Is tensorflow fully opensource? I stumble on a "built-in function"

Hello,

“session.py” has a line of code that reads “return tf_session.TF_SessionRun_wrapper(” but I just can’t find the source code of this function.
When trying to get information about this function, Python says “built-in function TF_SessionRun_wrapper is not a module, class, method, function, traceback, frame, or code object”

Am I missing something here? I need to make changes to tensorflow and I’m stuck right now. I was able to follow the whole work of a convolutional NN up to this point.

Thank you.

It Is a binding between python and C++ API: