Convert dumb quotes to smart quotes in Python

smartquotes.py

#!/usr/bin/env python3

""" 
Inspired by : https://gist.github.com/davidtheclark/5521432
Converts dumb quotes to smart quotes, -- to EM_DASH and ... to ELLIPSES.
"""

import sys
import re
import os

class bcolors:
    HEADER = '\033[95m'
    OKBLUE = '\033[94m'
    OKGREEN = '\033[92m'
    WARNING = '\033[93m'
    FAIL = '\033[91m'
    ENDC = '\033[0m'
    BOLD = '\033[1m'
    UNDERLINE = '\033[4m'

EN_DASH = "\u2013"
EM_DASH = "\u2014"
L_SQUOTE = "\u2018"
R_SQUOTE = "\u2019"
L_DQUOTE = "\u201C"
R_DQUOTE = "\u201D"

def error(msg):
    print( bcolors.FAIL + "ERROR: {}".format(msg) + bcolors.ENDC, file=sys.stderr  )
    sys.exit(1)

def exists(filen):
	return os.path.exists(filen) 

def dumb_to_smart_quotes(string):
    """Takes a string and returns it with dumb quotes, single and double,
    replaced by smart quotes. Accounts for the possibility of HTML tags
    within the string."""
    
    # Replace -- with EM_DASH
    string = string.replace('--', EM_DASH)
    # Replace ... with ELLIPSIS
    string = string.replace('...', ELLIPSIS)

    # Find dumb double quotes coming directly after letters or punctuation,
    # and replace them with right double quotes.
    string = re.sub(r'([a-zA-Z0-9.,?!;:\'\"])"', r'\1{}'.format(R_DQUOTE), string)
    # Find any remaining dumb double quotes and replace them with
    # left double quotes.
    string = string.replace('"', L_DQUOTE)
    # Reverse: Find any SMART quotes that have been (mistakenly) placed around HTML
    # attributes (following =) and replace them with dumb quotes.
    string = re.sub(r'={}(.*?){}'.format(L_DQUOTE, R_DQUOTE), r'="\1"', string)
    
    # Follow the same process with dumb/smart single quotes
    string = re.sub(r"([a-zA-Z0-9.,?!;:\"\'])'", r'\1{}'.format(R_SQUOTE), string)
    string = string.replace("'", L_SQUOTE)
    string = re.sub(r'={}(.*?){}'.format(L_SQUOTE, R_SQUOTE), r"='\1'", string)
    return string

if __name__ == '__main__':
	if len(sys.argv) < 3:
		print("Usage: python3 smartquote.py filename.txt smart.txt")
		sys.exit(1)
	
	filein = sys.argv[1]
	fileout = sys.argv[2]
	
	if filein == fileout:
		error( "Input file cannot be the same as output file" )
	
	if not exists(filein):
		error("File {} does not exist.".format(filein)) 
	if exists(fileout):
		error("File {} already exists.".format(fileout))
	
	result = ""
	with open(filein, 'r', encoding='utf-8') as f:
		contents = f.read()
		result = dumb_to_smart_quotes(contents)
		
	with open(fileout, 'w', encoding='utf-8') as f:
		f.write(result)