GUI event processing and exception handling

The testing API is designed such that GUI event processing is requested automatically wherever there may be pending GUI events. While events are being processed, the global exception handling is overridden temporarily in order to capture any unhandled exceptions. If any exceptions are caught, a test error is raised after all pending events are processed.

For example, locate(), perform() and inspect() all request pending events to be processed before and/or after interacting with GUI states. This automatic behavior is enabled by default, but it can be disabled via the auto_process_events flag (see Testing with modal dialogs for example).


In production environments, the GUI event loop typically blocks as it waits and processes new GUI events continuously, until the last window is closed. In tests, the GUI event loop needs to be run programmatically for a limited scope of commands and yield to test instructions every now and then while the GUI is still open.

In production environments, some unhandled exceptions are tolerated and suppressed by the GUI event loop, but some exceptions could cause the entire runtime to abort abnormally. Neither of these conditions are desirable in tests. Unhandled exceptions typically indicate bugs and should cause a test to error. Abortion of the Python runtime prevents running and collecting results from multiple tests in a test suite.