iron_gql

iron_gql is a GraphQL code generator and runtime. It reads a schema SDL and your query documents, and it generates a typed Python client with Pydantic models. You can connect GraphQL APIs to services, CLIs, background jobs, and tests without hand-written boilerplate.

Installation

pip install iron-gql            # runtime only (httpx + pydantic)
pip install iron-gql[codegen]   # + graphql-core for code generation

Key Features

• Query discovery. generate_gql_package scans your codebase for calls of the form <package>_gql("""..."""). It validates each statement and writes a module with typed helpers.
• Typed inputs and results. The generated Pydantic models match every selection set, enum, and input object that the discovered queries reference.
• Async runtime. runtime.GQLClient sends requests to GraphQL endpoints through httpx. When you set an ASGI target_app, the client calls the app in-process and does not use the network.
• Deterministic validation. graphql-core (a codegen dependency) validates every statement against the schema. It rejects operations that share a name but have different bodies.

Package Layout

• runtime.py contains the async GQLClient, the reusable GQLOperation base class, and the value serialization helpers.
• codegen/generate.py runs query discovery, validation, and module rendering.
• codegen/parser.py converts the GraphQL AST into typed helper structures for the renderer.

Getting Started

1. Describe your schema. Write the schema in an SDL file (schema.graphql). Include the root types that you use (query, mutation, subscription).
2. Write queries where you use them. Import the helper that the generator will create. Wrap each GraphQL statement in a call to this helper:

   from myapp.gql.client import client_gql
   
   get_user = client_gql(
       """
       query GetUser($id: ID!) {
           user(id: $id) {
               id
               name
           }
       }
       """
   )
   

   The generator derives the helper name (client_gql) from the package path that you ask it to build.
3. Generate the client module.

   from pathlib import Path
   
   from iron_gql.codegen import generate_gql_package
   
   generate_gql_package(
       schema_path=Path("schema.graphql"),
       package_full_name="myapp.gql.client",
       base_url_import="myapp.config:GRAPHQL_URL",
       scalars={"ID": "builtins:str"},
       to_camel_fn_full_name="myapp.inflection:to_camel",
       to_snake_fn=my_project_to_snake,
       debug_path=Path("iron_gql/debug/myapp.gql.client"),
       src_path=Path("."),
   )
   

   The call writes myapp/gql/client.py. The module contains:
   • an async client singleton,
   • Pydantic result and input models,
   • a query class per operation with typed execute methods,
   • overloads for the helper function, so editors can infer return types.
4. Call your API.

   async def fetch_user(user_id: str):
       query = get_user.with_headers({"Authorization": "Bearer token"})
       result = await query.execute(id=user_id)
       return result.user
   

Custom Scalars

The generator maps GraphQL scalars to Python types in two layers.

It maps built-in scalars automatically:

You configure custom scalars with the scalars parameter, in "module:type" format:

generate_gql_package(
    ...,
    scalars={
        "ID": "builtins:str",
        "Money": "decimal:Decimal",
        "ULID": "ulid:ULID",
    },
)

Custom scalar types must be Pydantic-compatible. That is, Pydantic must know how to parse the type from JSON (deserialization) and how to serialize it to JSON. Standard library types (datetime, Decimal, UUID, Enum) are compatible by default. Any type that implements __get_pydantic_core_schema__ is also compatible. The generator maps unknown scalars to object and writes a warning to the log.

Fragment Slots

Shared infrastructure code often owns a GraphQL operation, but it does not know which fields each caller needs on some field of that operation. A fragment slot lets each caller supply its own fragment for that field at call time. The operation does not have to name the fragments of its consumers in advance.

Mark a field with @slot in a query, mutation, or subscription. Do not use @slot inside a fragment definition. Give the field a static selection that selects __typename at the top level of its own selection set. This __typename must be unaliased and must have no directives. It must not come through an inline fragment or a fragment spread. The slot field itself cannot have @skip or @include: the operation always requests the slot field. If a caller wants no fragment data, it passes an empty list.

get_post_attachment = api_gql("""
    query GetPostAttachment($id: ID!) {
        post(id: $id) {
            id
            attachment @slot { __typename }
        }
    }
""")

A statement that holds exactly one fragment definition becomes a typed handle when some slot in the package can accept the fragment. A slot can accept a fragment when the fragment is spread-compatible with the type of the slot field. You can still spread the same fragment by name into other operations. For two kinds of statement, the helper returns a plain runtime.GQLOperation:

• a statement with one fragment definition that no slot accepts,
• a statement with several fragment definitions and no operation.

The fragments of these statements continue to work as building blocks for name spreads. They carry none of the obligations of a handle: self-containedness, a __typename of its own on polymorphic selections, and a non-empty selection. For a statement that contains an operation, the helper returns the class of that operation, as always.

