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updated docs strings and added README.md

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Vishesh 'ironeagle' Bangotra
2026-03-08 17:59:52 +05:30
parent 80f8defcc2
commit 1c48f58578
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78
openapi_first/__init__.py
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@@ -1,71 +1,77 @@
"""
# Summary
FastAPI OpenAPI First — strict OpenAPI-first application bootstrap for FastAPI.
---
## Summary
FastAPI OpenAPI First is a **contract-first infrastructure library** that
enforces OpenAPI as the single source of truth for FastAPI services.
The library removes decorator-driven routing and replaces it with
deterministic, spec-driven application assembly. Every HTTP route,
method, and operation is defined in OpenAPI first and bound to Python
handlers explicitly via operationId.
handlers explicitly via `operationId`.
---
## Installation
# Installation
Install using pip:
pip install openapi-first
```bash
pip install openapi-first
```
Or with Poetry:
poetry add openapi-first
```bash
poetry add openapi-first
```
---
## Quick start
# Quick Start
Minimal OpenAPI-first FastAPI application:
from openapi_first import app
import my_service.routes as routes
```python
from openapi_first import app
import my_service.routes as routes
api = app.OpenAPIFirstApp(
openapi_path="openapi.yaml",
routes_module=routes,
title="My Service",
version="1.0.0",
)
api = app.OpenAPIFirstApp(
openapi_path="openapi.yaml",
routes_module=routes,
title="My Service",
version="1.0.0",
)
```
OperationId-driven HTTP client:
from openapi_first.loader import load_openapi
from openapi_first.client import OpenAPIClient
```python
from openapi_first.loader import load_openapi
from openapi_first.client import OpenAPIClient
spec = load_openapi("openapi.yaml")
client = OpenAPIClient(spec)
spec = load_openapi("openapi.yaml")
client = OpenAPIClient(spec)
response = client.get_health()
response = client.get_health()
```
---
## Architecture
# Architecture
The library is structured around four core responsibilities:
- **loader**: Load and validate OpenAPI 3.x specifications (JSON/YAML)
- **binder**: Bind OpenAPI operations to FastAPI routes via operationId
- **app**: OpenAPI-first FastAPI application bootstrap
- **client**: OpenAPI-first HTTP client driven by the same specification
- **errors**: Explicit error hierarchy for contract violations
- `loader`: Load and validate OpenAPI 3.x specifications (JSON/YAML).
- `binder`: Bind OpenAPI operations to FastAPI routes via `operationId`.
- `app`: OpenAPI-first FastAPI application bootstrap.
- `client`: OpenAPI-first HTTP client driven by the same specification.
- `errors`: Explicit error hierarchy for contract violations.
---
## Public API
# Public API
The supported public API consists of the following top-level modules:
@@ -77,14 +83,14 @@ The supported public API consists of the following top-level modules:
---
## Design Guarantees
# Design Guarantees
- OpenAPI is the single source of truth
- No undocumented routes can exist
- No OpenAPI operation can exist without a handler or client callable
- All contract violations fail at application startup or client creation
- No hidden FastAPI magic or implicit behavior
- Deterministic, testable application assembly
- OpenAPI is the single source of truth.
- No undocumented routes can exist.
- No OpenAPI operation can exist without a handler or client callable.
- All contract violations fail at application startup or client creation.
- No hidden FastAPI magic or implicit behavior.
- Deterministic, testable application assembly.
---
"""

61
openapi_first/app.py
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@@ -1,32 +1,35 @@
"""
# Summary
OpenAPI-first application bootstrap for FastAPI.
---
## Summary
This module provides `OpenAPIFirstApp`, a thin but strict abstraction
that enforces OpenAPI as the single source of truth for a FastAPI service.
Notes:
**Core Principles:**
- The OpenAPI specification (JSON or YAML) defines the entire API surface
- Every operationId in the OpenAPI spec must have a corresponding Python handler function
- Handlers are plain Python callables (no FastAPI decorators)
- FastAPI route registration is derived exclusively from the spec
- FastAPI's autogenerated OpenAPI schema is fully overridden
- The OpenAPI specification (JSON or YAML) defines the entire API surface.
