossary for any term used on this page.