[Python] Get the first non-null value for a key in multiple mappings/dictionaries

I am wondering if there are cleaner ways to retrieve the first non-null value for a key in an iterable of mappings/dictionaries than what I am currently doing.

Use case


I have an object which stores its internal attributes in a TypedDict. I am implementing a flyweight pattern by allowing subclasses to freeze values and/or define defaults at the class-level to save memory.

from typing import ClassVar, TypedDict


class SomeBaseClass:
"""Some class that can define some behaviour."""

class State(TypedDict, total=False):
"""Defines the fields that the base class can have."""

a: int
b: str

state: State
"""Instance-level field values."""
frozen: ClassVar[State] = State()
"""Class-level frozen field values."""
defaults: ClassVar[State] = State()
"""Class-level default field values."""

def __init__(self, state: State) -> None:
"""Initialize the object with a state."""
self.state = state
if set(self.state).intersection(self.frozen):
msg = "Attempted to override frozen values."
raise ValueError(msg)

Current implementation(s)


To access an instance's attribute value, I need to return the first non-null value found in state, frozen, or default, and raise an error if no such value exists.

I have found two implementations that work (see property a and b). But I have a feeling that there might be a cleaner/slicker way to achieve this behaviour.

Option a: use .get()


@property
def a(self) -> int:
"""Return the first non-null value using `.get()`."""
if (
output := self.state.get("a", self.frozen.get("a", self.defaults.get("a")))
) is None:
msg = "`a` not found in state, frozen or defaults"
raise ValueError(msg)
return output

• Pros: retrieving the value (or lack thereof) can be achieved as a one-liner.
• Cons: the nested structure feels clunky and not incredibly readable

Option b: use next()


@property
def b(self) -> str:
"""Return the first non-null value of `b` using `next`."""
if (
output := next(
(
value
for mapping in (self.state, self.frozen, self.defaults)
if (value := mapping.get("b")) is not None
),
None,
)
) is None:
msg = "`b` not found in state, frozen or defaults"
raise ValueError(msg)
return output

• Pros: Extendable to more mappings (though not needed in my use case)
• Cons: takes more lines of code

Notes


I thought of using the or syntax:

return self.state.get("a") or self.frozen.get("a") or self.defaults.get("a")


This won't work as my values are allowed to be falsy (i.e. 0 for an int variable, "" for a str variable, an empty collection etc.)

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