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Tutorials›Python›Testing Python Code
Testing & Debugging · Chapter 26

Testing Python Code

Write small tests that protect behavior and make refactoring safer.

Why this matters

Tests help you know whether code still behaves correctly after changes.

Start with the idea

A useful test arranges input, performs an action, and checks an expected result. Start with pure functions because they are easy to test deterministically.

IdeaUnderstand the purpose
→
CodeRun the smallest example
→
PracticeChange it yourself
SubjectVision learning pattern: understand the idea before memorizing syntax.

Small working example

Run this example first. Do not change several things at once; confirm the basic behavior, then experiment.

Python 
def add(a, b):
    return a + b

def test_add():
    assert add(2, 3) == 5
What to notice

The example is intentionally small. Focus on the chapter’s main idea before adding extra syntax or framework code.

Common beginner mistake

Mistake

Writing tests that only exercise implementation details instead of observable behavior.

Better approach

Assert the contract the caller depends on, including important edge cases.

Quick recap

  • Tests protect behavior.
  • Assertions express expected results.
  • Small deterministic units are easiest to test.

Try it yourself

These are deliberately small. If you can complete them without copying the example, you are ready to continue.

  1. Test a function with normal input.
  2. Add one edge-case test.