Python server programming: test-driven development with pytest

As a powerful and popular programming language, Python is very suitable for server-side programming. One of the most important aspects of server-side programming is testing, because no matter how perfect your application is, it always needs to be tested to ensure its stability and correctness.

This is the time to use the Test Driven Development (TDD) method. TDD means writing test cases before actually writing the code. With this approach, you can more easily write stable, reliable applications because test cases can help you find and fix bugs. One of the highly respected testing frameworks is pytest.

In this article, we will discuss the process of test-driven development using pytest.

First, let's set up an example. We will create a web application that can get the definition of a word and retrieve it based on part of speech.

In this application, we need to write the following classes and methods:

Word class - represents a word, contains parts of speech and definition.

class Word:
    def __init__(self, word, part_of_speech, definition):
        self.word = word
        self.part_of_speech = part_of_speech
        self.definition = definition

Dictionary class - Represents a dictionary with methods for adding and querying word definitions.

class Dictionary:
    def __init__(self):
        self.words = []

    def add_word(self, word, part_of_speech, definition):
        new_word = Word(word, part_of_speech, definition)
        self.words.append(new_word)

    def search(self, query):
        results = []
        for word in self.words:
            if query in word.definition:
                results.append(word)
        return results

Now that we have these two classes, let’s start writing test cases.

We will use pytest to write test cases. pytest is a simple and flexible Python testing framework.

First, we need to install pytest. You can install pytest using pip:

pip install pytest

Next, let’s create a test_dictionary.py file in our project folder. The code in this file will contain the test cases we will use to test the Dictionary and Word classes.

We will first write a test case to test our Word class. We will use the assert statement to test whether the arguments for each word are stored correctly.

class TestWord:
    def test_init(self):
        w = Word('test', 'noun', 'this is a test')
        assert w.word == 'test'
        assert w.part_of_speech == 'noun'
        assert w.definition == 'this is a test'

We use the assert statement to check whether word, part_of_speech and definition are correctly set as input parameters for the word.

Now, we will write some test cases to test our Dictionary class.

class TestDictionary:
    def test_add_word(self):
        d = Dictionary()
        d.add_word('apple', 'noun', 'a fruit')
        assert len(d.words) == 1
        assert d.words[0].word == 'apple'
        assert d.words[0].part_of_speech == 'noun'
        assert d.words[0].definition == 'a fruit'

    def test_search(self):
        d = Dictionary()
        d.add_word('apple', 'noun', 'a fruit')
        d.add_word('banana', 'noun', 'another fruit')
        d.add_word('carrot', 'noun', 'a vegetable')
        results = d.search('fruit')
        assert len(results) == 2
        assert results[0].word == 'apple'
        assert results[1].word == 'banana'

Through these test cases, we can test whether our Dictionary class adds words correctly and returns results correctly when using the search method.

Now, we run the test cases to see if they pass. In the terminal, use the following command to run pytest:

pytest

If all tests pass, you should see output similar to the following:

============================== test session starts ==============================
platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-1.x.y
rootdir: /path/to/project
collected 3 items                                                              

test_dictionary.py ...                                                    [100%]

=============================== 3 passed in 0.01s ===============================

This means that our test cases passed , our Dictionary and Word classes work fine.

By using pytest for test-driven development, we can write test cases before writing code, which can help us ensure code quality and reliability. pytest is a very popular testing framework that is easy to use and powerful, and can meet most Python server-side programming testing needs.

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