graphql-codegen

graphql-codegen turns a GraphQL schema and your operations into a typed Python client with:

• no imposed transport;
• no validation overhead;
• no runtime dependencies¹.

Installation

uv add --dev graphql-codegen

The generated client depends on nothing but the standard library¹.

Quick start

Every example of this README is a file of bookshop, whose schema is schema.graphqls:

type Query {
  # …
  "The order with this identifier."
  order(id: ID!): Order
  # …
}

enum OrderStatus {
  PENDING
  SHIPPED
  DELIVERED
  CANCELED
}

type Order {
  id: ID!
  status: OrderStatus!
  # …
}

Write your operations in .graphql files:

query GetOrder($id: ID!) {
  order(id: $id) {
    id
    status
  }
}

The generator reads graphql-config, the file GraphQL editor extensions and linters already use²:

# An SDL file here, but globs, introspection results, and server URLs work too.
schema: schema.graphqls
documents: "*.graphql"
# This library's config, under its extension name.
extensions:
  pythonCodegen:
    # Where imports start, relative to this file's directory.
    moduleRoot: ..
    # Dotted name from the module root, so written to `bookshop/client`.
    package: bookshop.client

Generate the client:

graphql-codegen bookshop/graphql.config.yml

The generated package holds only the enum and input types the operations reach, so that it grows with your documents rather than with the schema. Each GraphQL document also gets its own Python module holding its operations (get_order_graphql.py here). Run them over any transport; your type checker verifies every variable and every field of the response:

from typing import Literal, assert_never, assert_type

from bookshop.client.runtime import Client
from bookshop.client.schema import OrderStatus
from bookshop.get_order_graphql import GetOrder
from bookshop.transport import transport

client = Client(transport)
data = client(GetOrder({"id": "o1"}))
order = data["order"]

# `Query.order`'s type is nullable so the type checker requires this test.
if order is None:
    print("No such order.")
else:
    # `Order.id: ID!` is a `str` on the wire.
    assert_type(order["id"], str)

    # An `enum` gets a generated alias of the `Literal` of its values.
    assert_type(order["status"], OrderStatus)
    assert_type(order["status"], Literal["PENDING", "SHIPPED", "DELIVERED", "CANCELED"])

    if "total" in order:
        # A field not selected in the GraphQL operation can never be there.
        assert_never(order)

    print(f"Order {order['id']} is {order['status']}.")

Typing

Type checking, not runtime validation

GraphQL is strongly typed, and the server:

• validates each operation against its schema before running it;
• responds with exactly the operation's shape.

Client-side validation of responses thus mostly adds overhead³. What a Python client lacks is the other half: knowing, while you write order["status"], that the key exists and holds an OrderStatus. That is a type checker's job, done once, before the code runs.

This library therefore generates exact types for your type checker and leaves each response as decoded from JSON. Only custom scalars with a codec are converted, and only fields asserted non-null are checked.

Operation types

Each operation gets a type for its variables and one for its data, both keyed by the names the GraphQL document uses. Wherever GraphQL lets a value be one of several things, its Python type is a union that a match checks for exhaustiveness:

• a selection on a union or an interface is one of several types, and becomes one type per concrete type, told apart by __typename (which the generator selects for you);
• an enum is one of several values, and becomes a closed Literal;
• a @oneOf input is one of several fields, and becomes a union of single-key types, so that a value with two keys fails type checking.

For instance, app.graphql selects a publication's length:

• in pages when it is printed;
• in minutes when it is an audiobook:

query GetPublication($id: ID!) {
  publication(id: $id) {
    title
    ... on Printed {
      pages: pageCount
      ... on Book {
        isbn
      }
    }
    ... on Audiobook {
      duration
    }
  }
}

Testing for a key narrows it to every type selecting that key, even one the server adds later. A match on __typename narrows it to one type:

def length(publication_id: str, /, *, client: Client) -> str:
    data = client(GetPublication({"id": publication_id}))
    publication = data["publication"]

    if publication is None:
        return "No such publication."

    if "pages" in publication:
        return f"{publication['title']} has {publication['pages']} pages."

    match publication["__typename"]:
        case "Audiobook":
            return f"{publication['title']} lasts {publication['duration']} minutes."
        case _ as never:
            assert_never(never)

A match handles a value it does not know as you see fit, with a case _: arm:

def status_label(status: OrderStatus, /) -> str:
    match status:
        case "PENDING":
            return "Being prepared"
        case "SHIPPED":
            return "On its way"
        case "DELIVERED":
            return "Delivered"
        case "CANCELED":
            return "Canceled"
        case _:
            # A member added after this client was generated.
            return "Unknown"

Or with case _ as never: assert_never(never), as the match on __typename above does, so that the type checker points at every match the addition misses once the client is regenerated.

No name clashes

Nothing prevents a schema or a document from using names that clash with Python keywords (class), standard library names (list, Literal), or the generator's own helpers.

The names the generator adds itself, such as _builtins or _GetBookData_book, are spelled around every name a module holds, so no name can shadow another, whatever names the schema and the documents use.

Client

Sans-IO

The small sans-IO runtime is copied into the generated package, and its public API is limited to:

from .client import (
    AsyncClient as AsyncClient,
    AsyncSubscriptionClient as AsyncSubscriptionClient,
    Client as Client,
    SubscriptionClient as SubscriptionClient,
)
from .error import (
    ClientError as ClientError,
    Error as Error,
    ExecutionError as ExecutionError,
    Location as Location,
    ProtocolError as ProtocolError,
    RequestError as RequestError,
    ResponseError as ResponseError,
    UnexpectedNullError as UnexpectedNullError,
)
from .injection import OMITTED as OMITTED
from .operation import Operation as Operation, Request as Request

A transport is a function from a request body to a response body, so any HTTP client, synchronous or asynchronous, works, and so does anything else that carries bytes. Client, AsyncClient, SubscriptionClient, and AsyncSubscriptionClient take the same generated operations, so one generation serves both synchronous and asynchronous code. Each client forwards every argument but the first (the request) to its transport, type checked against the transport's signature.

As an example, the bookshop's asynchronous transport uses httpx2 and accepts a timeout (and nothing else):

http = httpx2.AsyncClient(base_url="https://bookshop.example")
HEADERS = {"Accept": mime_type.GRAPHQL_RESPONSE, "Content-Type": mime_type.JSON}


async def transport(body: bytes, /, *, timeout: float | None = None) -> bytes:
    response = await http.post(
        "/graphql", content=body, headers=HEADERS, timeout=timeout
    )

    # GraphQL over HTTP sends a request error as a response with a 4xx status.
    if not response.headers.get("Content-Type", "").startswith(
        mime_type.GRAPHQL_RESPONSE
    ):
        response.raise_for_status()

    return response.content

A call through a client over this transport can thus pass a timeout:

from bookshop.app_graphql import GetBook
from bookshop.async_transport import AsyncClient
from bookshop.scalar import ISBN


async def title(isbn: ISBN, /, *, client: AsyncClient) -> str:
    data = await client(GetBook({"lookup": {"isbn": isbn}}), timeout=5.0)
    return data["book"]["title"]

async_transport.py also streams a subscription's Server-Sent Events, and transport.py does both with the standard library alone.

Subscriptions

A subscription client works over any transport yielding one body per event, Server-Sent Events, graphql-transport-ws, or multipart HTTP alike:

def watch(order_id: str, /, *, client: SubscriptionClient) -> list[OrderStatus]:
    """Follow the order until it is delivered, and return its statuses."""
    statuses: list[OrderStatus] = []
    events = client(OnOrderStatusChanged({"orderId": order_id}))

    # Closing the stream, however the loop ends, unsubscribes.
    with closing(events):
        for event in events:
            statuses.append(event["orderStatusChanged"]["status"])

            if statuses[-1] == "DELIVERED":
                break

    return statuses

Merging

Merging composes requests at runtime from operations written ahead of time, so every result keeps its exact type. A document built at runtime could only be typed loosely.

