Time: 0:00
Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).
This bug belongs to the scope and closure bugs pattern
Every callback reports the last value, a counter never increments, UnboundLocalError on a variable you can see.
What went wrong: In Python, default arguments are evaluated once at function definition time. The dictionary context={} is shared across all function calls.
The fix: Use context=None as the default and initialize a fresh dictionary inside the function: if context is None: context = {}
Remember: Never use a mutable object (like a dict or list) as a default function argument in Python.
The pattern behind it: scope and closure bugs.
Paste this into any web page or blog post. Visitors fix the bug right there, in their browser.
<iframe src="https://trybughunt.com/embed/1b38ba90-88c0-4e26-bca5-187057f8cb79" title="Debugging challenge: Mutable default context dictionary accumulates keys across calls" width="100%" height="580" loading="lazy" style="border:1px solid #d4d4d8;border-radius:12px;max-width:760px"></iframe>
<p style="font-size:14px">Debugging challenge from <a href="https://trybughunt.com/challenges/1b38ba90-88c0-4e26-bca5-187057f8cb79">TryBugHunt</a></p>