Warning if Component is passed a non-class?

[ZLLentz]

There was recently an issue downstream where, rather than passing a Device subclass into Component, a function that creates a device was passed in instead. This actually works ok... sometimes. It definitely breaks any code that needs to inspect the parent class's subdevices.

This isn't permitted in the Component docstring which specifies that it requires a class. Should we be throwing an appropriate warning here? E.g. if:

• we see a non-class callable
• we see something that doesn't subclass OphydObject
• some other criteria?

[danielballan]

I am in favor of doing cheap validation in Component.__init__. I am against strict type-checking because I think it is a feature that we can operate with other implementations that don't necessarily import ophyd or subclass OphydObject as long they present the required interface (e.g. ophyd-tango).

In a conversation with @RussBerg an idea was proposed to add something like a does_this_satisfy_the_required_device_api function. Could we run that on the first argument to Component?

[klauer]

If ophyd isn't imported, any validation done in Component doesn't matter, but agreed that we should not check if it's an OphydObj subclass.

Generic validation as you suggest, @danielballan, would fail for a callable in place of a class (e.g., cpt = Cpt(lambda: None), as you couldn't inspect the device API compatibility without first calling it to inspect the type. I think that's really at the core of this issue that @ZLLentz is bringing up.