A tuple of queries, or of mutations, runs in one call to the transport, each result typed by its own operation:

def book_and_similar(
    isbn: ISBN, text: str, /, *, client: Client
) -> tuple[str, list[str]]:
    # Two queries in one call to the transport.
    book_data, search_data = client(
        (
            GetBook({"lookup": {"isbn": isbn}}),
            Search({"text": text}),
        )
    )
    assert_type(book_data, GetBookData)
    assert_type(search_data, SearchData)

A list built at runtime also runs in one call to the transport, whatever its length. Its results then share one type, the union of its operations' data types:

def look_up(
    isbns: Sequence[ISBN], publication_ids: Sequence[str], /, *, client: Client
) -> tuple[GetBookData | GetPublicationData, ...]:
    """Fetch the books, then the publications, in one call to the transport."""
    return client(  # ty: ignore[unsound-return-statement]
        [
            *(GetBook({"lookup": {"isbn": isbn}}) for isbn in isbns),
            *(GetPublication({"id": id_}) for id_ in publication_ids),
        ]
    )

Errors

A response with errors raises an ExecutionError:

class ExecutionError(ResponseError, Generic[_Data_co]):
    """The server raised errors executing the request, but sent the rest of the data.

    A field that raised is `null`, as is its nearest nullable parent if it is non-null.
    """

    data: Final[Mapping[str, object] | None]

    def parse_data(self) -> _Data_co | None:
        """Return the data converted as in a response without errors, in a fresh copy.

When several operations were merged, an ExceptionGroup holds one for each operation that failed.

You can also have the client return the error instead of raising it, by calling returning_error() on the request:

• the result is then typed as either the data or the error, which you tell apart before using it;
• in a merge, you choose for each request whether its error is returned or raised;
• a subscription carries on past an event with errors.

def cancel(order_ids: list[str], /, *, client: Client) -> list[str]:
    """Cancel the orders in one call to the transport, and explain each failure."""
    results = client(
        [
            CancelOrder({"input": {"order": order_id}}).returning_error()
            for order_id in order_ids
        ]
    )
    # Each error has a note naming its operation and the variables sent.
    return [
        f"{error.__notes__[0]} {error!s}"
        for error in results
        if isinstance(error, ExecutionError)
    ]


def track(order_id: str, /, *, client: Client) -> str:
    result = client(GetOrder({"id": order_id}).returning_error())

    if isinstance(result, ExecutionError):
        data = result.parse_data()
        assert_type(data, GetOrderData | None)
        return f"Partially loaded: {data} ({result!s})."

Config

Custom scalars

Custom scalars travel as JSON values of the server's choosing, such as a date as a string:

extensions:
  pythonCodegen:
    # …
    # Each custom scalar's Python type, with its codec if any.
    scalars:
      DateTime:
        type: datetime.datetime
        codec:
          decode: ..scalar.decode_datetime
          encode: ..scalar.encode_datetime
      ISBN:
        type: ..scalar.ISBN
      Money:
        type: decimal.Decimal
        codec:
          decode: decimal.Decimal
          encode: str
      UUID:
        type: uuid.UUID
        codec:
          decode: uuid.UUID
          encode: str

• a path starting with .. is relative to the directory holding the package;
• a bare name, such as str, is a builtin;
• an unconfigured custom scalar is typed object.

Where no existing callable fits, write the codec yourself:

from datetime import datetime
from typing import NewType

ISBN = NewType("ISBN", str)
"""A string on the wire and in Python that a type checker tells apart from others."""


def decode_datetime(value: str, /) -> datetime:
    return datetime.fromisoformat(value)


def encode_datetime(value: datetime, /) -> str:
    return value.isoformat()

