I'm looking for a tool that will play nicely with Python. Except for my Python requirement, my question is the same as this one:
"I am looking for a tool which will take an XML instance document and output a corresponding XSD schema."
Currently, there is no module that will run within your python program and do this conversion. But I see the problem of creating a XSD schema from XML as a tooling problem. It's the kind of functionality that I'll use once, to get a schema started but after that I'll be maintaining the schema myself. From reading a single XML file the XSD generator will create a starting point for a real schema, it cannot infer all the functionality and options offered by XSD. Basically, I don't see the need to have this conversion run as a module inside of my code, generating new XSDs every time the XML changes. After all, it's the schema that defines the XML not the other way around.
As end-user pointed out you could use xsd.exe but you might also want to look at other tools such as trang (a bit old) for Java and stylusstudio (XML tool).