headgate

An async API-client base for Python, built on httpx2. Subclass AsyncBaseClient, declare what each endpoint answers, and every client you build gets the same production behaviour: retries that re-send only when it is safe, a shared rate budget that listens to the server, a per-origin circuit breaker, whole-call deadlines, credentials that never leave your origin, an opt-in cache that never stores them, typed events and OpenTelemetry, and errors that say exactly what happened. Each endpoint method stays one line.

A headgate controls how much water gets through a channel. This library does the same for requests, in one place, so that no client has to get it right on its own.

Requirements and install

Python 3.14 or newer (endpoint paths are t-strings) and httpx2, pydantic's fork of httpx. Upstream httpx is not supported: passing one of its objects raises a TypeError naming the mistake.

pip install headgate               # or: uv add headgate. Brings httpx2[http2], hishel and msgspec
pip install 'headgate[pydantic]'   # + the pydantic codec flavor
pip install 'headgate[curl]'       # + AsyncCurlTransport: a browser's TLS fingerprint, via curl_cffi
pip install 'headgate[otel]'       # + OpenTelemetryObserver (opentelemetry-api only)
pip install 'headgate[cache]'      # + what hishel's AsyncSqliteStorage needs (hishel[async]: anysqlite)

Quickstart

A client is a subclass that fixes its configuration in super().__init__() and exposes one method per endpoint. A path is a t-string, so every value is escaped as one segment; expect= declares what the endpoint answers.

from http import HTTPStatus
from typing import TYPE_CHECKING

import msgspec

from headgate import AsyncBaseClient, Cursor, Expect, HttpOptions, Rate, StaticHeaderAuth, nothing
from headgate.msgspec import decoder

if TYPE_CHECKING:
    import httpx2

    from headgate import Paginated


class Series(msgspec.Struct, frozen=True):
    """A data series: its id, title and observations."""

    id: str
    title: str
    values: list[float]


class Release(msgspec.Struct, frozen=True):
    """A scheduled data release."""

    id: int
    name: str


class ReleasePage(msgspec.Struct, frozen=True):
    """One page of releases, and the cursor of the next page (``None`` on the last)."""

    releases: list[Release]
    next_cursor: str | None = None


def releases_on(page: ReleasePage) -> list[Release]:
    """Return the releases on *page*."""
    return page.releases


def cursor_after(page: ReleasePage) -> str | None:
    """Return the cursor of the page after *page*."""
    return page.next_cursor


# Readers and pagers are values: build them once, at import, where a bad declaration fails.
SERIES = decoder(Series)
SERIES_OR_NONE = Expect(SERIES).on(HTTPStatus.NOT_FOUND, nothing)  # the API documents 404 as "no such series"
RELEASE_PAGES = Cursor(items=releases_on, next=cursor_after, param="cursor")


class CatalogClient(AsyncBaseClient):
    """The catalog API: one method per endpoint, each one line."""

    def __init__(self, api_key: str, *, transport: httpx2.AsyncBaseTransport | None = None) -> None:
        """Authenticate with *api_key*; *transport* replaces the network, as a test does."""
        super().__init__(
            base_url="https://api.catalog.example/v1",
            rate=Rate.per_second(10),
            auth=StaticHeaderAuth({"Authorization": f"Bearer {api_key}"}),
            http=HttpOptions(transport=transport),
        )

    async def get_series(self, series_id: str) -> Series:
        """Return the series *series_id*; a 404, undeclared here, raises ``UpstreamStatusError``."""
        return await self.get(t"series/{series_id}", expect=SERIES)

    async def find_series(self, series_id: str) -> Series | None:
        """Return the series *series_id*, or ``None`` when the API answers 404."""
        return await self.get(t"series/{series_id}", expect=SERIES_OR_NONE)

    def releases(self) -> Paginated[ReleasePage, Release]:
        """Return every release, fetched a page at a time as the iteration reaches it."""
        return self.paginate("releases", expect=decoder(ReleasePage), pages=RELEASE_PAGES)

