Required Queries

Learn the fundamentals of required queries in FastAPI.

🎯 Core Concept

Required query parameters must be provided in the request URL, otherwise the API returns a 422 validation error. They don't have default values.

📖 What You'll Learn

In this section, you'll understand:

• Required vs optional query parameters

• How it applies to real-world API development

• Using required parameters effectively

• Common patterns and best practices

💡 Required Query Parameters

from fastapi import FastAPI

app = FastAPI()

# Query parameter with NO default = REQUIRED
@app.get("/search")
async def search(q: str):
    # q is REQUIRED
    # GET /search ❌ 422 Error
    # GET /search?q=python ✅ Valid
    return {"query": q}

# Multiple required parameters
@app.get("/convert")
async def convert(value: float, from_unit: str, to_unit: str):
    # All three parameters are REQUIRED
    return {
        "value": value,
        "from": from_unit,
        "to": to_unit
    }

🔍 Making Parameters Optional

To make a parameter optional, add a default value:

from typing import Optional

# Required: no default
@app.get("/items")
async def get_items(category: str):
    # category is REQUIRED
    return {"category": category}

# Optional: has default
@app.get("/items")
async def get_items(category: str = "all"):
    # category is optional, defaults to "all"
    return {"category": category}

# Optional: can be None
@app.get("/items")
async def get_items(category: Optional[str] = None):
    # category is optional, can be None
    return {"category": category}

How it applies to real-world API development

Required parameters are used for:

• **Search queries**: Must provide search term

• **Conversions**: Must provide input units and output units

• **Authentication**: API key must be provided

• **Critical filters**: Category, date range for valid results

🔑 Key Takeaways

• ✅ Understand the purpose of required queries

• ✅ Know when to apply this pattern

• ✅ Recognize its benefits in real-world scenarios

• ✅ Be prepared to use it in your projects

Ready to explore more? Check out the advanced section for production patterns and edge cases.