Stage and Unstage

When a Device d is used in scan, it is “staged” and “unstaged.” Think of this as “setup” and “cleanup”. That is, before a device is triggered, read, or moved, the scan is expected to call d.stage(). And, at the end of scan, d.unstage() is called. (Whenever possible, unstaging is performed even if the scan is aborted or fails due to an error.)

The staging process is a “hook” for preparing a device for use. To add custom staging logic to a Device, subclass it and override stage and/or unstage like so.

class MyMotor(EpicsMotor):

    def stage(self):
        print('I am staging.')
        super().stage()

    def unstage(self):
        print('I am unstaging.')
        super().unstage()

It is crucial to call super(), as above, so that any built-in staging behavior is not overridden.

A common use for staging is to set certain signals to certain values for a scan and then set them back at the end. For example, a detector device might turn on “capture mode” at the beginning of the scan and then flip it back off (or back to its original setting, whatever that was) at the end. For this, ophyd provides a convenience, stage_sigs — a dictionary mapping signals to desired values. The device reads the initial values of these signals, stashes them, changes them to the desired value, and then restore the initial value when the device is unstaged. It is best to customize stage_sigs in the device’s __init__ method, like so:

class MyMotor(EpicsMotor):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.stage_sigs[self.user_offset] = 5

When a MyMotor device is staged, its user_offset value will be set to 5. When it is unstaged, it will be set back to whatever value it had right before it was staged.