I believe this accomplished by modifying
compilation-error-regexp-alist(-alist)?. If py-test is running in a raw
compilation-mode than the appropriate alists need to be added to the global font-lock-keywords & error-regexp-alists for compilation-mode.
However I believe when defining a compilation derived mode with
define-compilation-mode, it automatically references a variable for deriving mode specific error and font-lock information. IE
(define-compilation-mode pytest-compilation-mode "Pytest Compilation" "docs.")
Automatically generates a references to variables like:
I believe for this example it would be a
pytest-compilation-mode-font-lock-keywords which would cover metadata about the buffer like number of errors, if it finished, the same as
compilation-mode-font-lock-keywords does in the
compile library. Similarly
compilation-error-regexp-alist defines what errors look like (as well as hyperlink formatting, etc). Finally
compilation-error-regexp-alist-alist defines how to map to lines referenced within a given error. I recommend reading through the documentation for
M-x locate-library compile for more details.