How to run a session without a GUI¶

A session built on Session rather than redsun.qt.QtSession shows nothing, which is what a headless deployment or a test wants. Every device, presenter and service comes up, and nothing asks for a display.

Install with no extra¶

Session needs no Qt package, so a plain install is enough:

pip install redsun

The redsun[pyqt] and redsun[pyside] extras exist only to bring in a Qt binding for QtSession.

Write the session¶

Subclass Session instead of QtSession:

from redsun import AsDevice, AsPresenter, Session


class MyApp(Session):
    stage: AsDevice[MyMotor]
    stage_ctrl: AsPresenter[MyController]

Leave out the views: section and every AsView declaration, because a view shows something and there is nothing here to show it on.

Build, use, shut down¶

Build the session, use its components, and shut it down:

app = MyApp().build()
# app.stage_ctrl, app.stage and every other component are ready to use
app.shutdown()

build makes every component and every link, a coroutine slot included, and shutdown releases them. There is no run, because that method exists only on a session with a frontend: it also shows a window and starts that frontend's event loop.

Signals to main-thread slots wait in a queue

Without an event loop, a signal sent to a slot that runs on the main thread, as every slot of a view does, waits in a queue. Deliver what waits by calling psygnal.emit_queued() on the main thread, for example in a test after the action that sends the signal.

Load one from a session file¶

A session file that names no frontend builds on the class from_config is called on:

# session.yaml
session: my-lab

devices:
  stage:
    plugin_name: mylab
    plugin_id: my_motor

presenters:
  stage_ctrl:
    plugin_name: mylab
    plugin_id: my_controller

from redsun import Session

app = Session.from_config("session.yaml").build()

When you layer several files, leave frontend out of every one of them. If any file names a frontend, the merged configuration names it too, and the session is built on that frontend; Merging the sources explains how files combine.

Read a failure¶

A session file that names frontend: qt builds on redsun.qt.QtSession, so it needs a Qt binding installed. Without one you get this error:

ImportError: the configuration names frontend 'qt', which cannot be imported:
No module named 'app_model'. Install the packages it needs; the 'qt' frontend
comes with the 'pyqt' and the 'pyside' extra.

Install redsun[pyqt] or redsun[pyside], or drop frontend from the file.