Each scalar becomes a type alias, and one with a codec carries it for the client, which converts its values on the way in and out:

import builtins as _builtins
import typing as _typing
from .runtime import _reflection
from datetime import datetime as _datetime
from ..scalar import decode_datetime as _decode_datetime
from ..scalar import encode_datetime as _encode_datetime
from ..scalar import ISBN as _ISBN
from decimal import Decimal as _Decimal
from uuid import UUID as _UUID

type DateTime = _typing.Annotated[_datetime, _reflection.Codec(decode=_decode_datetime, encode=_encode_datetime)]

type ISBN = _ISBN

type Money = _typing.Annotated[_Decimal, _reflection.Codec(decode=_Decimal, encode=_builtins.str)]

type UUID = _typing.Annotated[_UUID, _reflection.Codec(decode=_UUID, encode=_builtins.str)]

def order(book_id: str, address: Address, /, *, client: Client) -> str:
    data = client(
        PlaceOrder(
            {"input": {"lines": [{"book": book_id}], "shippingAddress": address}}
        )
    )
    order = data["placeOrder"]
    assert_type(order["total"], Decimal)
    assert_type(order["placedAt"], datetime)
    return f"Order {order['id']}: {order['total']:.2f} at {order['placedAt']:%H:%M}."

Non-null fields

A directive brings the idea of Client Controlled Nullability to any server, since only the generated code is aware of it:

extensions:
  pythonCodegen:
    # …
    # Client directive asserting a schema-nullable field is not null.
    nonNullDirectiveName: nonNull

Asserted on book, the field's type is then not optional:

query GetBook(
  "An identifier or an ISBN."
  $lookup: BookLookup!
  $withReviews: Boolean! = false
) {
  book(lookup: $lookup) @nonNull {
    ...BookCard
    isbn
    pageCount
    reviews @include(if: $withReviews) {
      rating
      text
      postedAt
    }
  }
}

class GetBookData(_compat.TypedDict, closed=True):
    book: _typing.Annotated[_GetBookData_book, _reflection.NON_NULL]
    """The book with this identifier or ISBN."""

def describe(isbn: ISBN, /, *, client: Client) -> str:
    variables: GetBookVariables = {"lookup": {"isbn": isbn}}

    try:
        data = client(GetBook(variables))
    except UnexpectedNullError as error:
        assert error.path == ["book"]
        assert error.__notes__ == [f"Raised by `GetBook` with variables {variables!r}."]
        return "No such book."

    book = data["book"]
    author = book["author"]
    by = "an anthology" if author is None else f"by {author['name']}"
    assert_type(book["price"], Decimal)
    return f"{book['title']}, {by}, costs {book['price']:.2f}."

Structs

A selection set has a fixed depth, so data of unbounded depth, such as a tree, can only come back as a JSON scalar. Following the Struct RFC, a Struct types that scalar with an input type, which may be recursive:

extensions:
  pythonCodegen:
    # …
    # Interface of object types carrying a JSON payload shaped like an input.
    structInterfaceName: Struct

type Query {
  # …
  "The filter of the saved search with this name, exactly as it was saved."
  savedSearch(name: String!): BookFilterStruct
}

"A JSON payload with the shape of the input type the implementation is named after."
interface Struct {
  value: JSON
}

type BookFilterStruct implements Struct {
  value: JSON
}

"The books meeting a condition, or a combination of conditions."
input BookFilter @oneOf {
  genre: Genre
  author: ID
  priceBelow: Money
  and: [BookFilter!]
  or: [BookFilter!]
  not: BookFilter
}

BookFilterStruct's payload is then typed by BookFilter (the input type named after the struct minus the interface's name) regardless of its depth:

class _GetSavedSearchData_savedSearch(_compat.TypedDict, closed=True):
    value: _input.BookFilter | None

The same definition can also type what is sent, as ListBooks takes a BookFilter too:

query ListBooks($filter: BookFilter, $first: Int) {
  books(filter: $filter, first: $first) {
    ...BookCard
    genre
  }
}

def run_saved_search(name: str, /, *, client: Client) -> list[str]:
    data = client(GetSavedSearch({"name": name}))
    search = data["savedSearch"]

    if search is None:
        return []

    # Sent back as is.
    books = client(ListBooks({"filter": search["value"]}))
    return [book["title"] for book in books["books"]]

Injectors

Some input values are client's business rather than each client() call's.

