wgets

#!/usr/bin/env python

import urllib.parse as up, binascii as ba
import os, sys, types, subprocess, hashlib

def print_usage(extras=None, exit_with=1):
	p = lambda *a: print(*a, file=sys.stderr)
	cmd = os.path.basename(sys.argv[0])
	example_args = 'http://os.archlinuxarm.org/os/'\
		'ArchLinuxARM-sun4i-latest.tar.gz', 'cea5d785df19151806aa5ac3a917e41c'
	p(f'Usage: {cmd} [ wget_opts ] url checksum')
	p('Example: {} -c {} {}'.format(cmd, *example_args))
	if extras is not None:
		if isinstance(extras, types.StringTypes): extras = [extras]
		for line in extras: p(line)
	if exit_with is not None: sys.exit(exit_with)

class InputError(Exception):
	def print_usage(self):
		if not self.args: print_usage()
		msg = self.args[0].format(*self.args[1:])
		print_usage(f'ERROR: {msg}')

def process_cli(args=None):
	if args is None: args = sys.argv[1:]
	args = args or ['-h']
	for arg in args:
		if arg.startswith('-h') or arg == '--help': raise InputError
	args, checksum = args[:-1], args[-1]
	if not args: raise InputError('both url and checksum must be specified')
	args, url = args[:-1], args[-1]
	file_path = up.urlparse(url).path
	file_path = up.unquote_plus(file_path.rsplit('/', 1)[-1])
	if not file_path: raise InputError('failed to detect output filename from url')
	args = ['wget'] + args + ['-O', file_path, url]
	return file_path, checksum, args

def validate_digest( checksum,
		_hash_funcs = [ ('md5', 16), ('sha1', 20),
			('sha256', 32), ('sha512', 64), ('sha224', 28), ('sha384', 48) ] ):
	digest = ba.a2b_hex(checksum)
	for hash_name, hash_len in _hash_funcs:
		if len(digest) == hash_len: break
	else:
		raise InputError( 'Error: failed to pick hash'
			f' for value (len: {len(checksum)}): {checksum}' )
	return digest, hash_name, getattr(hashlib, hash_name)

class DigestError(Exception): pass
def check_hash(file_path, hash_func, digest, bs=4*2**20):
	chk = hash_func()
	with open(file_path, 'rb') as src:
		for chunk in iter(lambda: src.read(bs), b''): chk.update(chunk)
	if digest != (chk := chk.digest()):
		raise DigestError(f'{ba.b2a_hex(digest)} (expected) != {ba.b2a_hex(chk)} (real)')

def main(args=None):
	try:
		file_path, checksum, wget_args = process_cli(args)
		digest, hash_name, hash_func = validate_digest(checksum)
	except InputError as err: err.print_usage()
	print(f'Using hash: {hash_name}')
	print(f'Using output filename: {file_path}')
	subprocess.run(wget_args, check=True)
	check_hash(file_path, hash_func, digest)
	print('Checksum matched')

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