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FastAPI

Introduction

FastAPI is a modern, high-performance web framework for building APIs using Python. Designed with efficiency and developer productivity in mind, it takes full advantage of Python’s type hints to provide automatic validation, serialization, and robust error handling. FastAPI is particularly well-suited for building RESTful APIs, microservices, and backend services for real-time applications.

Core Features

  • Asynchronous Programming
  • Automatic Interactive API Documentation
  • Type-Driven Development
  • Data Validation with Pydantic
  • Built-in Dependency Injection
  • Security and Authentication
Tutorials

Official: FastAPI

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Getting Started

Installation
  • Installs FastAPI, the framework we’ll use to build our APIs.
  • Installs Uvicorn, the Asynchronous Server Gateway Interface (ASGI) that will run the FastAPI app and serve it to clients.
  • Installs PyTest, a testing framework that allows us to write and run test cases for our FastAPI endpoints efficiently.
pip install fastapi uvicorn pytest

Verify the installation

python -m uvicorn --help

Running a Basic Server

main.py: (NOTE: 檔名不能與專案名相同，例如 fastapi.py)

from fastapi import FastAPI
app = FastAPI()
@app.get("/")
async def read_root():
    return {"message": "Hello, World!"}

Command to start the API Server

uvicorn main:app --reload

Access the Application : http://127.0.0.1:8000/ 

Example Code

Quick Test

Constraint/Validation

Pyantic: Base Constraint

from pydantic import BaseModel, Field

class User(BaseModel):
    name: str = Field(..., max_length=50)
    age: int = Field(..., gt=0, le=100)
    email: str = Field(..., regex="^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$")

Pyantic: Custom Error Messages

from pydantic import BaseModel, Field

class User(BaseModel):
    name: str = Field(..., max_length=50, description="Name must be under 50 characters")
    age: int = Field(..., gt=0, le=100, description="Age must be between 1 and 100")
    email: str = Field(..., regex="^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$", description="Invalid email format")

Pyantic: Combining Multiple Constraints

from pydantic import BaseModel, Field

class User(BaseModel):
    username: str = Field(..., min_length=3, max_length=20, regex="^[a-zA-Z0-9_.]+$")

Pyantic: Integration with FastAPI Endpoints

from fastapi import FastAPI, HTTPException
from pydantic import BaseModel, Field

app = FastAPI()

class User(BaseModel):
    name: str = Field(..., max_length=50)
    age: int = Field(..., gt=0, le=100)
    email: str = Field(..., regex="^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$")

@app.post("/user/")
async def create_user(user: User):
    return user
Upload File
    FastAPI File Upload Download Tutorial

    Upload single file

    from fastapi import FastAPI, File, UploadFile, HTTPException
from fastapi.responses import JSONResponse
from typing import List
import os
import shutil
from pathlib import Path

# Create upload directory
UPLOAD_DIR = Path("uploads")
UPLOAD_DIR.mkdir(exist_ok=True)

app = FastAPI(title="FastAPI File Upload Service")

@app.post("/upload/single")
async def upload_single_file(file: UploadFile = File(...)):
    """Upload a single file with basic validation"""
    if file.filename == "":
        raise HTTPException(status_code=400, detail="No file selected")

    file_path = UPLOAD_DIR / file.filename

    with open(file_path, "wb") as buffer:
        shutil.copyfileobj(file.file, buffer)

    return {
        "filename": file.filename,
        "content_type": file.content_type,
        "size": file.size,
        "location": str(file_path)
    }

    Upload single file with validations

    validators.py:

    from pathlib import Path
from fastapi import UploadFile

class DocumentValidator:
    def __init__(self, max_size: int = 10 * 1024 * 1024):  # 10MB default
        self.max_size = max_size
        self.allowed_extensions = {'.pdf', '.txt', '.json'}

    async def validate_file(self, file: UploadFile) -> dict:
        """Check if the document file is valid"""
        result = {"valid": True, "errors": []}

        # Check if user selected a file
        if not file.filename or file.filename.strip() == "":
            result["valid"] = False
            result["errors"].append("No file selected")
            return result

        # Check file extension
        file_ext = Path(file.filename).suffix.lower()
        if file_ext not in self.allowed_extensions:
            result["valid"] = False
            result["errors"].append(
                f"File extension '{file_ext}' not allowed. Use: .pdf, .txt, or .json"
            )

        # Read file to check size
        content = await file.read()
        await file.seek(0)  # Reset file pointer for later use

        # Check file size
        file_size = len(content)
        if file_size > self.max_size:
            result["valid"] = False
            result["errors"].append(
                f"File too large ({file_size:,} bytes). Maximum: {self.max_size:,} bytes"
            )

        return result

    main.py

    from fastapi import FastAPI, File, UploadFile, HTTPException
from fastapi.responses import JSONResponse
from typing import List
import os
import shutil
import uuid
from pathlib import Path
from datetime import datetime
from validators import DocumentValidator

# Create upload directory
UPLOAD_DIR = Path("uploads")
UPLOAD_DIR.mkdir(exist_ok=True)

app = FastAPI(title="FastAPI File Upload Service")

# Create validator instance
doc_validator = DocumentValidator(max_size=25 * 1024 * 1024)  # 25MB limit

@app.post("/upload/single")
async def upload_single_file(file: UploadFile = File(...)):
    """Upload a single file with validation"""

    # Validate the file first
    validation = await doc_validator.validate_file(file)

    if not validation["valid"]:
        raise HTTPException(
            status_code=400,
            detail={
                "message": "File validation failed",
                "errors": validation["errors"]
            }
        )

    # Create unique filename to prevent conflicts
    file_ext = Path(file.filename).suffix
    unique_filename = f"{uuid.uuid4()}{file_ext}"
    file_path = UPLOAD_DIR / unique_filename

    try:
        with open(file_path, "wb") as buffer:
            shutil.copyfileobj(file.file, buffer)
    except Exception as e:
        raise HTTPException(
            status_code=500,
            detail=f"Failed to save file: {str(e)}"
        )

    return {
        "success": True,
        "original_filename": file.filename,
        "stored_filename": unique_filename,
        "content_type": file.content_type,
        "size": file.size,
        "upload_time": datetime.utcnow().isoformat(),
        "location": str(file_path)
    }

@app.get("/")
async def root():
    return {"message": "FastAPI File Upload Service is running"}

     

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