Learning goals

• Write comments in Python code.
• Know that Python does not run comments.
• Write comments that explain why, not only what.

What is a comment?

A comment is a note in your code. It is information for the reader. Python ignores it, so it is not run.

Everything after a hashtag # is a comment:

# This line is a comment. Python skips it.
otto.forward(50)

Where can I write a comment?

You can write a comment on its own line. You can also write it after an instruction:

otto.forward(50)  # Otto walks 50 steps

Python only ignores the part after the #. The instruction before it still runs.

Why use comments?

• Comments make your code easier to read.
• They help the reader understand what happens. This reader can be your teacher, a friend, or you next week!
• You can also use a comment to switch off an instruction for a moment. Python skips it:

otto.forward(50)
# otto.left(90)
otto.forward(50)

Here, Otto does not turn. He walks 100 steps in a straight line.

Guidelines for good comments

Do not only repeat the code. Tell why you do something, or how it works.

# Bad: repeats the code
otto.left(90)  # turn left 90

# Good: tells why
otto.left(90)  # Turn a quarter so the next side of the square can start

Keep comments short and clear. Do not comment on every line. Write them where the reader might get lost.

Summary

# comment on its own line
otto.forward(50)  # comment after an instruction