Compute checksums with external program vs. python stdlib, with python3 compatibility

compute_checksums_advanced.py

from __future__ import print_function

import argparse
import codecs
import hashlib
import os
import subprocess
import sys
import time
try:
    import xxhash
except ImportError:
    xxhash = None

def compute_checksum(algorithm, basedir, filename, output):
  if algorithm == 'xxhash':
    if xxhash is None:
        raise Exception("xxhash not available on this platform.  Try 'pip install xxhash'")
    h = xxhash.xxh64()
  else:
    h = hashlib.new(algorithm)

  with open(filename, "rb") as f:
    for chunk in iter(lambda: f.read(16384), b""):
        h.update(chunk)
    rpath = os.path.relpath(filename, basedir)
    checksum_txt = "%s  %s" % (h.hexdigest(), rpath)
    print(checksum_txt, file=output)

def run_checksum(algorithm, basedir, filename, output):
  if algorithm == 'xxhash':
    algorithm = 'xxh'
  cmd = algorithm + 'sum'
  rpath = os.path.relpath(filename, basedir)
  args = [cmd, rpath]
  process = subprocess.Popen(args, stdout=output, close_fds=False, cwd=basedir)
  if process.wait() != 0:
    raise subprocess.CalledProcessError(process.returncode, args)

def main():
  parser = argparse.ArgumentParser(description='Compute checksums for all files in CWD')
  parser.add_argument('--pure-python', action='store_true', help='Generate checksums with sha256')
  parser.add_argument('-a', '--algorithm', default='xxhash', help='Generate checksums')

  args = parser.parse_args()

  # TODO: Replace with the actual basedir of the backup directory
  basedir = os.getcwd()
  checksums_path = os.path.join(basedir, 'CHECKSUMS')

  t0 = time.time()
  with codecs.open(checksums_path, 'w', encoding='utf8') as output:
    for root, _, files in os.walk(basedir):
      for fname in files:
        cpath = os.path.join(root, fname)
        if cpath == checksums_path:
          continue
  
        if args.pure_python:
          compute_checksum(args.algorithm, basedir, cpath, output)
        else:
          run_checksum(args.algorithm, basedir, cpath, output)
  print("Total Elapsed time: %r" % (time.time() - t0,), file=sys.stderr)

if __name__ == '__main__':
  sys.exit(main())