Override perform_get() and perform_set() without a subclass

[klanclos]

This pull requests enables the developer to specify how GET and SET requests will be handled on a per-item basis without requiring the use of custom subclasses of mktl.Item.

[baileyji]

Is there documentation on what constitutes a valid performer method? Some things I immediately wonder about:

• Can it assume an asyncio event loop is running? If so can it use it?
• How long can it take?
• Does it need to be reentrant?
• Might it be called concurrently?
• How can a daemon author say, in essence, "performer method X is blocked during the execution of performer method Y"?
• What is the (implicit) contract around errors and exceptions it should raise, if any?
• Are there restrictions on the return values?
• I can envision many commands where a get query needs arguments, especially for engineering/debug commands. It doesn't look like there is any provision for that.

Also I believe that Daemon._perform_get_wrapper is superfluous code as the performer will either be a function or a bound method of its class instance.

A quick example:

from typing import Callable

class Bar:
    def __init__(self, name:str):
        self.name = name
    
    def core_method(self, *args):
        print(f"Core method of {repr(self)} name={self.name} called with args: {args}")


class Foo:
    def __init__(self, method:Callable, b:str):
        self.anothers_method = method
        self.b = b
    
    def call_anothers(self, arg):
        self.anothers_method(arg, self.b)
        
    def core_method(self, *args):
        print(f"Core method of {repr(self)} {self.b} called with args: {args}")
        

x = Bar('external')
y = Foo(x.core_method, 'bval, foo instance')

y.call_anothers('some arg to call_anothers')
y.core_method('some arg to y core method')

and a deep dive is here https://docs.python.org/3/howto/descriptor.html

[klanclos]

@baileyji The main thing I'm trying to accomplish with the wrapper method is to allow the implementation to define/provide a performer method that doesn't receive an Item instance as the first argument. If I used something like types.MethodType to replace the Item.perform_set reference, whatever I used there would receive 'self' (the mktl.Item instance) as the first argument.

I believe your example code effectively does the same thing as what's in the pull request; I store a reference to the externally provided method, regardless of whether it is a function or an instance method, and replace Item.perform_set with Item._perform_set_wrapper so the rest of the handling chain remains unaware.

I'll add language to the docstrings about the expected return values, I see that's missing. Thanks for the heads-up.

With respect to how the request is executed, I will add language to the docstrings for Item.perform_get() and Item.perform_set(), since it's relevant there as well-- and the documentation for the other methods should all be landing in the same place so it's only described once.

I can flesh out a similar structure for handling the raw request, in addition to this mechanism for overriding perform_get() and perform_set(); I'll look into that.

[klanclos]

This pull request is starting to get a bit wordy with the families of methods to handle the various ways of adding and setting requests, but I think it's reasonably complete at this point.