Update_Manifest.py

from collections import OrderedDict
import re, json, sys, os

def parseObj(tokens):
	if(type(tokens) == str):
		tokens = re.split(r'[\t\r\n]+', tokens)
		tokens.reverse()
	obj = OrderedDict()
	while len(tokens):
		nt = tokens.pop()
		if nt in ["}", ""]:
			return obj
		elif nt.startswith('"'):
			nt = json.loads(nt)
			val = tokens.pop()
			if val.startswith('"'):
				obj[nt] = json.loads(val)
			elif val == "{":
				obj[nt] = parseObj(tokens)
		else:
			raise Exception("nt = %r" % nt)

def dumpObj(obj, indent=0):
	s = ""
	for key in obj:
		s += "\t" * indent + json.dumps(key)
		if type(obj[key]) == OrderedDict:
			s += "\n" + "\t" * indent + "{\n" + dumpObj(obj[key], indent + 1) + "\t" * indent + "}\n"
		else:
			s += "\t\t%s\n" % json.dumps(obj[key])
	return s

with open(sys.argv[1], "r") as f:
	manifestpath = os.path.dirname(f.name)
	data = f.read()

manifest = parseObj(data)

print(dumpObj(manifest))

if "-username" in sys.argv:
	username = sys.argv[sys.argv.index("-username") + 1]
	appstate = manifest['AppState']
	for depotkey in appstate['StagedDepots']:
		cmd = \
			"DepotDownloader.exe -app " + appstate['appid'] + \
			" -depot " + depotkey + " -manifest " + appstate['StagedDepots'][depotkey]['manifest'] + \
			" -username " + username + " -remember-password -dir \"" + \
			manifestpath + "\\common\\" + appstate['installdir'] + "\" -validate"
		print(cmd)
		os.system(cmd)
		installed_depot = appstate['StagedDepots'][depotkey]
		del installed_depot['dlcappid']
		appstate['InstalledDepots'][depotkey] = installed_depot
		del appstate['StagedDepots'][depotkey]
	appstate['StateFlags'] = "4"
	appstate['UpdateResult'] = "0"
	appstate['StagingSize'] = "0"
	appstate['buildid'] = appstate['TargetBuildID']
	appstate['BytesStaged'] = appstate['BytesToStage']
	appstate['BytesDownloaded'] = appstate['BytesToDownload']
	
	print(dumpObj(manifest))
	
	with open(sys.argv[1], "w") as f:
		f.write(dumpObj(manifest))

Oh, so you didn't need an answer for your question what is KeyError: 'StagedDepots'? cool :)