In-code documentation


  • What can I do to make my code more easily understandable?

  • What information should go into comments?

  • What are docstrings and what information should go into docstrings?

In this episode we will learn how to write good documentation inside your code.

Exercise - Writing good comments

In-code-1: Comments

Let’s take a look at two example comments (comments in Python start with #):

Comment A

# now we check if temperature is below -50
if temperature < -50:
    print("ERROR: temperature is too low")

Comment B

# we regard temperatures below -50 degrees as measurement errors
if temperature < -50:
    print("ERROR: temperature is too low")

Which of these comments is more useful? Can you explain why?

Sometimes version control is better than a comment

Examples for code comments where Git is a better solution

Keeping zombie code “just in case” (rather use version control):

# do not run this code!
# if temperature > 0:
#     print("It is warm")

Instead: Remove the code, you can always find it back in a previous version of your code in Git.

Emulating version control:

# John Doe: threshold changed from 0 to 15 on August 5, 2013
if temperature > 15:
    print("It is warm")

Instead: You can get this information from git log or git show or git   annotate or similar.

What are “docstrings” and how can they be useful?

Here is function fahrenheit_to_celsius which converts temperature in Fahrenheit to Celsius, implemented in a couple of different languages. Your language is missing? Please contribute an example.

The first set of examples uses regular comments:

# This function converts a temperature in Fahrenheit to Celsius.
def fahrenheit_to_celsius(temp_f: float) -> float:
    temp_c = (temp_f - 32.0) * (5.0/9.0)
    return temp_c

The second set uses docstrings or similar concepts. Please compare the two (above and below):

def fahrenheit_to_celsius(temp_f: float) -> float:
    Converts a temperature in Fahrenheit to Celsius.

    temp_f : float
        The temperature in Fahrenheit.

        The temperature in Celsius.

    temp_c = (temp_f - 32.0) * (5.0/9.0)
    return temp_c

Read more:

Docstrings can do a bit more than just comments:

  • Tools can generate help text automatically from the docstrings.

  • Tools can generate documentation pages automatically from code.

It is common to write docstrings for functions, classes, and modules.

Good docstrings describe:

  • What the function does

  • What goes in (including the type of the input variables)

  • What goes out (including the return type)

Naming is documentation: Giving explicit, descriptive names to your code segments (functions, classes, variables) already provides very useful and important documentation. In practice you will find that for simple functions it is unnecessary to add a docstring when the function name and variable names already give enough information.


  • Comments should describe the why for your code not the what.

  • Writing docstrings can be a good way to write documentation while you type code since it also makes it possible to query that information from outside the code or to auto-generate documentation pages.