- Every `operationId` in the OpenAPI spec must have a corresponding
Python handler function.
- Handlers are plain Python callables (no FastAPI decorators).
- FastAPI route registration is derived exclusively from the spec.
- FastAPI's autogenerated OpenAPI schema is fully overridden.
**Responsibilities:**
- Loads and validates an OpenAPI 3.x specification
- Dynamically binds HTTP routes to handler functions using operationId
- Registers routes with FastAPI at application startup
- Ensures runtime behavior matches the OpenAPI contract exactly
- Loads and validates an OpenAPI 3.x specification.
- Dynamically binds HTTP routes to handler functions using `operationId`.
- Registers routes with FastAPI at application startup.
- Ensures runtime behavior matches the OpenAPI contract exactly.
**Constraints:**
- This module intentionally does NOT: Generate OpenAPI specs, generate client code, introduce a new framework or lifecycle, or alter FastAPI dependency injection semantics.
- This module intentionally does NOT:
- Generate OpenAPI specs.
- Generate client code.
- Introduce a new framework or lifecycle.
- Alter FastAPI dependency injection semantics.
"""
from fastapi import FastAPI
@@ -42,16 +45,19 @@ class OpenAPIFirstApp(FastAPI):
Notes:
**Responsibilities:**
- `OpenAPIFirstApp` subclasses FastAPI and replaces manual route registration with OpenAPI-driven binding
- All routes are derived from the provided OpenAPI specification, and each operationId is mapped to a Python function in the supplied routes module
- `OpenAPIFirstApp` subclasses `FastAPI` and replaces manual route
registration with OpenAPI-driven binding.
- All routes are derived from the provided OpenAPI specification,
and each `operationId` is mapped to a Python function in the
supplied routes module.
**Guarantees:**
- No route can exist without an OpenAPI declaration
- No OpenAPI operation can exist without a handler
- Swagger UI and `/openapi.json` always reflect the provided spec
- Handler functions remain framework-agnostic and testable
- No route can exist without an OpenAPI declaration.
- No OpenAPI operation can exist without a handler.
- Swagger UI and `/openapi.json` always reflect the provided spec.
- Handler functions remain framework-agnostic and testable.
Example:
```python
from openapi_first import OpenAPIFirstApp
@@ -77,15 +83,20 @@ class OpenAPIFirstApp(FastAPI):
Args:
openapi_path (str):
Filesystem path to the OpenAPI 3.x specification file. This specification is treated as the authoritative API contract.
Filesystem path to the OpenAPI 3.x specification file. This
specification is treated as the authoritative API contract.
routes_module (module):
Python module containing handler functions whose names correspond exactly to OpenAPI operationId values.
Python module containing handler functions whose names correspond
exactly to OpenAPI `operationId` values.
**fastapi_kwargs (Any):
Additional keyword arguments passed directly to `fastapi.FastAPI` (e.g., title, version, middleware, lifespan handlers).
Additional keyword arguments passed directly to
`fastapi.FastAPI` (e.g., title, version, middleware, lifespan
handlers).
Raises:
OpenAPIFirstError:
If the OpenAPI specification is invalid, or if any declared operationId does not have a corresponding handler function.
If the OpenAPI specification is invalid, or if any declared
`operationId` does not have a corresponding handler function.
"""
# Initialize FastAPI normally
super().__init__(**fastapi_kwargs)

47
openapi_first/binder.py
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@@ -1,33 +1,35 @@
"""
# Summary
OpenAPI-driven route binding for FastAPI.
---
## Summary
This module is responsible for translating an OpenAPI 3.x specification
into concrete FastAPI routes. It enforces a strict one-to-one mapping
between OpenAPI operations and Python handler functions using operationId.
between OpenAPI operations and Python handler functions using `operationId`.
Notes:
**Core Responsibility:**
- Read path + method definitions from an OpenAPI specification
- Resolve each operationId to a Python callable
- Register routes with FastAPI using APIRoute
- Fail fast when contract violations are detected
- Read path + method definitions from an OpenAPI specification.
