"""
################################################################################
Usage: python dirdiff.py dir1-path dir2-path
Compare two directories to find files that exist in one but not the other.
This version uses the os.listdir function and list difference.  Note that
this script checks only filenames, not file contents--see diffall.py for an
extension that does the latter by comparing .read() results.
################################################################################
"""

import os, sys

def reportdiffs(unique1, unique2, dir1, dir2):
    """
    Generate diffs report for one dir: part of comparedirs output
    """
    if not (unique1 or unique2):
        print('Directory lists are identical')
    else:
        if unique1:
            print('Files unique to', dir1)
            for file in unique1:
                print('...', file)
        if unique2:
            print('Files unique to', dir2)
            for file in unique2:
                print('...', file)

def difference(seq1, seq2):
    """
    Return all items in seq1 only;
    a set(seq1) - set(seq2) would work too, but sets are randomly 
    ordered, so any platform-dependent directory order would be lost
    """
    return [item for item in seq1 if item not in seq2]


def comparedirs(dir1, dir2, files1=None, files2=None):
    """
    Compare directory contents, but not actual files;
    may need bytes listdir arg for undecodable filenames on some platforms
    """
    print('Comparing', dir1, 'to', dir2)
    files1  = os.listdir(dir1) if files1 is None else files1
    files2  = os.listdir(dir2) if files2 is None else files2
    unique1 = difference(files1, files2)
    unique2 = difference(files2, files1)
    reportdiffs(unique1, unique2, dir1, dir2)
    return not (unique1 or unique2)               # true if no diffs

def getargs():
    "Args for command-line mode"
    try:
        dir1, dir2 = sys.argv[1:]                 # 2 command-line args
    except:
        print('Usage: dirdiff.py dir1 dir2')
        sys.exit(1)
    else:
        return (dir1, dir2)

if __name__ == '__main__':
    dir1, dir2 = getargs()
    comparedirs(dir1, dir2)