Test-Driven Development (TDD) in Python

Test-Driven Development (TDD) in Python#

Test-Driven Development (TDD) is a software development process that relies on the repetition of a very short development cycle:

  1. Write a test that fails

  2. Write the minimal code to make it pass

  3. Refactor if necessary

“Make incremental changes.” → TDD is an incremental process. Commit often.

This tutorial walks you through a simple example using a Line class.

Step 1: Write the Failing Test#

Create a file called test_line.py:

# test_line.py
from line import Line

def test_length():
    line = Line(0, 0, 3, 4)
    assertEqual(line.length(), 5)

This test assumes that the Line class and a length() method exist and that it calculates the distance between (0,0) and (3,4), which is 5.

At this point, the test will fail because we haven’t written line.py.

Step 2: Make the Test Pass#

Now create line.py:

# line.py
import math

class Line:
    def __init__(self, x1, y1, x2, y2):
        self.x1 = x1
        self.y1 = y1
        self.x2 = x2
        self.y2 = y2

    def length(self):
        dx = self.x2 - self.x1
        dy = self.y2 - self.y1
        return math.sqrt(dx**2 + dy**2)

Now, run the test again:

python test_line.py

The test should now pass.

Step 3: Refactor#

Now that your test passes, you can safely refactor the code. For example, you might want to rename the length() method to get_length(). Pycharm has a built-in refactor function that helps you with this. Right-click on the method name and select Refactor–> Rename. Type in the new name for the method. Note that Pycharm automatically changes the name everywhere it occurs.

Make sure to run tests after each change to verify correctness.

Summary#

TDD helps you:

  • Define clear goals before you start coding

  • Prevent overengineering

  • Catch bugs early

  • Write better structured and testable code

Happy testing!

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