Sorry, but I believe you were misdirected*.  mock, as opposed to unittest.mock, pytest, flask, and werkzeug  are 3rd party modules.  The error report seems to be missing part of the stacktrace at both ends.  What line is your file resulted in the error?  (This time we can guess == @patch. @What called _lookup_app_object in flask?  (I have no idea.)

However, once contextlib._GeneratorContextManager calls next(self.gen), the rest of the trace is outside the stdlib.  At the end, the RuntimeError is raised by flask, not by python, because the flask _app_ctx_stack.top does not exist.  If you do not understand their error message, ask flask people.

* I assume that cjw296 *glanced* at your report, saw 'RuntimeError', and too quickly assumed 'cpython error'.  flask could have defined, for instance, FlaskRuntimeError for its error reporting.