io-adapters

A small utility library for decoupling I/O from business logic by combining dependency injection with lightweight, automatically generated fakes.

Install

uv add io-adapters

API Reference

io-adapters API docs

Testing use cases that involve I/O is inherently difficult because they depend on:

• external state (filesystems, databases, services)

• side effects that are hard to observe directly

• slow or flaky infrastructure

A common mitigation is to combine:

• Dependency Injection (DI)

• The Repository / Adapter pattern

This allows business logic to depend on an abstract interface rather than concrete I/O.

However, in practice this usually requires:

• writing and maintaining bespoke fake implementations

• keeping fake behaviour in sync with real implementations

• duplicating boilerplate across domains

For small or medium-sized projects, this overhead can outweigh the benefits.

Simply register each I/O function with one of the register decorators and the functionality will be added to the RealAdapter object, on top of that a stub will be added to the FakeAdapter object too so you can pass in either to your usecase and the functionality will work.

Example

from enum import Enum
from pathlib import Path

from io_adapters import (
    IoAdapter,
    RealAdapter,
    add_domain,
    get_fake_adapter,
    get_real_adapter,
    register_domain_read_fn,
    register_domain_write_fn,
)


# you can use any hashable object to register an I/O function
class FileFormat(Enum):
    JSON = "json"


add_domain("orders")
add_domain("payment")


@register_domain_read_fn("orders", "str")
def read_str(path: str | Path, **kwargs: dict) -> str: ...


# stack decorators to register the same function to multiple domains
@register_domain_read_fn("orders", FileFormat.JSON)
@register_domain_read_fn("payment", FileFormat.JSON)
def read_json(path: str | Path, **kwargs: dict) -> dict: ...


@register_domain_write_fn("orders", "str")
def write_str(data: dict, path: str | Path, **kwargs: dict) -> None: ...


@register_domain_write_fn("orders", FileFormat.JSON)
@register_domain_write_fn("payment", FileFormat.JSON)
def write_json(data: dict, path: str | Path, **kwargs: dict) -> None: ...


def some_usecase(adapter: IoAdapter, path: str) -> None:
    adapter.read(path, "str")
    # Some business logic
    new_path = f"{path}_new.json"

    adapter.write({"a": 1}, new_path, FileFormat.JSON)


# in production inject the real adapter
orders_adapter: RealAdapter = get_real_adapter("orders")
some_usecase(orders_adapter, "some/path/to/file.json")


# in testing inject the fake which has all the same funcitonality as the 
# `RealAdapter` and assert that the fakes end state is as expected
fake = get_fake_adapter("orders")
some_usecase(fake, "some/path/to/file.json")
assert fake.files["some/path/to/file.json"] == {"a": 1}

Repo map

├── .github
│   └── workflows
│       ├── ci_tests.yaml
│       └── publish.yaml
├── .pytest_cache
│   └── README.md
├── docs
│   └── source
│       └── conf.py
├── src
│   └── io_adapters
│       ├── __init__.py
│       ├── _adapters.py
│       ├── _clock.py
│       ├── _container.py
│       ├── _io_funcs.py
│       └── _registries.py
├── tests
│   ├── __init__.py
│   ├── test_adapters.py
│   ├── test_adapters_apis.py
│   └── test_container.py
├── .pre-commit-config.yaml
├── README.md
├── pyproject.toml
├── ruff.toml
└── uv.lock
::