Python recursive override path function and mixin class

Here’s a recursive utility function in Python that allows overriding nested paths through objects that can be mutable mappings, mutable sequences or dataclass instances.

The function allows doing things like this:

override_paths(
  {
    "foo0": "bar",
      "lvl1": {
        "foo1": "bar1",
        "list1": [1, 2, 3, 4, 5, 6],
        "foobar1": Foobar(foo="hello1"),
    },
  },
  foo0="123",
  lvl1__foo1="456",
  lvl1__list1__3=23,
  lvl1__foobar1__foo="hello2",
)

{
  "foo0": "123",
  "lvl1": {
    "foo1": "456",
    "list1": [1, 2, 3, 23, 5, 6],
    "foobar1": Foobar(foo="hello2"),
  },
}

There is also a OverridableMixin that makes it convenient to do things like this inline:

Foobar().overridden(foo__bar__baz=123)

import dataclasses
from copy import deepcopy
from dataclasses import is_dataclass
from itertools import chain
from typing import (
    TYPE_CHECKING,
    Any,
    ClassVar,
    MutableMapping,
    MutableSequence,
    Protocol,
    Self,
    TypeVar,
)


class Dataclass(Protocol):
    """Identifies an instance of any dataclass."""

    __dataclass_fields__: ClassVar[dict[str, Any]]


if TYPE_CHECKING:
    Overridable = MutableMapping | MutableSequence | Dataclass  # pragma: no cover
else:
    # Having to add `| object` to the end of this union to keep PyCharm happy as
    # the PyCharm type checker can't figure out the Dataclass Protocol above.
    # Doing this inside a TYPE_CHECKING condition so that mypy still validates
    # the protocol correctly.
    Overridable = MutableMapping | MutableSequence | Dataclass | object


# Using TypeVar instead of the inline type parameter syntax because pydocstyle
# seems unable to cope with the newer syntax.
T = TypeVar("T", bound=Overridable)


def override_paths(
    target: T,
    *args: tuple[str | list[str], Any],
    **kwargs: Any,
) -> T:
    """
    Override keys, indexes or fields specified via a flat path syntax.

    E.g.

    override_paths({"foo": {"bar": 123}}, foo__bar=456)
    {"foo": {"bar": 456}}

    The override paths can be given as either kwargs, or as args as tuples with
    the path and the value as pairs. The path can be a __ string or a list of
    strings.

    override_paths({"foo": {"bar": 123}}, ("foo__bar", 456))
    {"foo": {"bar": 456}}

    override_paths({"foo": {"bar": 123}}, (["foo, "bar"], 456))
    {"foo": {"bar": 456}}

    Mutable mappings, mutable sequences and dataclass instances can be
    overridden.
    """
    target = deepcopy(target)
    for key_path, value in chain(args, kwargs.items()):
        if not key_path:
            return value
        if isinstance(key_path, str):
            key_path: list[str] = key_path.split("__")  # type: ignore[no-redef]
        path_entry: str = key_path[0]
        if isinstance(target, MutableMapping):
            if path_entry not in target:
                raise KeyError(f"Key '{path_entry}' not found in target")
            target[path_entry] = override_paths(
                target[path_entry],
                (key_path[1:], value),
            )
            continue
        if path_entry.isdigit() and isinstance(target, MutableSequence):
            override_index = int(path_entry)
            if len(target) - 1 < override_index:
                raise IndexError(f"index {override_index} out of range in target")
            target[override_index] = override_paths(
                target[int(path_entry)],
                (key_path[1:], value),
            )
            continue
        if has_dataclass_field(target, path_entry):
            setattr(
                target,
                path_entry,
                override_paths(
                    getattr(target, path_entry),
                    (key_path[1:], value),
                ),
            )
            continue
        raise KeyError(f"Key / index / field '{path_entry}' not found in target")
    return target


def has_dataclass_field(
    target: Any,
    field_name: str,
) -> bool:
    """
    Check if an object is a dataclass that has a dataclass field with the given
    name.
    """
    return is_dataclass(target) and any(
        True for f in dataclasses.fields(target) if f.name == field_name
    )


@dataclasses.dataclass
class OverridableMixin:
    """
    Mixin class which adds a convenience method to override paths on the
    instance and get back an overridden copy.

    This makes it convenient to override paths inline with a new instance
    without needing a variable referencing the instance, e.g.

    Foobar().overridden(foo__bar__baz=123)

    Can be dropped in place to get an instance of Foobar with that path
    overridden with the new value.
    """

    def overridden(
        self,
        **kwargs: Any,
    ) -> Self:
        """
        Get a copy of this instance with the given paths overridden.
        """
        return override_paths(self, **kwargs)

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