Updated function testing

docs/src/development/testing/functionaltesting.md

@@ -1,3 +1,140 @@
-## Functional testing
+# Functional testing
 
-- Each function is compared to specifications to returnthe required outputs
+Each function is compared to specifications to ascertain that it returns the required outputs.
+
+## Framework
+
+For Functional testing, [Pytest](https://docs.pytest.org/en/latest/index.html) is preferred.
+
+## Setting up
+
+Before installing `pytest`, ensure you have python installed.
+Manage your environment using `conda`, `venv`, `pyenv`, `poetry` or `shell.nix`(NixOS).
+
+### Installation
+
+First, we start by creating an environment(`conda` specific.)
+```bash
+# New environment
+conda create --name <env_name>
+```
+
+To install `pytest`, open your terminal:
+```bash
+pip install pytest 
+```
+
+For an existing project with a `requirements.txt` or `environment.yml` file
+```bash
+# requirements.txt
+conda create --name <env_name> --file requirements.txt
+
+# or
+
+# environment.yml
+conda env create -f environment.yml
+```
+
+### Usage
+
+To use `pytest` you need to create a `tests` directory in your project.
+Inside the test directory, you can create tests.
+
+In the test file, import the pytest module.
+```bash
+import pytest
+```
+
+## Creating tests
+
+The test files have a naming convention.
+The file name has to have prefix `test_` or suffix `_test.py`.
+For example: `test_*.py` and `*_test.py` respectively.
+
+**NOTE:** Using the prefix `test_` is preferred.
+
+The test functions also do have a naming convention.
+The function has to have a prefix `test_.`
+For example:
+```python
+...
+
+# test function
+def test_addition():
+    ...
+```
+
+To create new functional tests using pytest:
+
+Function in `Project/example.py` module.
+```python
+...
+
+# function module
+def addition(a, b):
+    return a + b 
+```
+
+Test function in `tests/` directory.
+```python
+import pytest
+import Project.example
+
+# test function
+def test_addition():
+    c = addition(3, 2)
+    assert c == 5
+
+...
+```
+
+## Running tests
+
+To run the tests, open your terminal and navigate to your test directory:
+```bash
+pytest
+```
+
+The above command will run all tests present in the `tests/` directory.
+
+To run a single test use:
+```bash
+pytest tests/test_example.py
+```
+
+For more, refer to [invoking pytest](https://docs.pytest.org/en/latest/how-to/usage.html#how-to-invoke-pytest).
+
+## Pytest fixtures
+
+Fixtures are functions that provide resources or perform actions required for test execution.
+By encapsulating these setup and teardown steps into fixtures:
+- It streamlines the test code and promotes code re-usability
+- It improves the overall readability and maintainability of the test suite.
+
+Fixture example that contains raster path that can be used by multiple tests: 
+```python
+import pytest
+
+# fixtures
+@pytest.fixture()
+def vector_data():
+    path = 'vector.geojson'
+    return path
+
+@pytest.fixture()
+def raster_data():
+    path = 'raster.tif'
+    return path
+
+def test_mask_raster(raster_data, vector_path):
+    raster = mask_raster(raster_data, vector_path)
+    ...
+```
+
+## Pytest in Django projects
+
+`pytest-django` is a pytest plugin used to test django applications.
+
+## Pytest with BDD
+
+BDD library for pytest.