- Resolve each `operationId` to a Python callable.
- Register routes with FastAPI using `APIRoute`.
- Fail fast when contract violations are detected.
**Design Constraints:**
- All routes MUST be declared in the OpenAPI specification
- All OpenAPI operations MUST define an operationId
- Every operationId MUST resolve to a handler function
- Handlers are plain Python callables (no decorators required)
- No implicit route creation or inference is allowed
- All routes MUST be declared in the OpenAPI specification.
- All OpenAPI operations MUST define an `operationId`.
- Every `operationId` MUST resolve to a handler function.
- Handlers are plain Python callables (no decorators required).
- No implicit route creation or inference is allowed.
**Constraints:**
- This module intentionally does NOT: Perform request or response validation, generate Pydantic models, modify FastAPI dependency injection, or interpret OpenAPI semantics beyond routing metadata.
- This module intentionally does NOT:
- Perform request or response validation.
- Generate Pydantic models.
- Modify FastAPI dependency injection.
- Interpret OpenAPI semantics beyond routing metadata.
"""
from fastapi.routing import APIRoute
@@ -45,21 +47,26 @@ def bind_routes(app, spec: dict, routes_module) -> None:
spec (dict):
Parsed OpenAPI 3.x specification dictionary.
routes_module (module):
Python module containing handler functions. Each handler's name MUST exactly match an OpenAPI operationId.
Python module containing handler functions. Each handler's name MUST
exactly match an OpenAPI `operationId`.
Raises:
MissingOperationHandler:
If an operationId is missing from the spec or if no corresponding handler function exists in the routes module.
If an `operationId` is missing from the spec or if no corresponding
handler function exists in the routes module.
Notes:
**Responsibilities:**
- Iterates through the OpenAPI specification paths and methods
- Resolves each operationId to a handler function, and registers a corresponding APIRoute on the FastAPI application
- Iterates through the OpenAPI specification paths and methods.
- Resolves each `operationId` to a handler function, and registers
a corresponding `APIRoute` on the FastAPI application.
**Guarantees:**
- Route registration is deterministic and spec-driven. No route decorators are required or supported. Handler resolution errors surface at application startup.
- Route registration is deterministic and spec-driven. No route
decorators are required or supported. Handler resolution errors
surface at application startup.
"""
paths = spec.get("paths", {})

22
openapi_first/client.py
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@@ -1,10 +1,8 @@
"""
# Summary
OpenAPI-first HTTP client for contract-driven services.
---
## Summary
This module provides `OpenAPIClient`, a thin, strict HTTP client that
derives all callable operations directly from an OpenAPI 3.x specification.
@@ -49,20 +47,22 @@ class OpenAPIClientError(OpenAPIFirstError):
class OpenAPIClient:
"""
OpenAPI-first HTTP client (httpx-based).
OpenAPI-first HTTP client (`httpx`-based).
Notes:
**Responsibilities:**
- This client derives all callable methods directly from an OpenAPI 3.x specification. Each operationId becomes a method on the client instance.
- This client derives all callable methods directly from an
OpenAPI 3.x specification. Each `operationId` becomes a method
on the client instance.
**Guarantees:**
- One callable per operationId
- Explicit parameters (path, query, headers, body)
- No implicit schema inference or mutation
- Returns raw `httpx.Response` objects
- No response validation or deserialization
- One callable per `operationId`.
- Explicit parameters (path, query, headers, body).
- No implicit schema inference or mutation.
- Returns raw `httpx.Response` objects.
- No response validation or deserialization.
Example:
```python

30
openapi_first/errors.py
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@@ -1,10 +1,8 @@
"""
# Summary
Exceptions for OpenAPI-first FastAPI applications.
---
## Summary
This module defines a small hierarchy of explicit, intention-revealing
exceptions used to signal contract violations between an OpenAPI
specification and its Python implementation.
@@ -12,10 +10,10 @@ specification and its Python implementation.
