Trailing Comments

February 23, 2021

Today’s exercise is based on a blog entry that Reddit pointed me to. Sean is trying to write a program that finds lines of code in his Python programs that have trailing comments at the ends of lines, which he considers bad style. His naive program improperly flagged a line as containing a trailing comment when the comment marker is embedded in a quoted string:

x = "# This is fine"
"This \# is fine too"
x = "" # This is not

Sean gets a little bit sidetracked in his blog entry, and never quite solves the problem; I’m not convinced the code he writes is correct (though he seems to think it is), and I’m also not convinced that trailing comments are a bad thing. Nevertheless, the task makes an interesting exercise, especially when we also handle quoted escape sequences.

Your task is to write a program that identifies lines of code with trailing comments. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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Posted by programmingpraxis
Filed in Exercises
6 Comments »

6 Responses to “Trailing Comments”

  1. Daniel said
    February 23, 2021 at 10:39 PM

    @programmingpraxis, I believe the solution you posted considers full comment lines (those starting with a “#”, possibly preceded by whitespace) as trailing comments. For example, the line # comment would seemingly be counted as a trailing comment. Have I interpreted your code correctly, and should there be a distinction between 1) full comment lines and 2) lines that have code followed by a comment? I’ve assumed that a trailing comment detector would only detect the latter.

    Here’s a solution in Python, which takes a Python file as input and outputs the line numbers with trailing comments. I’ve utilized Python’s built-in tokenize module. This accommodates comment markers in either single or double quoted strings, and also handles trailing comments that occur at the end of multi-line strings after the triple quotes.

    trailing_comments.py

    import sys
import tokenize

assert len(sys.argv) == 2

with tokenize.open(sys.argv[1]) as f:
    line = -1
    tokens = tokenize.generate_tokens(f.readline)
    for token in tokens:
        # A trailing comment is a comment token that starts on the same line that the
        # preceding token ended on.
        if token.type == tokenize.COMMENT and token.start[0] == line:
            print(line)
        line = token.end[0]

    Example usage:

    example.py

    [sourcode lang=”text”]
    x = ‘# this is not a trailing comment’
    “this # is not part of a trailing comment”
    x = “” # this is a trailing comment

    this is not (full comment line)

    x = “””
    :-) # this multi-line string text is not part of a trailing comment
    “”” # this text is
    [/sourcecode]

    [sourcode lang=”text”]
    $ python3 trailing_comments.py example.py
    [/sourcecode]

    Output:

    3
7
  2. Daniel said
    February 23, 2021 at 10:41 PM

    I misspelled “sourcecode” in my example usage. Here’s another attempt.

    Example usage:

    example.py

    x = ‘# this is not a trailing comment’
    "this # is not part of a trailing comment"
    x = "" # this is a trailing comment
    # this is not (full comment line)
    x = """
    :-) # this multi-line string text is not part of a trailing comment
    """ # this text is

    $ python3 trailing_comments.py example.py

    Output:

    3
7
  3. Daniel said
    February 23, 2021 at 10:44 PM

    Another attempt.

    example.py

    x = ‘# this is not a trailing comment’
    "this # is not part of a trailing comment"
    x = "" # this is a trailing comment
    # this is not (full comment line)
    x = """
    :-) # this multi-line string text is not part of a trailing comment
    """ # this text is

    $ python3 trailing_comments.py example.py

    Output:

    3
7
  4. Daniel said
    February 23, 2021 at 10:47 PM

    A last attempt to correct formatting of my example usage.

    example.py

    x = '# this is not a trailing comment'
"this # is not part of a trailing comment"
x = ""  # this is a trailing comment
# this is not (full comment line)
x = """
:-)  # this multi-line string text is not part of a trailing comment
""" # this text is

    $ python3 trailing_comments.py example.py

    Output:

    3
7
  5. Kevin said
    February 24, 2021 at 8:15 PM

    A solution in Racket:

    (define (trailing-comments path)
  (define shebang "#!")
  (let next ((lines (port->lines (open-input-file path))) (count 1) (out null))
    (if (null? lines)
        (display (~a "lines: " (if (null? out) "(none)" (string-join (map number->string (reverse out))))))
        (let ((line (car lines)))
          (let ((chars (reverse (string->list (if (and (= count 1) (string-prefix? line shebang)) (string-trim line shebang #:right? #f) line)))))
            (let ((icomm (index-of chars #\#)) (iquot (index-of chars #\")))
              (next (cdr lines) (add1 count) (if (and icomm (or (not iquot) (< icomm iquot))) (cons count out) out))))))))

    Test source code file, trailing-comments.py:

    #!/usr/bin/env python

# comments start with the octothorpe
x = "But having one in a string, like this: #, doesn't count."
"This one: \#, doesn't count either."

# user input
Fahrenheit = int(raw_input("Enter a temperature in Fahrenheit: "))
Celsius = (Fahrenheit - 32) * 5.0/9.0 # convert to celsius
print "Temperature:", Fahrenheit, "Fahrenheit = ", Celsius, " C"

    Testing:

    > (trailing-comments "trailing-comments.py")
lines: 3 7 9
  6. Zack said
    February 25, 2021 at 3:02 PM

    Here is my take on this using Julia 1.5: https://pastebin.com/afzBbauw

    This solution works with a single line, but it’s not too difficult to adapt it to run on a whole program. Cheers

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