I'm writing an script in Python to automatically update the structures in my Liferay portal and I want to do it via the json REST API.

I make a request to get an structure (method getStructure), and it worked.

But when I try to do an structure update in the portal it shows me the following error:

ValueError: Content-Length should be specified for iterable data  
of type class 'dict' {'serviceContext': "{'prueba'}", 'serviceClassName': 
'com.liferay.portlet.journal.service.JournalStructureServiceUtil', 'name': 'FOO', 
'xsd': '... THE XSD OBTAINED VIA JSON ...', 'serviceParameters': 
'[groupId,structureId,parentStructureId,name,description,xsd,serviceContext]', 
'description': 'FOO Structure', 'serviceMethodName': 'updateStructure', 
'groupId': '10133'}

What I'm doing is the next:

urllib.request.Request(url = URL, data = data_update, headers = headers)

URL is http://localhost:8080/tunnel-web/secure/json
The headers are configured with basic authentication (it works, it is tested with the getStructure method).
Data is:

data_update = {
    "serviceClassName" : "com.liferay.portlet.journal.service.JournalStructureServiceUtil", 
    "serviceMethodName" : "updateStructure", 
    "serviceParameters" : "[groupId,structureId,parentStructureId,name,description,xsd,serviceContext]", 
    "groupId" : 10133, 
    "name" : FOO, 
    "description" : FOO Structure, 
    "xsd" : ... THE XSD OBTAINED VIA JSON ..., 
    "serviceContext" : "{}" }

Does anybody know the solution? Have I to specify the length for the dictionary and how? Or this is a bug?