Take idempotency keys. They make retries safe: if the connection drops after the server placed an order, the transport sends it again, and the key, unique to the order, tells the server it already placed it rather than charging the customer twice. GraphQL has no built-in idempotency, so implementing it usually means adding the key as an argument or an input field of each mutation that needs it. When many different mutation operations require idempotency, it becomes the concern of all their client() calls, each having to get hold of a key. This applies to other concepts too, such as database transaction IDs.

An injector handles such a value in one place instead: when client is constructed. The client then passes it to every variable or input field with the name given in the config. No variables' type accepts it, so that no client() call can pass one by mistake:

extensions:
  pythonCodegen:
    # …
    # Values the client injects, which no call can pass.
    injectorNames: [idempotencyKey]

PlaceOrderInput holds one, for instance:

input PlaceOrderInput {
  "Makes placing the same order twice harmless: the client sends a new one per order."
  idempotencyKey: UUID

The generated package's injection module types the injectors you must supply:

import collections.abc as _abc
import typing as _typing
from .runtime import _compat
from .runtime import injection as _injection
from .runtime import OMITTED as _OMITTED
from . import _scalar

class InjectorFunctions(_compat.TypedDict, closed=True):
    """The functions supplying each injected value, by name.

    Where the value may be null, one returning `OMITTED` leaves it out, and one returning `None` sends `null`."""
    idempotencyKey: _typing.NotRequired[_abc.Callable[[], _scalar.UUID | None | _OMITTED]]

def injectors(functions: InjectorFunctions, /) -> _injection._Injectors:
    """Return what a client calls to supply the injected values, each serialized as its type says."""
    return _injection._Injectors(functions, injector_functions_type=InjectorFunctions)

The client gets its injector once, when built:

if __name__ == "__main__":
    client = Client(
        transport,
        injectors=injectors({"idempotencyKey": uuid4}),
    )

And no call passes a key:

def order(book_id: str, address: Address, /, *, client: Client) -> str:
    data = client(
        PlaceOrder(
            {"input": {"lines": [{"book": book_id}], "shippingAddress": address}}
        )
    )

The client injects new values on each call, so a retry belongs in the transport, which sends the same body, key included, again:

def transport(body: bytes, /, *, timeout: float | None = None) -> bytes:
    retries = 2

    while True:
        try:
            response = _post(
                "/graphql", body, accept=mime_type.GRAPHQL_RESPONSE, timeout=timeout
            )
        except HTTPError as error:
            # GraphQL over HTTP sends a request error as a response with a 4xx status.
            if error.headers.get_content_type() != mime_type.GRAPHQL_RESPONSE:
                raise

            response = error
        except ConnectionError:
            # The response was lost.
            if not retries:
                raise

            retries -= 1
            continue

        with response:
            return response.read()

Colocation

By default, each document's operations and fragments go into a module of the generated package's document subpackage, such as document/get_order.py for get_order.graphql, which the subpackage re-exports (lazily from Python 3.15). However, each generated operation is a module-level constant rather than a method of one client class, so it can live anywhere. In particular, it can live next to the GraphQL document it comes from, so that a feature's .graphql files, their generated modules, and the code calling their operations sit side by side and evolve together:

extensions:
  pythonCodegen:
    # …
    # Name pattern of each document's module, written next to the document.
    documentSiblingModule: "{document}_graphql"

This pattern puts get_order.graphql's operations and fragments in get_order_graphql.py. Code calling these operations imports them from there:

from bookshop.client.schema import OrderStatus
from bookshop.get_order_graphql import GetOrder

Python API

generate() does the same as the command:

class _Params(TypedDict, closed=True):
    document: DocumentNode
    schema: GraphQLSchema
    config: Config


def generate(**args: Unpack[_Params]) -> dict[PurePosixPath, bytes]:
    """Pure function returning the content of each file of the generated package by its path."""

The public API is limited to:

from graphql_codegen.config import Config as Config
from graphql_codegen.document_sibling_module import (
    DocumentSiblingModule as DocumentSiblingModule,
)
from graphql_codegen.generate import generate as generate
from graphql_codegen.package_location import PackageLocation as PackageLocation
from graphql_codegen.scalar import Codec as Codec, Scalar as Scalar