💡 Problem Formulation: Testing code is an essential aspect of software development, ensuring functionality correctness across updates. In Python, one of the convenient ways to perform tests is using the doctest module. This module searches for pieces of text that look like interactive Python sessions in docstrings and then executes them to verify that they work exactly as shown. In this article, we will explore the methods by which interactive Python examples can be tested using doctest, with a focus on ensuring that a function, when given a specified input, produces the expected output.
Method 1: Inline Doctest in Docstrings
This straightforward approach allows you to include tests within the docstrings of your functions or classes. Doctest then executes these examples and checks the output against the expected results. This method encourages a well-documented codebase and ensures examples in your documentation remain accurate over time.
Here’s an example:
def add(a, b):
"""
Adds two numbers and returns the sum.
>>> add(2, 3)
5
"""
return a + b
import doctest
doctest.testmod()Output of this code snippet would indicate that all tests have passed or failed with specific feedback.
In this example, the function add() includes a simple test in its docstring demonstrating its usage. When doctest.testmod() is called, it checks if calling add(2, 3) indeed returns 5, as specified in the docstring.
Method 2: External Documentation File Testing
For comprehensive documentation, doctest can verify code examples in separate files, such as a README or any markdown/text file, maintaining a clear separation between documentation and code.
Here’s an example:
"""
This is a content of an external documentation file where the next line is a doctest example:
>>> from my_module import multiply
>>> multiply(4, 5)
20
"""
import doctest
doctest.testfile("example.txt")Output of this code would mirror the outcomes of tests, much like inline examples.
This snippet suggests that by providing the filename (e.g., “example.txt”) to doctest.testfile(), doctest will search through the file looking for examples to run. Here, it verifies that the multiply function returns 20 when called with 4 and 5 as arguments.
Method 3: Running Doctest as a Script
Doctest can be run as its own script at the command line, scanning all files that match a certain pattern and executing contained tests. This method is useful for integrating into a larger testing suite or continuous integration system.
Here’s an example:
# Assuming this code is in a file named 'example.py'
def subtract(x, y):
"""
Subtract y from x and return the result.
>>> subtract(10, 5)
5
"""
return x - y
# At the bottom of the file
if __name__ == "__main__":
import doctest
doctest.testmod()Command line: $ python -m doctest example.py
The output will indicate if tests passed, otherwise it will show failures.
The example contains a test within the subtract function’s docstring, which is executed when the command runs the file with doctest. This ensures that subtract(10, 5) returns 5.
Method 4: Using Doctest with Test Files for Module Validation
Doctest can also test individual modules by specifying a separate test file containing the doctests. This allows for a modular testing approach, ideal for larger projects.
Here’s an example:
"""
Save this doctest in a test file:
>>> from my_module import divide
>>> divide(10, 2)
5.0
"""
import doctest
doctest.testmod("my_module", verbose=True)Verbose output will report all test cases, passed and failed alike.
By running doctest.testmod("my_module", verbose=True), the module named my_module is tested against the doctests written in its associated test file. This snippet ensures that the divide function works as expected and also gives detailed output because of the verbose flag.
Bonus One-Liner Method 5: Command Line Flag Execution
For quick one-off tests, Python’s command line flag can be used to execute doctest without writing additional code.
Here’s an example:
$ python -m doctest -v my_module.py
Command line execution: Outputs a verbose log of all tests in the module.
This example illustrates running doctest directly from the command line against a specific Python file (in this case, my_module.py). By adding the -v flag, you get a verbose printout of all tests being executed, which can be useful for debugging purposes.
Summary/Discussion
- Method 1: Inline Doctest in Docstrings. Ideal for small to medium projects. Strength: promotes documentation. Weakness: can clutter code with extensive tests.
- Method 2: External Documentation File Testing. Best suited for projects with comprehensive documentation separate from the code. Strength: keeps documentation and code separate. Weakness: less discoverable for developers not familiar with documentation.
- Method 3: Running Doctest as a Script. Good for automation in larger tests suites. Strength: can be easily integrated into CI systems. Weakness: requires command-line proficiency.
- Method 4: Using Doctest with Test Files. Useful for larger projects with many modules. Strength: offers modular testing. Weakness: needs test files maintenance.
- Bonus Method 5: Command Line Flag Execution. Quick, great for one-off tests or continuous testing during development. Strength: no extra code. Weakness: not integrated into a larger test suite.