Query Validation

Learn the fundamentals of query validation in FastAPI.

🎯 Core Concept

Query parameter validation ensures that incoming query parameters match expected types and constraints. FastAPI uses Query() to define validation rules.

📖 What You'll Learn

In this section, you'll understand:

• Using Query() for validation

• String constraints (length, regex)

• Numeric constraints (range)

• Documentation and error messages

💡 Query Validation Example

from fastapi import FastAPI, Query
from typing import Optional

app = FastAPI()

@app.get("/search")
async def search(
    q: str = Query(..., min_length=1, max_length=50),
    skip: int = Query(0, ge=0),
    limit: int = Query(10, ge=1, le=100),
    tag: Optional[str] = Query(None)
):
    # q: required, 1-50 characters
    # skip: >= 0 (default 0)
    # limit: 1-100 (default 10)
    # tag: optional
    return {"q": q, "skip": skip, "limit": limit}

Real-World Usage

Query validation is essential for:

• **Data integrity**: Ensure valid ranges

• **Security**: Prevent invalid requests

• **Documentation**: Swagger shows constraints

• **User feedback**: Clear error messages

🔑 Key Takeaways

• ✅ Understand the purpose of query validation

• ✅ 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.