azure-functions-parser

A lightweight decorator that adds FastAPI-like request parsing and validation to Azure Functions HTTP triggers.

Features

• Automatic parsing and validation of query parameters
• Automatic parsing and validation of JSON request bodies using Pydantic models
• Automatic response serialization for different return types (dict, Pydantic models, strings, etc.)
• Zero configuration required - just add the decorator
• Full type hints and async support

Installation

pip install azure-functions-parser

Usage

import azure.functions as func
from azure.functions_parser import validate_request

@validate_request
def main(req: func.HttpRequest, some_query_param: int = 1):
    ...

Quick Start

You have an API endpoint that takes user data in the request body, and an optional query parameter.

Before

import azure.functions as func
import json

def main(req: func.HttpRequest) -> func.HttpResponse:
    body = req.get_json()
    try:
        name = body["name"]
        age = int(body["age"])
        format = req.params.get("format", "json")

        if format not in ["json", "csv"]:
            raise ValueError(f"Invalid argument for 'format': {format}")
    except Exception as e:
        return func.HttpResponse(str(e), status_code=400)

    return func.HttpResponse(
        json.dumps({"name": name, "age": age, "format": format}),
        status_code=200
    )

After

from typing import Literal
from pydantic import BaseModel
import azure.functions as func
from azure.functions_parser import validate_request

class User(BaseModel):
    name: str
    age: int

@validate_request
def main(req: func.HttpRequest, user: User, format: Literal["json", "csv"] = "json"):
    return {"name": user.name, "age": user.age, "format": format}

The request body, User, and query parameter, format, will be parsed and validated for incoming requests. If there's a validation error, a 400 response will be issued automatically, with descriptive errors.

Suppose you get an incoming request with the following data:

• body: {"age": 25.5} ('name' is missing, and 'age' is a float)
• params: ?format=xml (should be 'json' or 'csv')

The following error will be sent back to the client with a 400 code.

{
  "errors": [
    {
      "param": "format",
      "reason": "Input should be 'json' or 'csv'",
      "type": "literal_error",
      "input": "xml"
    },
    {
      "param": "name",
      "reason": "Field required",
      "type": "missing"
    },
    {
      "param": "age",
      "reason": "Input should be a valid integer, got a number with a fractional part",
      "type": "int_from_float",
      "input": 25.5
    }
  ]
}

Usage Guide

Query Parameters

Query parameters are automatically parsed based on your function's parameters:

@validate_request
def handler(req, name: str, age: int = 18, format: str = "json"):
    return {"name": name, "age": age, "format": format}

This will:

• Require name as a required query parameter
• Accept an optional age parameter that defaults to 18
• Accept an optional format parameter that defaults to "json"
• Automatically convert parameters to the correct type
• Return a 400 error with validation details if parameters are invalid or missing

Request Body

Use a Pydantic model to parse and validate the JSON request body. Your function may have at most one, and only one, parameter annotated as a Pydantic model.

from pydantic import BaseModel, EmailStr

class UserProfile(BaseModel):
    name: str
    age: int
    email: EmailStr
    interests: list[str] | None = None

@validate_request
def create_user(req: func.HttpRequest, user: UserProfile):
    return {"name": user.name, "age": user.age}

Response Handling

The decorator automatically handles different return types.

Any return value that is not a func.HttpResponse will be wrapped in a 200 OK response, with your return value as the response body.

@validate_request
def handler1(req: func.HttpRequest):
    return {"message": "OK"}  # Returns JSON response

@validate_request
def handler2(req: func.HttpRequest):
    return "Hello, World!"  # Returns plain text response

@validate_request
def handler3(req: func.HttpRequest):
    return UserProfile(...)  # Returns JSON-serialized Pydantic model

@validate_request
def handler4(req: func.HttpRequest):
    return func.HttpResponse(...)  # Returns custom HttpResponse directly

Error Handling

Invalid requests return detailed 400 responses:

{
  "errors": [
    {
      "param": "format",
      "reason": "Input should be 'json' or 'csv'",
      "type": "literal_error",
      "input": "xml"
    },
    {
      "param": "name",
      "reason": "Field required",
      "type": "missing"
    },
    {
      "param": "age",
      "reason": "Input should be a valid integer, got a number with a fractional part",
      "type": "int_from_float",
      "input": 25.5
    }
  ]
}

Async Support

The decorator works with both sync and async handlers:

@validate_request
async def handler(req: func.HttpRequest, user: UserProfile):
    # ... async operations ...
    return {"status": "success"}

Contributing

Contributions are welcome!

License

This project is licensed under the MIT License - see the LICENSE file for details.