A script to remove comments from LaTeX source

strip_comments.py

import ply.lex, argparse, io

#Usage
# python stripcomments.py input.tex > output.tex
# python stripcomments.py input.tex -e encoding > output.tex

#This utility is released under the WTFPL license: http://www.wtfpl.net/about/

def strip_comments(source):
    tokens = (
                'PERCENT', 'BEGINCOMMENT', 'ENDCOMMENT', 'BACKSLASH',
                'CHAR', 'BEGINVERBATIM', 'ENDVERBATIM', 'NEWLINE', 'ESCPCT',
             )
    states = (
                ('linecomment', 'exclusive'), 
                ('commentenv', 'exclusive'), 
                ('verbatim', 'exclusive')
            )
    
    #Deal with escaped backslashes, so we don't think they're escaping %.
    def t_ANY_BACKSLASH(t):
        r"\\\\"
        return t
        
    #One-line comments
    def t_PERCENT(t):
        r"\%"
        t.lexer.begin("linecomment")
     
    #Escaped percent signs
    def t_ESCPCT(t):
        r"\\\%"
        return t
    
    #Comment environment, as defined by verbatim package       
    def t_BEGINCOMMENT(t):
        r"\\begin\s*{\s*comment\s*}"
        t.lexer.begin("commentenv")
    
    #Verbatim environment (different treatment of comments within)   
    def t_BEGINVERBATIM(t):
        r"\\begin\s*{\s*verbatim\s*}"
        t.lexer.begin("verbatim")
        return t
    
    #Any other character in initial state we leave alone    
    def t_CHAR(t):
        r"."
        return t
        
    def t_NEWLINE(t):
        r"\n"
        return t
    
    #End comment environment    
    def t_commentenv_ENDCOMMENT(t):
        r"\\end\s*{\s*comment\s*}"
        #Anything after \end{comment} on a line is ignored!
        t.lexer.begin('linecomment')
    
    #Ignore comments of comment environment    
    def t_commentenv_CHAR(t):
        r"."
        pass
        
    def t_commentenv_NEWLINE(t):
        r"\n"
        pass
    
    #End of verbatim environment    
    def t_verbatim_ENDVERBATIM(t):
        r"\\end\s*{\s*verbatim\s*}"
        t.lexer.begin('INITIAL')
        return t
        
    #Leave contents of verbatim environment alone
    def t_verbatim_CHAR(t):
        r"."
        return t
        
    def t_verbatim_NEWLINE(t):
        r"\n"
        return t
    
    #End a % comment when we get to a new line
    def t_linecomment_ENDCOMMENT(t):
        r"\n"
        t.lexer.begin("INITIAL")
        #Newline at the end of a line comment is stripped.
    
    #Ignore anything after a % on a line        
    def t_linecomment_CHAR(t):
        r"."
        pass
        
    lexer = ply.lex.lex()
    lexer.input(source)
    return u"".join([tok.value for tok in lexer])
    
def main():
    parser = argparse.ArgumentParser()
    parser.add_argument('filename', help = 'the file to strip comments from')
    parser.add_argument('--encoding', '-e', default='utf-8')
    
    args = parser.parse_args()
    
    with io.open(args.filename, encoding=args.encoding) as f:
        source = f.read()
    
    print(strip_comments(source))
    
if __name__ == '__main__':
    main()

The % at the end of 2nd line need to be preserved or the code can't run, how to solve that?

\makeatletter
\def\alloc@#1#2#3#4#5%
 {\ifnum\count1#1<#4% make sure there's still room
    \allocationnumber\count1#1
    \global\advance\count1#1\@ne
    \global#3#5\allocationnumber
    \wlog{\string#5=\string#2\the\allocationnumber}%
  \else\ifnum#1<6
    \def\etex@dummy@definition{}% <-- code added
    \begingroup \escapechar\m@ne
    \expandafter\alloc@@\expandafter{\string#2}#5%
  \else\errmessage{No room for a new #2}\fi\fi
 }
\makeatother

UPDATE:
I've modified the snippet to solve my problem, along with @m3phisto's suggestion.
https://gist.github.com/dzhuang/dc34cdd7efa43e5ecc1dc981cc906c85
Thank you for the useful code.

For anyone stumbling over this in the future: latexpand can reliably remove comments, too.

Wow, I completely forgot about this and didn't see all these comments. Thanks to everyone who has made improvements. I've added a comment to clarify the licensing situation.

To remove all the comments from a latex file, another option is to use use arxiv-latex-cleaner. Actively maintained, 1.2k GitHub stars, written in Python but no need to know Python.