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Python / FastAPI Basics Interview Questions

What are the key differences between Pydantic v1 and v2, and how does FastAPI use Pydantic v2?

FastAPI 0.100+ fully supports Pydantic v2, which is a complete rewrite in Rust offering 5–50× speed improvements. Several APIs changed between v1 and v2.

Pydantic v1 vs v2 key changes
v1 (old)v2 (current)Notes
.dict().model_dump()Serialise model to dict
.json().model_dump_json()Serialise to JSON string
.parse_obj().model_validate()Create model from dict
.schema().model_json_schema()Get JSON schema
@validator@field_validatorField-level validation
@root_validator@model_validatorModel-level validation
class Config:model_config = ConfigDict()Model configuration
orm_mode=Truefrom_attributes=TrueEnable ORM object input
from pydantic import BaseModel, field_validator, model_validator, ConfigDict
from typing import Self

class OrderItem(BaseModel):
    product_id: int
    quantity:   int
    unit_price: float

    @field_validator("quantity")
    @classmethod
    def quantity_positive(cls, v: int) -> int:
        if v <= 0:
            raise ValueError("quantity must be positive")
        return v

class Order(BaseModel):
    items: list[OrderItem]
    discount: float = 0.0

    model_config = ConfigDict(from_attributes=True)  # replaces orm_mode

    @model_validator(mode="after")
    def discount_valid(self) -> Self:
        if self.discount < 0 or self.discount > 1:
            raise ValueError("discount must be between 0 and 1")
        return self

    @property
    def total(self) -> float:
        subtotal = sum(i.quantity * i.unit_price for i in self.items)
        return round(subtotal * (1 - self.discount), 2)

# Serialisation
order = Order(items=[OrderItem(product_id=1, quantity=2, unit_price=9.99)])
print(order.model_dump())        # dict
print(order.model_dump_json())   # JSON bytes
print(order.model_json_schema()) # JSON Schema

What is the Pydantic v2 replacement for the v1 .dict() method?
What replaces orm_mode=True in Pydantic v2?

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