Simple morse-code implementation for strings usings '.' and '-' symbols

morse.py

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
#
# simple morse-code implementation for strings usings '.' and '-' symbols
#
# (c) 2018 Bernd Busse

import sys


ALPHABET = {
    'A': '.-',
    'B': '-...',
    'C': '-.-.',
    'D': '-..',
    'E': '.',
    'F': '..-.',
    'G': '--.',
    'H': '....',
    'I': '..',
    'J': '.---',
    'K': '-.-',
    'L': '.-..',
    'M': '--',
    'N': '-.',
    'O': '---',
    'P': '.--.',
    'Q': '--.-',
    'R': '.-.',
    'S': '...',
    'T': '-',
    'U': '..-',
    'V': '...-',
    'W': '.--',
    'X': '-..-',
    'Y': '-.--',
    'Z': '--..',
    '1': '.----',
    '2': '..---',
    '3': '...--',
    '4': '....-',
    '5': '.....',
    '6': '-....',
    '7': '--...',
    '8': '---..',
    '9': '----.',
    '0': '-----',
}
INVERSE = {v: k for k, v in ALPHABET.items()}

class EncodingError(RuntimeError):
    pass


class DecodingError(RuntimeError):
    pass


def usage(executable):
    sys.stderr.write("usage: {} -e TEXT\n".format(executable))
    sys.stderr.write("       {} -d CODE\n".format(executable))
    sys.stderr.flush()


def encode(message):
    code = []

    for char in message:
        if char == ' ':
            code.append("    ")
        else:
            try:
                code.append(ALPHABET[char.upper()])
            except KeyError as e:
                raise EncodingError("Invalid character: '{}'".format(char))

    return ' '.join(code)


def decode(code):
    message = []
    space = False

    for char in code.split(' '):
        if char == '' and not space:
            message.append(' ')
            space = True
        elif char == '': pass
        elif char == '--··--':
            message.append("[SP]")
        elif char == '·-·-·-':
            message.append("[EOM]")
        else:
            try:
                message.append(INVERSE[char])
                space = False
            except KeyError as e:
                raise DecodingError("Invalid character: '{}'".format(char))

    return ''.join(message)


if __name__ == '__main__':
    if len(sys.argv) != 3:
        usage(sys.argv[0])
        exit(1)

    if sys.argv[1] == "-e":
        # Run encoding
        code = encode(sys.argv[2])
        print("Morsecode: {:s}".format(code))
    elif sys.argv[1] == "-d":
        # Run decoding
        message = decode(sys.argv[2])
        print("Message: {:s}".format(message))
    else:
        usage(sys.argv[0])
        exit(1)