Use it with async with, which closes the connection pool on the way out:

async def main(api_key: str) -> tuple[float, Series | None, list[str]]:
    """Read one series, look up one that may not exist, and list every release."""
    async with CatalogClient(api_key) as catalog:
        gdp = await catalog.get_series("GDP")
        retired = await catalog.find_series("RETIRED")  # None: the API answered 404
        names = [release.name async for release in catalog.releases()]
    return gdp.values[-1], retired, names

Nothing in those methods mentions retries, rate limits, timeouts or credentials, and nothing needs to: get_series("GDP") is paced by the client's budget, bounded by a 60-second deadline, retried on a 503 or a dropped connection, refused fast while the origin's breaker is open, and fails with an error that names the client, the request line, the status, the request id and every attempt. examples/ has this client, the same client in both codec flavors, and a fuller one (orders_client.py): a minted OAuth token, an idempotent POST, a declared business refusal, Link-header pages and a streamed export.

What every client gets

How the pieces fit together, for contributors and the curious: design.

The contracts

1. Declared answers, or raise. A call returns what its reader read from a 2xx or from a status it declared, and raises a ClientError for anything else. There is no raw mode and no None for "it failed".
2. Transport only. A client does I/O and decoding. Retries, pacing and the breaker live in the transport stack; business decisions belong to the caller.
3. Wire types are data. Responses decode into plain typed values (msgspec structs or pydantic models) with no behaviour.
4. No logging; typed events instead. headgate writes no log records. It reports to the observers you install, with sanitized data only, and its errors carry what you need to diagnose a failure.
5. No environment reads. Credentials are passed in. No proxy variable, .netrc or SSL_CERT_FILE is read: the HTTP client and its transport are built with trust_env=False.

Configuration

AsyncBaseClient(*, base_url, rate, auth=None, policy=TransportPolicy(), http=HttpOptions()). rate is a Rate (a budget of the client's own) or a RateBudget shared with other clients. TransportPolicy says what the stack does to every request, HttpOptions how requests are sent and read. Both are frozen and validate on construction.

class OrdersClient(AsyncBaseClient):
    """The shop's order API."""

    def __init__(
        self,
        client_id: str,
        client_secret: str,
        *,
        observers: tuple[Observer, ...] = (),
        transport: httpx2.AsyncBaseTransport | None = None,
    ) -> None:
        """Mint tokens with *client_id* and *client_secret*; report to *observers*; *transport* replaces the network."""
        super().__init__(
            base_url="https://api.shop.example/v2",
            rate=Rate.per_second(20),
            auth=ShopTokenAuth(client_id, client_secret),
            policy=TransportPolicy(
                retry=RetryPolicy(attempts=4),  # four sends at most, the first included
                deadline=30.0,  # each call, every attempt and wait included
                idempotency=IdempotencyKeys(),  # every POST and PATCH carries a key, so it is retried safely
                rate_limits=(RateRule("/exports", Rate.per_minute(6)),),  # a slower lane for the export endpoint
                routes=(Route("/exports", deadline=300.0),),  # and a longer deadline
            ),
            http=HttpOptions(
                transport=transport,
                timeout=Timeouts(read=20.0),
                user_agent="shop-sync/1.4",
                observers=observers,
            ),
        )

Every option, its validation and the precedence of call, view, route and policy: configuration.

Per-call options

Every verb (get, post, put, patch, delete, head) takes the URL, expect=, and optionally retry=False or a RetryPolicy, follow_redirects=, cache_ttl=, timeout=, deadline=, idempotency_key=, and the request options params=, headers=, json=, data=, content= and files=. client.with_options(headers=..., timeout=..., deadline=..., retry=...) returns a view with other defaults that shares the pool, the budgets and the breakers; the client it came from owns the pool.

