It would watch your program during execution and record each function calls input and output. And then create tests for each function using those inputs and outputs.
You could always go over the created tests manually and fix them or review them but if nothing else it could be a good start.
A lower barrier way of trying this out could be to run MonkeyType in combination with OP's hypothesis-auto (which uses type annotations to generate the tests).
[1] https://github.com/google/pytype
[2] https://github.com/dropbox/pyannotate
If the library can use the observed and recorded testcase to extrapolate far beyond your manual call, this could be a really interesting way of testing python code.
Because the common lisp environment already defines special variables that contain the last form evaluated and the return value of the last form evaluated, when you evaluate code at the REPL and it succeeds, you call the (testy:test) function, which captures them and wraps them up as a test for the package you're working on, adding it to the database of tests for the package.
Not only was it useful for development (no need to specify your tests up front, you write your code until it works, then that state "is" the test), but it was also helpful for fixing bugs that you hadn't thought of up front (you fix the bug and then that state is captured as a new test as well).