Python script to return directories with sizes less than specified minimum

dirless.py

#!/usr/bin/env python

import sys
import os
from functools import reduce

def sized(source, size):
    m = {}

    def fileSize(root, path):
        joined = os.path.join(root, path)
        return 0 if os.path.islink(joined) else os.path.getsize(joined)

    for root, dirs, files in os.walk(source, topdown = False):
        # Size all the files here...
        total = reduce((lambda acc, path: acc + fileSize(root, path)), files, 0)
        # Add the (already sized) children
        total = reduce((lambda acc, path: acc + m[os.path.join(root, path)]), dirs, total)
        # Store for the root
        m[root] = total

    if m[source] < size:
        print(source)

def usage():
    print("Usage: " + __file__ + " <lt byte size> <root path>")
    print("Also supports stdin.")
    exit(1)

if __name__ == "__main__":
    size = 0
    try:
        size = int(float(sys.argv[1]))
    except:
        usage()

    if len(sys.argv) < 2:
        usage()

    if len(sys.argv) > 2 and os.path.isdir(sys.argv[2]):
        source = sys.argv[2]
        for path in os.listdir(source):
            path = os.path.join(source, path)
            if os.path.isdir(path):
                sized(path, size)
    else:
        for path in sys.stdin:
            path = path.rstrip()
            if os.path.isdir(path):
                sized(path, size)
            else:
                sys.stderr.write("Error: " + path + " is not a path\n")
                exit(1)