...

kitty/prewarm.py

@@ -202,7 +202,7 @@ class PrewarmProcess:
             self.poll_to_send(bool(output_buf))
             for (fd, event) in self.poll.poll(2):
                 if event & error_events:
-                    raise PrewarmProcessFailed('Failed doing I/O with prewarm process: {event}')
+                    raise PrewarmProcessFailed(f'Failed doing I/O with prewarm process: {event}')
                 if fd == self.write_to_process_fd and event & select.POLLOUT:
                     n = os.write(self.write_to_process_fd, output_buf)
                     output_buf = output_buf[n:]

kitty_tests/prewarm.py

@@ -62,7 +62,7 @@ def socket_child_main(exit_code=0, initial_print=''):
             status = wait_for_child_death(pty.child_pid, timeout=5)
             if status is None:
                 os.kill(pty.child_pid, signal.SIGKILL)
-            self.assertIsNotNone(status, 'prewarm wrapper process did not exit')
+            self.assertIsNotNone(status, f'prewarm wrapper process did not exit. Screen contents: {pty.screen_contents()}')
             with suppress(AttributeError):
                 self.assertEqual(os.waitstatus_to_exitcode(status), exit_code, pty.screen_contents())