Get a folder/directory size in Python 3. Run: [ python DirectorySize.py '/usr/bin' ] (without braces) to get size in '/usr/bin' folder.

DirectorySize.py

import sys
import os

def calculateSize(path):
    totalSize = 0

    if len(path) > 2:
        print('Extra argument given.')
        sys.exit(0)
    elif len(path) == 1:
        path = os.path.abspath(os.getcwd())
    else:
        path = os.path.abspath(str(path[1]))
    for file in os.listdir(path):
        totalSize = totalSize + os.path.getsize(os.path.join(path, file))
    print(str(totalSize/(1024*1024))+" MBytes")

calculateSize(sys.argv)