Smaracters (Faubi version 1.0)

#!/usr/bin/python3
# Faubi implementation of smaracters. Version 1.0
# Example usage: python smaracters.py "this is an example message" -f lowercase_latin.json
# lowercase_latin.json: http://pastebin.com/1AsrDEkX
import sys, argparse, json

argparser = argparse.ArgumentParser(description='Writes text using phpbb emoji (i.e. smaracters)')

argparser.add_argument('-l', '--line-length', type=int, default=40, help='Max number of emoji to display per line')
argparser.add_argument('-s', '--space-length', type=int, default=1, help='Number of spaces to put between characters')
argparser.add_argument('-f', '--char-file', action='append', help='A json file to load characters from')
argparser.add_argument('message', help='The text to write with emoji. If not present will load from stdin')

args = argparser.parse_args()

letter_height = 0

characters = {}

if not args.char_file:
    sys.exit('No character files specified')

# Load character data from supplied json files
for json_filename in args.char_file:
    try:
        with open(json_filename) as json_file:
            json_data = json.load(json_file)
    except FileNotFoundError:
        sys.exit('No such file: {0}'.format(json_filename))
    except ValueError:
        sys.exit('Unable to parse file: {0}'.format(json_filename))
    try:
        letter_height = max(letter_height, json_data['letter_height'])
        for character, value in json_data['characters'].items():
            if character not in characters:
                characters[character] = value
    except IndexError:
        sys.exit('Invalid data in file: {0}'.format(json_filename))

# Make all characters the same height by adding black rows to top of short characters
for character in characters:
    char_height = len(characters[character])
    if char_height < letter_height:
        char_width = len(characters[character][0])
        characters[character] = [' '*char_width]*(letter_height-char_height) + characters[character]

emoji = {' ': ':D', '#': ':mrgreen:'}

if args.message:
    message = args.message
else:
    message = sys.stdin.readline()[:-1]

current_line = ['']*letter_height
current_length = 0
new_length = 0

# Loop through once before printing anything to find invalid characters
for character in message:
    if character not in characters:
        sys.exit('Unsupported character: {0}'.format(character))

# Loop through characters, adding each to the current line
for index, character in enumerate(message):
    char_lines = characters[character]

    char_width = len(char_lines[0])

    if char_width > args.line_length:
        sys.exit('Character wider than line length limit: {0}'.format(character))

    new_length += char_width

    # Line break if new character would make line too long
    if new_length > args.line_length:
        for line_number, line in enumerate(current_line):
            print(' '.join(emoji[character] for character in line))
            current_line[line_number] = ''
        print()
        new_length = char_width
    else:
        # Add space before character if not at beginning of line
        if index != 0:
            for line_number in range(len(current_line)):
                current_line[line_number] += ' '*args.space_length
            new_length += args.space_length

    # Add character to current line
    for line_number, char_line in enumerate(char_lines):
            current_line[line_number] += char_line

    current_length = new_length

# Print last line
if current_line[0] != '':
    for line_number, line in enumerate(current_line):
        print(' '.join(emoji[character] for character in line))
        current_line[line_number] = ''