6.15 Hands-on Exercises

Practice Programs

• Write a function demonstrating UnboundLocalError, then fix it using the global keyword.
• Given a nested list, show the difference between copy.copy() and copy.deepcopy() by mutating a nested element.
• Write a closure using nonlocal to implement a simple counter.

Mini Projects

• Configuration loader — build a small config loader that reads settings from environment variables with sensible defaults, using 6.11 Variables in DevOps as a starting point.
• Reference tracer — build a tool that takes any object and prints its id(), type(), and sys.getrefcount() before and after a mutation, to make the concepts from 6.2 Objects and Variable References and 6.6 Garbage Collection concrete.

Quick Interview Answer

“These exercises reinforce the chapter’s core ideas hands-on: the UnboundLocalError-then-fix exercise exercises scope and the global keyword; the shallow-vs-deep-copy exercise exercises reference sharing on nested mutable objects; the nonlocal counter exercises closures and enclosing scope; and the reference tracer ties identity, type, and reference counting together into one small diagnostic tool.”

Common Mistakes

• Skipping the shallow-vs-deep-copy exercise as “just theory” — it’s one of the most common real bugs when working with nested config dicts or lists of records.
• Building the reference tracer without accounting for the temporary reference sys.getrefcount()’s own call creates, and misreading the count as a result.