Loading FASTAPI::COSMOS...
“FastAPI uses Pydantic V2 for request validation and response serialization. In V2, the validation and JSON parsing engine was rewritten entirely in Rust (pydantic-core), executing data validation at compiled C/Rust speeds (~5x to 50x faster than pure Python) while enforcing strict static typing and generating interactive OpenAPI 3.1 JSON schemas.”
The 20x speedup of Pydantic V2 using pydantic-core in Rust, strict type coercion, and JSON schema extraction.
from pydantic import BaseModel, EmailStr, Field
from datetime import datetime
class UserCreate(BaseModel):
username: str = Field(min_length=3, max_length=32)
email: EmailStr
age: int = Field(ge=18, le=120)
created_at: datetime = Field(default_factory=datetime.utcnow)
model_config = {
"str_strip_whitespace": True,
"json_schema_extra": {"example": {"username": "ada_lovelace", "email": "ada@cosmos.org", "age": 28}}
}HTTP Request Body (JSON bytes) enters FastAPI endpoint
Rust pydantic-core parses JSON directly into validated C structures without Python object allocations
Applies field constraints (gt=0, regex, email validation, datetime parsing)
Constructs validated Pydantic model and injects it into endpoint parameter
Response model serializes back to JSON bytes at native Rust speeds via model_dump_json()
Pydantic V2 response_model serialization compiles to direct Rust bytecode, skipping Python dictionary transformation overhead.