How to distinguish unit tests from integration tests and what are their main differences?

Unit tests check individual, isolated parts of code — usually functions or methods — without dependencies on external systems. They are fast, simple, and help ensure that a specific piece of logic works correctly.

Integration tests verify the interaction between multiple components or modules of a system, including working with databases, networks, files, and other external services. They are more complex and slower but are important for verifying the system's integrity.

Main differences:

• Scope of testing:

  • Unit — individual functions/classes.
  • Integration — interaction between modules.

• Dependencies:

  • Unit — minimal, often using mocks.
  • Integration — real dependencies and environment.

• Speed:

  • Unit — fast.
  • Integration — slower.

Example of a unit test in Python using unittest:

import unittest
from mymodule import add

class TestAdd(unittest.TestCase):
    def test_add_positive(self):
        self.assertEqual(add(2, 3), 5)

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

An integration test might, for example, verify that a function correctly writes data to a database and reads it back.