Fixtures establish and release test resources. Isolation keeps one test’s changes from determining another test’s outcome.
Before you start
You should know Python functions, collections and exceptions. Use a small isolated script or interactive session to trace the example. Pay attention to when objects are created and when work executes; iteration, binding and mutation can happen at different points in a program.
The practical goal is to reason through this situation: Create a temporary directory per test and clean it up afterward. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Create isolated resources
Give each test its own temporary directory and controlled input.
Step 2: Inject the failure
Provide malformed data and assert the documented exception rather than waiting for accidental environmental failure.
Step 3: Verify cleanup
Release handles and inspect partial outputs after both success and failure.
Worked scenario
Create a temporary directory per test and clean it up afterward.
A file-processing test writes input into its temporary directory, invokes processing and checks the result there. A failure case also checks that incomplete output is absent or clearly marked under the contract. It never modifies a real user directory, so repeated and reordered execution remains meaningful.
Common mistake
A shared mutable fixture can make failures depend on execution order.
Verify the behavior
Run tests independently and in reversed order; confirm no files or shared state leak between cases.
Interview exercise
Test a file-processing failure.
Answer and reasoning
Inject malformed input, assert the expected exception and verify output and resource cleanup without touching real user files.
Continue learning
Compare the scenario with the Python interview questions and test your understanding with the Python MCQs. For terminology and implementation details, consult the reference material.