IMAGE_URL = api_gql("""
    fragment ImageUrl on ImageAttachment {
        url
    }
""")

Pass a handle, or a sequence of handles, into execute. The keyword argument is the snake_case form of the name of the slot field, or of its alias. For example, mainAttachment @slot becomes main_attachment=. Then read the typed model of each fragment from the slot node with handle.read(node):

result = await get_post_attachment.execute(id="p-1", attachment=IMAGE_URL)
if result.post is not None:
    image = IMAGE_URL.read(result.post.attachment)
    if image is not None:
        print(image.url)

read returns None in exactly two situations:

• The node is None because the server sent null.
• The runtime type of the node is outside the fragment's own selection.

If you read with a handle that you never passed to that slot, read raises an error. It does not return None, because a wiring bug must not look like a legitimate mismatch.

Fragments are isolated from each other. Each fragment reads exactly its own selection. It never receives the fields that the fragment of another caller selected.

You can reach slot data only through read. The data is not part of the fields of the result model, so model_dump() does not include it. A dumped result does not round-trip. To validate the dump again, you must supply a fragments context. Without one, validation fails with an error. With or without one, the data of the fragments is gone.

For each slot field type, the generator writes one compatibility base class, named {FieldType}Fragment. With this class, shared code can be generic over any fragment that is compatible with that field. The shared code does not have to know the concrete shape of the fragment:

async def read_attachment[T: pydantic.BaseModel](
    post_id: str, fragment: AttachmentFragment[T]
) -> T | None:
    result = await get_post_attachment.execute(id=post_id, attachment=fragment)
    if result.post is None:
        return None
    return fragment.read(result.post.attachment)

A fragment that is not spread-compatible with the slot causes a type error. The type checker finds the mismatch before you ship your code.

The runtime validates every fragment that you pass into a slot at the response boundary. For queries and mutations, validation occurs inside execute. For subscriptions, validation occurs on each received message. Malformed data causes an immediate error. The malformed data never surfaces later from read.

Three rules apply:

• A handle must be self-contained. It cannot spread other fragments, and it cannot reference variables ($name). The handle travels to the server as its own text, next to an operation that declares nothing for it. Fragments that no slot accepts are free of both rules. They can spread other fragments by name, and they take their variables from the operations that spread them.
• An operation that declares a compatible slot cannot define or spread any fragment name that a handle ships. This covers the handle's own name and its transitive dependencies. The generator rejects the combination as soon as the handle exists, whether or not anyone passes the handle. To remove the conflict, rename one of the two. Operations without a compatible slot are outside this rule. The same fragment can work in both roles across different operations.
• The slot keyword argument in execute is mandatory, and there is no default. To send no fragments, pass an empty list explicitly.

Customization Hooks

• Naming conventions. Supply to_camel_fn_full_name (a module:path string) and a to_snake_fn callable. These functions align the casing with your own alias_generator.
• Endpoint configuration. The generator writes base_url_import verbatim into the generated module. Set it to a global string, a configuration object, or a helper that returns the GraphQL endpoint.

Runtime Highlights

• GQLClient accepts an ASGI target_app. You can use the same runtime for production HTTP calls and for in-process ASGI execution.
• GQLOperation.with_headers clones the operation object. The original does not change, so each call can have its own headers.
• Upload scalars map to iron_gql.FileVar. When variables contain FileVar instances, the client automatically sends a multipart upload (see the GraphQL multipart request spec).
• serialize_var converts variables to JSON-compatible structures with the Pydantic TypeAdapter. It supports custom scalar types, nested models, dicts, and lists.

Example

The example/ directory contains a complete working setup. It has a GraphQL schema with queries, mutations, enums, interfaces, unions, and fragments. It also has the generation script and sample query definitions. See example/generate.py for the codegen call, and example/main.py for query usage.

Testing

In tests, replace the generated client to send queries to a test server or an ASGI app. Use monkeypatch or any other patch of the module attribute:

from iron_gql import runtime
from myapp.gql import api

async def test_get_user(monkeypatch):
    test_client = runtime.GQLClient(
        base_url="http://testserver",
        target_app=my_asgi_app,
    )
    monkeypatch.setattr(api, "API_CLIENT", test_client)

    result = await get_user.execute(id="1")
    assert result.user.name == "Alice"

The generated query classes resolve the client by module attribute name at call time. As a result, the replacement is sufficient. The name of the attribute is always {PACKAGE}_CLIENT. For the package myapp.gql.api, the attribute is API_CLIENT.

Validation and Troubleshooting

• Error messages show the file and the line of the statement that caused the error.
• Operations that share a name must have identical bodies. To remove the conflict, rename the operations or merge them.
• The generated helper raises LookupError for a statement that it does not know. After you add or edit a statement, regenerate the package.