# import unittest

**[import unittest](https://docs.python.org/3/library/unittest.html)** is a Python import that provides a framework for writing and running unit tests. Use it to test code systematically and catch regressions.

## Example

```python
# Python
# description: write and run unit tests

import unittest

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

class TestMath(unittest.TestCase):
    def setUp(self):
        """Called before each test."""
        self.result = 0
    
    def test_add_positive(self):
        self.assertEqual(add(2, 3), 5)
    
    def test_add_negative(self):
        self.assertEqual(add(-1, -2), -3)
    
    def test_add_zero(self):
        self.assertEqual(add(0, 5), 5)

if __name__ == "__main__":
    unittest.main()

# Run: Python test.py
# Output: ... (3 tests passed)
```

## Common assertions

- `self.assertEqual(a, b)`: a == b
- `self.assertNotEqual(a, b)`: a != b
- `self.assertTrue(x)`: x is true
- `self.assertFalse(x)`: x is false
- `self.assertIn(a, b)`: a in b
- `self.assertRaises(Exception)`: code raises exception
- `setUp()`: run before each test
- `tearDown()`: run after each test