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Testing
Lesson 67 of 77
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PYTHON EXPERTChapter 11 · Python Expert

Testing your code

Tests are code that checks code. The simplest test is an assert: assert add(2, 2) == 4 does nothing when true and raises AssertionError when false. The built-in unittest module groups tests into classes with helpful checks like assertEqual, and tools such as pytest run them for you. Writing a few tests for a tricky function catches a bug the moment you introduce it.

Worked example

How it reads

  • Each test_ method checks one behaviour
  • assertEqual(actual, expected) explains exactly what differed when it fails
  • Good tests cover the edges: empty input, spaces, zero, negatives
Cloud tip: Found a bug? Write a test that fails because of it first, then fix the code. The bug can never quietly come back.
main.py
PYTHON
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Console
Run your code to see its output here.
YOUR TURN

Add assert not is_leap(1900) and assert is_leap(2000), then fix is_leap so every assert passes (years divisible by 100 are not leap years unless they are also divisible by 400). The program should still print tests done.

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