I found myself in a situation where I had constructed a ProcessState class for storing the results of a cron job that runs on AWS. Passing the instance of ProcessState works just fine when it all happens in a single package. But then I needed to store the state in our database temporarily while the lambda kicks over to another procedure within a step function.

I needed two things:

1. A succinct way to export the state.
2. A way to load the state from the db and re-instantiate my ProcessState class

Two built-in Python methods did the trick: the __dict__ Special Attribute and the @classmethod decorator.

Starting File

Here's ProcessState:

import logging
from logging import Logger
import datetime
from .types.unit_dict import UnitDict


class ProcessState:
    """
    Object for storing and formatting Process State
    """

    def __init__(
        self,
        result: list[UnitDict] = [],
        complete: bool = False,
        errors: list[str] = [],
    ):
        self.result = result
        self.complete = complete
        self.errors = errors
        self.logger = logging.getLogger("ProcessLogger")

    def __repr__(self):
        return f"ProcessState('{self.result}', {self.complete}, {self.errors}"
        
    # Methods for updating and formatting date here.

From the type hints, you'll see that we're dealing here with properties of basic types: lists and a boolean. I've also instantiated the class with a logger.

Exporting with the Dict Attribute

If I want to get all of those properties, I can simply call the __dict__ attribute directly:

ps = ProcessState()

# Update the state here.

# Get the dictionary
dict = ps.__dict__

If I'm only storing values I want to pass along, this works great!

The wrinkle comes with the logger. I'm not interested in passing that along outside of my application. But the result will include reference to the logger.

So I'll have to handle filtering it out with a few helper functions:

class ProcessState:
    """
    Object for storing and formatting Process State
    """
    
    ...

    def get_dict_from_state(self):
        result = {k: v for k, v in self.__dict__.items() if self._is_value_property(v)}
        result["created_at"] = datetime.datetime.now().isoformat()

        return result

    def _is_value_property(self, v):
        if isinstance(v, Logger):
            return False
        return True

I've also gone ahead and added a "created_at" property.

Now, calling ps.get_dict_from_state() would provide only the errors, complete state, and result. Excellent!

Loading a Dictionary Into Our Class

Say I pass the result off to my DB. I then query the db for those values again.

Looking at my __init__ method above, I could almost get away with passing in the dictionary as-is with a spread operator like so:

ps = ProcessState(**dict)

However, my dict now includes a created_at property which I haven't included in the __init__ function!

If it were important to persist that field, I could add it to my init function. Or I could make use of an extra **kwargs at then end of my init signiture to catch any arguements not already defined.

BUT say that I also need to do some data massaging, such as instantiating another class within ProcessStates own initialization?

@classmethod is the answer.

I'll show how I'm using it, then how to construct it:

ps = ProcessState.from_dict(dict)

very similar to calling the __init__ method, though we're not tied to one signature. In fact, we could have multiple @classmethods for different scenarios, lending more flexibility to our instance construction.

The simplest way to implement thiswould be to pass everything in:

class ProcessState:
    """
    Object for storing and formatting Process State
    """
    
    . . . 
    
    @classmethod
    def from_dict(cls, values: dict) -> "CollectionState":
        return cls(**values)

However, as I mentioned, we have an unwanted attribute. Here I'll filter it out with list comprehension:

    @classmethod
    def from_dict(cls, values: dict) -> "CollectionState":
        filtered values = [x for x in values if x in ["errors", "results", "complete"]] 
        return cls(**values)

With that, we're back up and running, ready to use ProcessState in our function!