Errors

Every failure is a ClientError, so an application boundary can map the tree onto its own answers:

ClientError                      also: an unparseable URL, a pagination loop
├── TransportError               a network failure that outlasted retries
│   ├── ClientTimeoutError       a timeout, or the call's deadline
│   └── CircuitOpenError         the origin's breaker refused the call unsent
├── UpstreamStatusError          an undeclared status; a 5xx that outlasted retries; a refusal in a body
│   ├── AuthError                401 or 403: our credential was refused
│   └── RateLimitError           429, a quota refusal, or a server pause past the deadline
├── ResponseParseError           a 2xx body the reader could not read
├── ResponseTooLargeError        a body past max_response_bytes
├── ForbiddenDestinationError    a hop the SSRF guard refused
└── ClientConfigurationError     our own setup is wrong

Every error carries method, url (credentials redacted), request_id, attempts, retryable, retry_after and a note per attempt (attempt 2/3: 503 Service Unavailable (Retry-After 2.0s) after 0.41s). An UpstreamStatusError adds status_code, the provider's code and its RFC 9457 problem. All of them pickle.

def gateway_status(error: ClientError) -> HTTPStatus:
    """Return the status an API gateway answers with when a call to an upstream failed with *error*."""
    match error:
        case ClientTimeoutError():
            return HTTPStatus.GATEWAY_TIMEOUT  # the upstream was too slow, or the call's deadline passed
        case ClientConfigurationError():
            return HTTPStatus.SERVICE_UNAVAILABLE  # our own setup is wrong; an operator must act
        case _:
            return HTTPStatus.BAD_GATEWAY  # the upstream failed, refused us, or sent something unreadable

More in errors.

Testing your client

headgate.testing tests a client through its real stack, with nothing private patched: a FakeTransport stands in for the network, and a TestClock makes every wait instant.

import httpx2
from quickstart import CatalogClient

from headgate.testing import FakeTransport, TestClock, isolated_breakers

GDP = {"id": "GDP", "title": "Gross domestic product", "values": [27.4, 28.1]}


async def test_find_series_reads_a_404_as_none() -> None:
    """Return ``None`` for the 404 the endpoint documents, signing the request with the key."""
    fake = FakeTransport().route("GET", "/v1/series/RETIRED", 404)
    async with CatalogClient("test-key", transport=fake) as catalog:
        assert await catalog.find_series("RETIRED") is None
    assert fake.calls[0].headers["Authorization"] == "Bearer test-key"


async def test_a_503_is_retried_after_the_wait_it_asks_for() -> None:
    """Retry a 503 once its ``Retry-After`` has passed, on a test clock that makes the wait instant."""
    fake = FakeTransport().route(
        "GET",
        "/v1/series/GDP",
        httpx2.Response(503, headers={"Retry-After": "2"}),
        httpx2.Response(200, json=GDP),
    )
    clock = TestClock()
    with clock.install(), isolated_breakers():  # the clients built in the block use them
        catalog = CatalogClient("test-key", transport=fake)
    async with catalog:
        series = await catalog.get_series("GDP")
    assert series.values == [27.4, 28.1]
    assert clock.sleeps == [2.0]

Fault schedules, cassettes and switching the policy off: testing.

Codec flavors

The core imports no codec. A client picks one by import, and both flavors expose decode, decoder and to_builtins with the same behaviour: strict typing (a field that changes type fails), unknown fields ignored, and a mismatch raised as DecodeError, which the client turns into ResponseParseError.

Development

uv sync                                   # Python 3.14 and every dev dependency
uv run ruff check && uv run ruff format --check
uv run ty check
uv run python -m pytest --cov             # 100% line and branch coverage is required
uv run python -m pytest benchmarks        # the benchmarks, as CI runs them
uv run mutmut run                         # mutation testing; slow, run it before a release

Lint selects every ruff rule, ty treats every rule as an error, and suppression comments are not accepted. See CONTRIBUTING.md for the rules a change must keep, CODE_OF_CONDUCT.md for how we work together, and SECURITY.md to report a vulnerability privately. The changes in each release are in CHANGELOG.md.

Licence

MIT. src/headgate/_curl.py is derived from httpx-curl-cffi (BSD-3-Clause); its notice is kept in the file.