I want to find all builds which started some user by his username. Is it possible to do with the help of Python?
Yes. you could use Python, if you have access to the jenkins master.
The userId is in build.xml which is usually in your jenkins jobs/you_job_name/builds/your_build_number/build.xml
You could iterate over all the jobs and their builds and get the answer from there.
Get information about the Jenkins builds related to specified username