Test-Driven Development in Python: Best Practices, and Detailed Explanation
Tags: tdd, python, programming, tutorial
Published: 2023-06-29 15:47:37


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Hey there, Python lovers! Are you looking for a way to improve your software development process and build more maintainable code? Look no further than test-driven development (TDD) in Python! In this article, we’ll explore the benefits of TDD, best practices for implementing it, and some fun example to get you started.

## Why You Should Give TDD a Try:

TDD can provide a variety of benefits when developing Python applications. By writing tests before writing code, you can catch errors and bugs early, reducing the time and cost required for testing and debugging. This leads to better code quality, faster development time, better collaboration among team members, and improved confidence in your code. Who wouldn’t want those benefits?

## Best Practices for TDD in Python:


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/m4vyzw0pei8rhx6072k6.png)



The TDD process is often described as “_Red-Green-Refactor_”. The “_Red_” phase involves writing a test that fails, which means that the code being tested hasn’t been written yet. The “_Green_” phase involves writing the minimum amount of code required to pass the test. Once the test passes, the “_Refactor_” phase involves improving the code’s design and efficiency without changing its behavior.

1. Write tests first: This ensures that your code meets the requirements of the project and works as expected.

2. Write small and simple tests: Testing one aspect of the code at a time makes it easier to identify and fix errors and bugs.

3. Test edge cases: Testing unexpected inputs and outputs ensures that your code can handle anything that comes its way.

4. Use [unittest](https://docs.python.org/3/library/unittest.html): Python’s built-in testing framework is easy to use and helps you write and run tests quickly.

5. Run tests frequently: Catch errors and bugs as soon as they are introduced by running tests frequently during development.

## Tools that maybe handy:

Test Coverage: Test coverage is a metric that measures the percentage of the code that is covered by tests. Achieving high test coverage is important, as it ensures that all parts of the code are thoroughly tested. Python provides several tools, such as [coverage.py](https://coverage.readthedocs.io/en/7.2.7/), that can be used to measure test coverage.

Test Doubles: Test doubles are objects that are used in place of other objects during testing. For example, a mock object can be used to simulate the behavior of a real object. Python provides several libraries, such as [unittest.mock](https://docs.python.org/3/library/unittest.mock.html), that can be used to create test doubles.

Test-Driven Data Analysis (TDDA): Test-driven data analysis is a variant of TDD that is used in data science. TDDA involves writing tests for data cleaning, data preprocessing, and statistical models. By using TDDA, data scientists can ensure that their results are reproducible and that their code is correct.

HTTP interaction replay: It is like recording and playing back conversations between your Python program and a web server. It’s useful for testing, debugging, and mocking web-based applications. When you use a library like [VCR.py](https://vcrpy.readthedocs.io/en/latest/) it records the HTTP requests and responses, and then plays them back later when you need them.

## TDD with python in action:

Let’s consider a feature for an e-commerce application where a customer can add a product to their cart. Here’s an example of how you might use TDD to develop this feature in Python:

**Write a failing test**: The first step in TDD is to write a test that fails. In this case, we might write a test that checks whether a customer’s cart is updated correctly when they add a product:

```
def test_add_to_cart():
 customer = Customer()
 product = Product(‘123’, ‘Widget’, 9.99)
 customer.add_to_cart(product)
 assert customer.cart == {‘123’: {‘name’: ‘Widget’, ‘price’: 9.99, ‘quantity’: 1}}
```

**Run the test**: When you run the test, it should fail because we haven’t implemented the add_to_cart method yet.

Write the implementation: Now we can write the implementation code for the add_to_cart method. It might look something like this:

```

class Customer:
    def __init__(self):
        self.cart = {}

    def add_to_cart(self, product):
        if product.id in self.cart:
            self.cart[product.id]['quantity'] += 1
        else:
            self.cart[product.id] = {'name': product.name, 'price': product.price, 'quantity': 1}

```

**Run the test again**: Now that we’ve implemented the add_to_cart method, we can run the test again. This time, it should pass.

**Refactor**: If necessary, we can now refactor our code to make it more efficient, readable, and maintainable. For example, we might extract the cart item creation logic into a separate method:

```
class Customer:
    def __init__(self):
        self.cart = {}

    def add_to_cart(self, product):
        if product.id in self.cart:
            self.cart[product.id]['quantity'] += 1
        else:
            self.cart[product.id] = self._create_cart_item(product)

    def _create_cart_item(self, product):
        return {'name': product.name, 'price': product.price, 'quantity': 1}

```
**Repeat**: We can now continue this cycle of writing failing tests, implementing the code to make them pass, and refactoring as necessary until we’ve fully developed and tested the feature.


**Conclusion**:

In conclusion, test-driven development is a great way to develop Python programs that are reliable, maintainable, and bug-free. By writing test cases before writing the implementation, we can catch errors and bugs early in the development process and avoid costly debugging later on. With practice, you’ll find that TDD becomes an intuitive and efficient way to develop software.