# Unit Testing

A unit test tests a small "unit" of code - usually a function or method - independently from the rest of the program.

Some key advantages of unit testing include:

* **Isolates code** - This allows testing individual units in isolation from other parts of the codebase, making bugs easier to identify.
* **Early detection** - Tests can catch issues early in development before code is deployed, saving time and money.
* **Regression prevention** - Existing unit tests can be run whenever code is changed to prevent new bugs or regressions.
* **Facilitates changes** - Unit tests give developers the confidence to refactor or update code without breaking functionality.
* **Quality assurance** - High unit test coverage helps enforce quality standards and identify edge cases.

Every language has its unit testing framework. In Python, some popular ones are

* unittest
* pytest
* doctest
* testify

**Example:**

**calc.py**

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

def subtract(a,b):
    return a-b

def multiply(a,b):
    return a * b

def divide(a, b):
    return a / b
```

**main.py**

```python
from calc import add, subtract, multiply, divide

print(add(1,2))
print(subtract(1,2))
print(multiply(1,2))
print(divide(1,2))
```

**test\_calc.py**

```python
from calc import *
import unittest 

class TestMathOperations(unittest.TestCase):
    def test_add(self):
        self.assertEqual(add(10, 5), 15)
        self.assertEqual(add(-1, 1), 0)
        self.assertEqual(add(-1, -1), -2)

    def test_subtract(self):
        self.assertEqual(subtract(10, 5), 5)
        self.assertEqual(subtract(-1, 1), -2)
        self.assertEqual(subtract(-1, -1), 0)

    def test_multiply(self):
        self.assertEqual(multiply(10, 5), 50)
        self.assertEqual(multiply(-1, 1), -1)
        self.assertEqual(multiply(-1, -1), 1)

    def test_divide(self):
        self.assertEqual(divide(10, 5), 2)
        self.assertEqual(divide(-1, 1), -1)
        self.assertEqual(divide(-1, -1), 1)
        self.assertRaises(ZeroDivisionError, divide, 10, 0)  # Test for division by zero

if __name__ == '__main__':
    unittest.main()
```


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