immutable state

[jamesdbrock]

I want to drop some notes here in this issue about immutable state.

When we call set_state we want to pass in a new state, which means creating a new immutable state object which possibly re-uses some of the immutable state from the old state object. Here are some patterns I developed for doing that. I don't know if this is the most Pythonic way of doing immutable data structures but this is what I came up with. The tricky part is doing shallow copies of class object properties which are lists.

class StatePersist:
    """
    State which should persist after the app closes and opens.
    """
    def __init__(self):
        self.modelpath : str = ""
        self.imagepath : str = ""
        self.sensitivity : int = 50 
        self.pixelsmillimeter : int = 50.0
        self.filters: list[StateFilterPersist] = []

    def __copy__(self):
        cls = self.__class__
        result = cls.__new__(cls)
        result.__dict__.update(self.__dict__)
        result.filters = self.filters[:]
        return result
        """
        When we shallow copy, we also want to make a shallow copy of the member lists.

        https://docs.python.org/3/library/copy.html

        https://stackoverflow.com/questions/1500718/how-to-override-the-copy-deepcopy-operations-for-a-python-object
        """


def handle_filteradd(self, event=None):
        state = copy.copy(self.state)
        state.filters.append(StateFilter())
        state.persist = copy.copy(self.state.persist)
        state.persist.filters.append(StateFilterPersist())
        state.discover = discover_state(state.persist, self.state.discover)
        match self.state.infer:
            case StateInferred():
                state.infer = copy.copy(self.state.infer)
                state.infer.filters.append(None)
        self.persist_state_save(state.persist)
        self.set_state(state)

[jamesdbrock]

Maybe stuff in this post would be helpful for re-wx idioms https://medium.datadriveninvestor.com/exploring-the-best-functional-programming-modules-in-python-74489b67c4c4

[jamesdbrock]

FUNCTIONAL PYTHON, PART I: TYPOPÆDIA PYTHONICA

https://www.tweag.io/blog/2022-09-08-fp1-typopaedia-pythonica/

FUNCTIONAL PYTHON, PART II: DIAL M FOR MONOID

https://www.tweag.io/blog/2023-01-19-fp2-dial-m-for-monoid/