4 exercises that build on the Python Comments lesson, from easy to hard. Each one is checked in your browser when you press Check. Stuck? Use the hint before peeking at the solution.
What this lesson covers: A comment is text after a # that Python ignores, used to explain why code does something, leave reminders, or temporarily switch a line off. By the end you can write single-line, inline and block comments, comment out code while debugging, tell comments from docstrings, avoid the multi-line comment myth, and write comments that explain why instead of what. You will practise: Silence the debug lines; Which hashes are comments?; Fix the swallowed return; Strip the comment from a line.
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Python Comments practice exercises
1. Silence the debug lines
Write codeEasy✓ Completed
The program prints two DEBUG lines that were useful while testing. Comment them out with # so the program prints only Total: 45. Keep the total variable.
Your code
prices = [10, 15, 20]
print("DEBUG prices:", prices)
total = sum(prices)
print("DEBUG total computed")
print("Total:", total)
Hint: Put # at the very start of each line that prints DEBUG. Leave the other lines alone.
One possible solution
prices = [10, 15, 20]
# print("DEBUG prices:", prices)
total = sum(prices)
# print("DEBUG total computed")
print("Total:", total)
2. Which hashes are comments?
Predict the outputEasy✓ Completed
Some # characters start comments and some are just text inside strings. Type exactly what the program prints.
Read this code
code = "#42" # a string, not a comment
# print("hidden")
print(code, "# ok")
print("Done") # print("Done again")
Hint: A # inside quotes is plain text. A # outside quotes hides the rest of that line from Python.
Actual output
#42 # ok
Done
3. Fix the swallowed return
Fix the bugMedium✓ Completed
average(numbers) should return the average of a list, but it returns None. A comment is hiding part of the code. Fix it without deleting the explanation.
Your code
def average(numbers):
total = sum(numbers) # add them up; return total / len(numbers)
print(average([2, 4, 6]))
Hint: Everything after the # on that line is a comment, including the return statement. Move it to its own line.
One possible solution
def average(numbers):
total = sum(numbers) # add them up
return total / len(numbers)
print(average([2, 4, 6]))
4. Strip the comment from a line
Write codeHard✓ Completed
Write strip_comment(line) that returns a line of Python source without its comment and without trailing spaces. A # only starts a comment when it is not inside quotes (single or double). Examples: x = 5 # five becomes x = 5, and color = "#fff" # white becomes color = "#fff".
Your code
def strip_comment(line):
# naive version: breaks when # is inside a string
return line.split("#")[0].rstrip()
print(strip_comment("x = 5 # five"))
Hint: Walk through the characters with a loop and remember whether you are inside quotes. Cut the line at the first # you meet while not inside quotes.
One possible solution
def strip_comment(line):
quote = None
for i, ch in enumerate(line):
if quote:
if ch == quote:
quote = None
elif ch in "\"'":
quote = ch
elif ch == "#":
return line[:i].rstrip()
return line.rstrip()
print(strip_comment("x = 5 # five"))