Notes:
**Design Principles:**
- Errors represent programmer mistakes, not runtime conditions
- All errors are raised during application startup
- Messages are actionable and suitable for CI/CD output
- Exceptions are explicit rather than reused from generic built-ins
- Errors represent programmer mistakes, not runtime conditions.
- All errors are raised during application startup.
- Messages are actionable and suitable for CI/CD output.
- Exceptions are explicit rather than reused from generic built-ins.
These errors should normally cause immediate application failure.
"""
@@ -27,8 +25,11 @@ class OpenAPIFirstError(Exception):
Notes:
**Responsibilities:**
- This exception exists to allow callers, test suites, and CI pipelines to catch and distinguish OpenAPI contract violations from unrelated runtime errors
- All exceptions raised by the OpenAPI-first core should inherit from this type
- This exception exists to allow callers, test suites, and CI
pipelines to catch and distinguish OpenAPI contract violations
from unrelated runtime errors.
- All exceptions raised by the OpenAPI-first core should inherit
from this type.
"""
pass
@@ -40,12 +41,15 @@ class MissingOperationHandler(OpenAPIFirstError):
Notes:
**Scenarios:**
- An OpenAPI operation does not define an operationId
- An operationId is defined but no matching function exists in the provided routes module
- An OpenAPI operation does not define an `operationId`.
- An `operationId` is defined but no matching function exists in
the provided routes module.
**Guarantees:**
- This represents a violation of the OpenAPI-first contract and indicates that the specification and implementation are out of sync
- This represents a violation of the OpenAPI-first contract and
indicates that the specification and implementation are out of
sync.
"""
def __init__(self, *, path: str, method: str, operation_id: str | None = None):

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openapi_first/loader.py
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@@ -1,10 +1,8 @@
"""
# Summary
OpenAPI specification loading and validation utilities.
---
## Summary
This module is responsible for loading an OpenAPI 3.x specification
from disk and validating it before it is used by the application.
@@ -14,14 +12,18 @@ must never reach the routing or runtime layers.
Notes:
**Design Principles:**
- OpenAPI is treated as an authoritative contract
- Invalid specifications fail fast at application startup
- Supported formats are JSON and YAML
- Validation errors are surfaced clearly and early
- OpenAPI is treated as an authoritative contract.
- Invalid specifications fail fast at application startup.
- Supported formats are JSON and YAML.
- Validation errors are surfaced clearly and early.
**Constraints:**
- This module intentionally does NOT: Modify the OpenAPI document, infer missing fields, generate models or code, or perform request/response validation at runtime.
- This module intentionally does NOT:
- Modify the OpenAPI document.
- Infer missing fields.
- Generate models or code.
- Perform request/response validation at runtime.
"""
import json
@@ -41,7 +43,8 @@ class OpenAPISpecLoadError(OpenAPIFirstError):
Notes:
**Guarantees:**
- This error indicates that the OpenAPI document is unreadable, malformed, or violates the OpenAPI 3.x specification
- This error indicates that the OpenAPI document is unreadable,
malformed, or violates the OpenAPI 3.x specification.
"""
pass
@@ -52,7 +55,8 @@ def load_openapi(path: str | Path) -> dict[str, Any]:
Args:
path (str | Path):
Filesystem path to an OpenAPI specification file. Supported extensions: .json, .yaml, .yml.
Filesystem path to an OpenAPI specification file. Supported
extensions: `.json`, `.yaml`, `.yml`.
Returns:
dict[str, Any]:
@@ -60,13 +64,16 @@ def load_openapi(path: str | Path) -> dict[str, Any]:
Raises:
OpenAPISpecLoadError:
If the file does not exist, cannot be parsed, or fails OpenAPI schema validation.
If the file does not exist, cannot be parsed, or fails OpenAPI
schema validation.
Notes:
**Guarantees:**
- The specification is parsed based on file extension and validated using a strict OpenAPI schema validator
- Any error results in an immediate exception, preventing application startup
- The specification is parsed based on file extension and validated
using a strict OpenAPI schema validator.
- Any error results in an immediate exception, preventing
application startup.
"""
spec_path = Path(path)