Mock testing an api for post method

I am trying to write a mock test for an api that someone else coded, and my duty is to test a module's post method. The module is written to do a job of a handler and it uses BaseHandler as super class.

What do i need is how/what to mock while writing the mock test for a http post method. Should I mimic a post request? Or make a real one? Or what else?

I am not allowed to share any codes, but the code uses tornado to make connection to localhost, mongodb for database read-write and is written using Python 3.6.

Any ideas are welcome. Thanks from now.