parent-lookup

Table of Contents

• Installation
• Usage
• Development Setup
• Meta
• Contributing

parent-lookup is a utility Python package enabling a child object to dynamically lookup its parent at runtime.
It comprises of a small set of decorators and methods allowing bidirectional parent-child relationships without hard-coded circular dependencies or circular imports.

Installation

pip install parent-lookup

Usage

API:

from parent_lookup import TParent, is_child_lookup, lookup_registry


class Child:
    def __init__(self) -> None:
        pass

    @overload
    def find_parent(self, parent_type: type[TParent]) -> TParent | None:
        pass

    @overload
    def find_parent(self, parent_type: TParent) -> TParent | None:
        pass

    def find_parent(self, parent_type: type[TParent] | TParent) -> TParent | None:
        return lookup_registry.lookup_parent(self, parent_type)


class Parent:
    def __init__(self) -> None:
        self._childs: list[Child] = []

    def __new__(
        cls,
    ) -> Parent:
        instance = super().__new__(cls)
        lookup_registry.register_parent(instance)
        return instance

    def add_child(self, child: Child) -> None:
        self._childs.append(child)

    @property
    @is_child_lookup
    def childs(self) -> list[Child]:
        return self._childs


# Create two objects: A parent object and a child object
parent = Parent()
child = Child()
# Add child to parent
parent.add_child(child)
# Lookup parent from child
found_parent = child.find_parent(Parent)
assert found_parent is parent

Descriptor API (optional alternative):

from parent_lookup import ParentLookup, is_child_lookup, lookup_registry


class Parent:
    def __init__(self) -> None:
        self._childs: list[Child] = []

    def __new__(cls) -> Parent:
        instance = super().__new__(cls)
        lookup_registry.register_parent(instance)
        return instance

    def add_child(self, child: Child) -> None:
        self._childs.append(child)

    @property
    @is_child_lookup
    def childs(self) -> list[Child]:
        return self._childs


class Child:
    parent: ParentLookup[Parent] = ParentLookup(Parent)


parent = Parent()
child = Child()
parent.add_child(child)

assert child.parent is parent

For more examples and usage, please refer to parent-lookup's documentation.

Development Setup

1. Install uv

This project uses uv as package manager. If you haven't already, install uv, preferably using it's "Standalone installer" method:
..on Windows:

powershell -ExecutionPolicy ByPass -c "irm https://astral.sh/uv/install.ps1 | iex"

..on MacOS and Linux:

curl -LsSf https://astral.sh/uv/install.sh | sh

(see docs.astral.sh/uv for all / alternative installation methods.)

Once installed, you can update uv to its latest version, anytime, by running:

uv self update

2. Clone the repository

Clone the parent-lookup repository into your local development directory:

git clone https://github.com/ClaasRostock/parent-lookup path/to/your/dev/parent-lookup

Change into the project directory after cloning:

cd parent-lookup

3. Install dependencies

Run uv sync -U to create a virtual environment and install all project dependencies into it:

uv sync -U

Note: Using --no-dev will omit installing development dependencies.

Explanation: The -U option stands for --update. It forces uv to fetch and install the latest versions of all dependencies, ensuring that your environment is up-to-date.

Note: uv will create a new virtual environment called .venv in the project root directory when running uv sync -U the first time. Optionally, you can create your own virtual environment using e.g. uv venv, before running uv sync -U.

4. (Optional) Activate the virtual environment

When using uv, there is in almost all cases no longer a need to manually activate the virtual environment.
uv will find the .venv virtual environment in the working directory or any parent directory, and activate it on the fly whenever you run a command via uv inside your project folder structure:

uv run <command>

However, you still can manually activate the virtual environment if needed. When developing in an IDE, for instance, this can in some cases be necessary depending on your IDE settings. To manually activate the virtual environment, run one of the "known" legacy commands:
..on Windows:

.venv\Scripts\activate.bat

..on Linux:

source .venv/bin/activate

5. Install pre-commit hooks

The .pre-commit-config.yaml file in the project root directory contains a configuration for pre-commit hooks. To install the pre-commit hooks defined therein in your local git repository, run:

uv run pre-commit install

All pre-commit hooks configured in .pre-commit-config.yaml will now run each time you commit changes.

pre-commit can also manually be invoked, at anytime, using:

uv run pre-commit run --all-files

To skip the pre-commit validation on commits (e.g. when intentionally committing broken code), run:

uv run git commit -m <MSG> --no-verify

To update the hooks configured in .pre-commit-config.yaml to their newest versions, run:

uv run pre-commit autoupdate

6. Test that the installation works

To test that the installation works, run pytest in the project root folder:

uv run pytest

Meta

Copyright (c) 2026 Claas Rostock. All rights reserved.

Claas Rostock - @LinkedIn - claas.rostock@dnv.com

Distributed under the MIT license. See LICENSE for more information.

https://github.com/ClaasRostock/parent-lookup

Contributing

1. Fork it (https://github.com/ClaasRostock/parent-lookup/fork)
2. Create an issue in your GitHub repo
3. Create your branch based on the issue number and type (git checkout -b issue-name)
4. Evaluate and stage the changes you want to commit (git add -i)
5. Commit your changes (git commit -am 'place a descriptive commit message here')
6. Push to the branch (git push origin issue-name)
7. Create a new Pull Request in GitHub

For your contribution, please make sure you follow the STYLEGUIDE before creating the Pull Request.