Python Comments vs Docstrings

When you're writing Python code, you often need to leave notes for yourself or for other developers. Two primary ways to do this are through comments and docstrings. Although they might seem similar at first glance, they serve different purposes and follow different conventions. Understanding when to use each can make your code cleaner, more maintainable, and easier for others to understand.

Comments are annotations in the source code that are ignored by the Python interpreter. They start with a # symbol and can be placed at the end of a line of code or on a line by themselves. Comments are useful for explaining why a particular piece of code exists, or for temporarily disabling code during debugging.

# This function calculates the area of a circle
def area(radius):
    # Using math.pi for better precision
    return 3.14159 * radius ** 2  # Approximate value of pi

In the example above, the comments help clarify the purpose of the function and the choice of using an approximate value for pi. Comments are best for short, inline explanations that aren't intended to be part of the formal documentation.

Docstrings, short for documentation strings, are string literals that appear right after the definition of a function, method, class, or module. Unlike comments, docstrings are not ignored by Python; they are stored as part of the object's __doc__ attribute. This makes them accessible programmatically, which is why they are the standard way to document your Python code.

def area(radius):
    """Calculate the area of a circle.

    Args:
        radius (float): The radius of the circle.

    Returns:
        float: The area of the circle.
    """
    return 3.14159 * radius ** 2

Here, the docstring provides a structured description of what the function does, its parameters, and its return value. This is especially useful because tools like Sphinx can automatically generate documentation from docstrings.

One key difference is that comments are for developers reading the source code, while docstrings are for users of your code who might be accessing it through an interactive help system or generated documentation. If you've ever used help() in the Python REPL, you've seen docstrings in action.

>>> help(area)
Help on function area in module __main__:

area(radius)
    Calculate the area of a circle.

    Args:
        radius (float): The radius of the circle.

    Returns:
        float: The area of the circle.

Another important distinction is that docstrings should follow a specific format. The most common styles are Google-style, NumPy/SciPy-style, and reStructuredText. Using a consistent style makes your documentation easier to read and parse.

When writing docstrings, it's good practice to include: - A brief summary of what the object does. - A description of parameters, if any. - Information about the return value, if applicable. - Any exceptions that might be raised. - Examples of usage, if helpful.

Comments should be used sparingly to avoid cluttering your code. If you find yourself writing a long comment to explain a complex algorithm, consider whether the code itself could be made clearer. Sometimes, refactoring is better than commenting.

Docstrings, on the other hand, should be comprehensive for public functions, classes, and modules. Even for private helper functions, a short docstring can be valuable for anyone who needs to understand or modify the code later.

In terms of tooling, docstrings are supported by documentation generators like Sphinx and pydoc, which can produce HTML, PDF, or other formats from your code. Comments do not have this capability, as they are not accessible programmatically.

It's also worth noting that multiline comments in Python are often achieved using triple-quoted strings, but these are not true comments—they are string literals that are not assigned to any variable. The Python interpreter does execute them, but since they aren't stored or used, they have no effect on the program. However, this is not the recommended way to write comments; use # for actual comments.

# This is a proper comment
"""
This is a string literal, not a comment.
It will be executed but ignored if not assigned.
"""

In practice, you should use docstrings for all your functions, classes, and modules that are part of the public API of your project. Use comments for internal explanations, reminders, or temporarily commenting out code.

Another advantage of docstrings is that they can be tested using doctest, a module that searches for pieces of text that look like interactive Python sessions and then executes them to verify that they work exactly as shown. This helps ensure that your examples in docstrings are accurate and up to date.

def add(a, b):
    """Add two numbers together.

    Examples:
        >>> add(2, 3)
        5
        >>> add(-1, 1)
        0
    """
    return a + b

You can run doctests by executing python -m doctest your_module.py in the terminal. This is a lightweight way to include tests directly in your documentation.

To summarize, both comments and docstrings have their place in Python programming. Use comments to clarify complex code or leave notes for other developers, and use docstrings to provide formal documentation for your functions, classes, and modules. By following these guidelines, you'll make your code more understandable and maintainable.

Remember that good documentation is a hallmark of professional code. Taking the time to write clear docstrings and thoughtful comments will pay off in the long run, especially when you or others need to revisit the code months or years later.