Morse Code Conversion Challenge

Write a program which reads a file containing Morse Code and outputs the conversion.

Specifications


The first argument is a path to a file.   
The file contains multiple lines.
Each line is a string of Morse code.


Each letter is separated by a space.
Each word is separated by two spaces.



Constraints


The input file is correctly formatted.
The Morse Code strings are alphanumeric.


Sample Input


.- ...- ..--- .-- .... .. . -.-. -..-  ....- .....  
-... .... ...--



Sample Output


  AV2WHIECX 45
  BH3


Source

My Solution:

import sys
import re

morse_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',
    '-----' : '0',
    '.----' : '1',
    '..---' : '2',
    '...--' : '3',
    '....-' : '4',
    '.....' : '5',
    '-....' : '6',
    '--...' : '7',
    '---..' : '8',
    '----.' : '9'
}

def decode(morse):
    decoded = ''
    line = re.split('\s', morse)
    for letter in line:
        decoded += morse_alphabet.get(letter)
    return decoded

def main(filename):
    with open(filename) as input_file:
        for line in input_file:
            print(decode(line))

if __name__ == "__main__":
    try:
        main(sys.argv[1])
    except:
        sys.exit("No argument provided / file not found.")


Applying the insight gained from my previous question. It seems fairly succinct but I wonder is it pythonic?
My remarks are similar to last time.  morse_alphabet should be MORSE_ALPHABET.  Repeated string concatenation (decoded += …) is bad for performance.  The main task could be accomplished using a one-liner substitution.

MORSE_ALPHABET = {
    ' '     : ' ',        # Note: Changed the key from '' to ' '
    '.-'    : 'A',
    '-...'  : 'B',
    '-.-.'  : 'C',
    …
}

def decode(morse):
    return re.sub(' ?( |[.-]*)', lambda m: MORSE_ALPHABET.get(m.group(1)), morse)


The main code would be simpler if you used fileinput.  Despite the current Pokémon craze, you should not Catch 'Em All, but only catch the exceptions that you intend to handle.

def main():
    try:
        for line in fileinput.input():
            print(decode(line), end='')
    except IOError as e:
        sys.exit(e)

if __name__ == "__main__":
    main()