Python Comments
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Single-Line Comments
A # marks the start of a single-line comment; the interpreter ignores everything from that character to the end of the line. They're the quickest way to leave yourself a short note without affecting how the code runs.
Example: Single-Line Comments
# This is a comment
print("Comments are ignored by the interpreter")Multi-Line Comments
There's no dedicated multi-line comment syntax in Python — developers either stack several # lines together, or repurpose an unassigned triple-quoted string ("""...""") as a block comment, since a standalone string statement has no effect when executed.
Example: Multi-Line Comments
# This is line one
# This is line two
"""
This triple-quoted string
acts as a block comment
"""
print("done")Documenting Code
Good comments explain *why* a piece of code exists — a tricky workaround, a business rule, a non-obvious edge case — rather than restating *what* the code already makes obvious. That distinction is what makes comments genuinely useful to the next person reading the file, including future you.
Example: Documenting Code
# Using a set here because duplicate ids would break the billing report
unique_ids = set()
print(unique_ids)Commenting Out Code
Wrapping a line or block in # prevents it from executing without deleting it, which is a common way to isolate which part of a script is causing a bug during debugging. Most editors offer a keyboard shortcut to comment/uncomment a whole selection at once.
Example: Commenting Out Code
print("this runs")
# print("this is commented out and does not run")Comment Best Practices
Comments that go stale are worse than no comment at all, since they actively mislead the next reader about what the code does. Keep them short, update them whenever the code they describe changes, and delete ones that no longer add information.
Example: Comment Best Practices
# Retry limit tuned after the 2024 outage postmortem
MAX_RETRIES = 5
print(MAX_RETRIES)Chapter Quiz — Complete all 14 topics to unlock
0/14 topics done
Complete these topics first: