pydantic

Pydantic Skill

Pydantic v2 is a high-performance Python data validation library with first-class support for type hints, environment configuration via BaseSettings, and a complete rewrite from v1 with significant API changes.

Code Style Rules

• Use PEP 604 for unions: T | None (not Optional[T])
• Use model_config = ConfigDict(...) not inner class Config
• Use model_validate() for untrusted external input (not __init__)
• Use Annotated[T, Field(...)] over Field(...) as a default value

Quick Reference

ConfigDict

from pydantic import BaseModel, ConfigDict

class MyModel(BaseModel):
    model_config = ConfigDict(
        from_attributes=True,       # replaces orm_mode=True (v1)
        extra="forbid",             # "allow", "ignore", or "forbid"
        validate_assignment=True,   # re-validate on attribute set
        arbitrary_types_allowed=True,
        populate_by_name=True,      # allow field name or alias
        str_strip_whitespace=True,
        frozen=True,                # make model immutable
    )

Validators

from pydantic import BaseModel, field_validator, model_validator, computed_field

class Order(BaseModel):
    quantity: int
    unit_price: float
    discount: float = 0.0

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

    @field_validator("discount")
    @classmethod
    def discount_in_range(cls, v: float) -> float:
        if not (0.0 <= v <= 1.0):
            raise ValueError("discount must be between 0 and 1")
        return v

    @model_validator(mode="before")
    @classmethod
    def check_raw_data(cls, data: dict) -> dict:
        # runs before individual field validation
        if "unit_price" in data and data["unit_price"] < 0:
            raise ValueError("unit_price cannot be negative")
        return data

    @model_validator(mode="after")
    def check_total(self) -> "Order":
        # runs after all fields are validated; self is fully populated
        if self.quantity * self.unit_price > 1_000_000:
            raise ValueError("order total exceeds limit")
        return self

    @computed_field
    @property
    def total(self) -> float:
        return self.quantity * self.unit_price * (1 - self.discount)

Serialization

model = Order(quantity=2, unit_price=9.99)

# model_dump options
model.model_dump()
model.model_dump(by_alias=True)          # use field aliases in output
model.model_dump(exclude_unset=True)     # omit fields not explicitly set
model.model_dump(exclude_none=True)      # omit None-valued fields
model.model_dump(include={"quantity"})   # only these fields
model.model_dump(exclude={"discount"})   # all except these fields
model.model_dump(mode="json")            # JSON-serializable types

# JSON string
model.model_dump_json()
model.model_dump_json(by_alias=True, exclude_none=True)

# Custom field serializer
from pydantic import field_serializer

class Item(BaseModel):
    price: float

    @field_serializer("price")
    def serialize_price(self, v: float) -> str:
        return f"${v:.2f}"

# Custom model serializer
from pydantic import model_serializer

class Item(BaseModel):
    name: str
    price: float

    @model_serializer
    def custom_serialize(self) -> dict:
        return {"label": self.name, "cost": self.price}

TypeAdapter — Bulk Validation

from pydantic import TypeAdapter

# Significantly faster than calling MyModel(...) in a loop
adapter = TypeAdapter(list[MyModel])
items = adapter.validate_python(raw_list)          # from Python objects
items = adapter.validate_json(json_string)         # from JSON bytes/str

# Also works for non-model types
int_adapter = TypeAdapter(int)
int_adapter.validate_python("42")  # returns 42

Custom Types — Annotated Pattern

from typing import Annotated
from pydantic import BeforeValidator, PlainSerializer, WithJsonSchema
from pydantic.functional_validators import AfterValidator

def parse_comma_list(v: str | list) -> list[str]:
    if isinstance(v, str):
        return [item.strip() for item in v.split(",")]
    return v

def validate_positive(v: int) -> int:
    assert v > 0, "must be positive"
    return v

CommaList = Annotated[
    list[str],
    BeforeValidator(parse_comma_list),
    WithJsonSchema({"type": "string", "description": "Comma-separated values"}),
]

PositiveInt = Annotated[int, AfterValidator(validate_positive)]

TagList = Annotated[
    list[str],
    BeforeValidator(parse_comma_list),
    PlainSerializer(lambda v: ",".join(v), return_type=str),
]

BaseSettings — Environment Configuration

from pydantic_settings import BaseSettings, SettingsConfigDict

class AppSettings(BaseSettings):
    model_config = SettingsConfigDict(
        env_file=".env",
        env_file_encoding="utf-8",
        env_nested_delimiter="__",   # DATABASE__HOST → database.host
        case_sensitive=False,
        extra="ignore",
    )

    debug: bool = False
    secret_key: str
    allowed_hosts: list[str] = ["localhost"]

settings = AppSettings()   # reads from env + .env file

Priority chain: explicit args > env vars > .env file > defaults

See BaseSettings Deep Reference for nested models, secrets directory, multiple env files, and custom sources.

PrivateAttr — Non-Validated Internal State

from pydantic import BaseModel, PrivateAttr

class Connection(BaseModel):
    host: str
    port: int = 5432
    _client: object = PrivateAttr(default=None)

    def connect(self) -> None:
        self._client = create_client(self.host, self.port)

Constrained Types via Field

from pydantic import BaseModel, Field

class Product(BaseModel):
    name: str = Field(min_length=1, max_length=100)
    price: float = Field(gt=0, le=999_999.99)
    stock: int = Field(ge=0)
    sku: str = Field(pattern=r"^[A-Z]{3}-\d{4}$")
    tags: list[str] = Field(min_length=0, max_length=10)

Special Types

from pydantic import EmailStr, SecretStr, AnyUrl, AnyHttpUrl
from pydantic import PastDate, FutureDate, AwareDatetime, ByteSize

class UserProfile(BaseModel):
    email: EmailStr              # validates email format
    password: SecretStr          # hidden in repr/serialization
    website: AnyHttpUrl | None = None
    birth_date: PastDate | None = None
    appointment: AwareDatetime | None = None  # timezone-aware datetime
    avatar_size: ByteSize = "5MB"             # "5MB", "1GiB", etc.

V1 → V2 Migration Quick Reference

V1	V2
class Config: orm_mode = True	model_config = ConfigDict(from_attributes=True)
@root_validator	@model_validator(mode='before'/'after')
@validator	@field_validator
.dict()	.model_dump()
.json()	.model_dump_json()
.parse_obj(data)	.model_validate(data)
.parse_raw(json_str)	.model_validate_json(json_str)
__fields__	model_fields
Field(alias=..., allow_population_by_field_name=True)	Field(alias=...) + ConfigDict(populate_by_name=True)

See V1→V2 Migration Guide for full mapping, breaking changes, and coexistence patterns.

Performance

# Prefer model_validate() for untrusted/external data — skips Python __init__ overhead
obj = MyModel.model_validate(raw_dict)

# TypeAdapter for batch operations — do NOT use list comprehension with __init__
adapter = TypeAdapter(list[MyModel])
items = adapter.validate_python(raw_list)   # single validation pass

# Selective model_dump — avoid serializing the whole model if you only need a few fields
subset = obj.model_dump(include={"id", "name"})

Workflow

Step 1: Define Models

Create BaseModel subclasses with type-annotated fields. Use ConfigDict at the top of each model to set behavior. Prefer Annotated[T, Field(...)] over Field(...) as default values for reusability.

Step 2: Add Validators

Add @field_validator for single-field rules. Use @model_validator(mode='before') to preprocess raw input dicts and @model_validator(mode='after') for cross-field invariants on the fully constructed model. Add @computed_field for derived read-only properties.

Step 3: Handle Serialization

Use model_dump(exclude_unset=True) when patching. Use by_alias=True when the API consumer expects camelCase or snake_case aliases. Use @field_serializer or @model_serializer for custom output formats.

Step 4: Configure Settings

Create a BaseSettings subclass in pydantic_settings for environment-driven config. Set env_nested_delimiter="__" to support nested models from flat env vars. Load once at startup and inject as a dependency.

Step 5: Validate

Run mypy or pyright with the pydantic plugin. Use TypeAdapter when validating collections for performance. Run tests to confirm validator error messages are user-friendly.

Guardrails

• Use ConfigDict not inner class Config -- class Config is the V1 pattern and silently ignored in some V2 contexts.
• Use model_validate() for untrusted input -- it is faster and safer than calling __init__ directly; triggers full validation pipeline.
• Use BaseSettings for environment configuration -- never read os.environ manually in application code; BaseSettings handles type coercion, defaults, and .env loading.
• Use TypeAdapter for bulk operations -- never use a list comprehension calling MyModel(...) or MyModel.model_validate(...) in a loop; a single TypeAdapter(list[MyModel]).validate_python(data) is significantly faster.
• Never mix V1 and V2 patterns in the same model -- mixing @validator (V1) with @field_validator (V2) in the same class causes silent misbehavior or errors.
• Use Annotated[T, Field(...)] over Field(...) as a default value -- the Annotated pattern allows reuse as a type alias and avoids mutable default pitfalls.
• Use SecretStr for passwords and tokens -- prevents accidental leakage in logs, repr, and serialization.
• Avoid model_dump(mode='json') in hot paths -- use model_dump_json() directly when you need a JSON string; it skips the intermediate dict.

Validation Checkpoint

Before delivering Pydantic code, verify:

• model_config = ConfigDict(...) used (not inner class Config)
• model_validate() used for external/untrusted data (not bare __init__)
• TypeAdapter used for bulk list/collection validation
• No V1 patterns (@validator, @root_validator, .dict(), .json()) in new code
• BaseSettings used for environment configuration, not os.environ
• Passwords/tokens use SecretStr
• Annotated[T, Field(...)] pattern used for constrained fields
• @computed_field used for derived properties (not plain @property)

Example

Task: User registration model with validation + app settings loaded from environment.

from __future__ import annotations  # NOT used — pydantic needs runtime type eval

from typing import Annotated
from pydantic import (
    BaseModel,
    ConfigDict,
    EmailStr,
    Field,
    SecretStr,
    computed_field,
    field_validator,
    model_validator,
)
from pydantic_settings import BaseSettings, SettingsConfigDict


# --- Reusable constrained type aliases ---
Username = Annotated[str, Field(min_length=3, max_length=32, pattern=r"^\w+$")]
Password = Annotated[SecretStr, Field(min_length=8)]


# --- Domain model ---
class UserRegistration(BaseModel):
    model_config = ConfigDict(str_strip_whitespace=True)

    username: Username
    email: EmailStr
    password: Password
    confirm_password: Password

    @field_validator("username")
    @classmethod
    def username_not_reserved(cls, v: str) -> str:
        reserved = {"admin", "root", "system"}
        if v.lower() in reserved:
            raise ValueError(f"'{v}' is a reserved username")
        return v

    @model_validator(mode="after")
    def passwords_match(self) -> "UserRegistration":
        if self.password != self.confirm_password:
            raise ValueError("passwords do not match")
        return self

    @computed_field
    @property
    def display_name(self) -> str:
        return self.username.capitalize()


# --- Settings from environment ---
class DatabaseSettings(BaseModel):
    host: str = "localhost"
    port: int = 5432
    name: str = "app"
    user: str = "postgres"
    password: SecretStr = SecretStr("postgres")

    @computed_field
    @property
    def url(self) -> str:
        pwd = self.password.get_secret_value()
        return f"postgresql+asyncpg://{self.user}:{pwd}@{self.host}:{self.port}/{self.name}"


class AppSettings(BaseSettings):
    model_config = SettingsConfigDict(
        env_file=".env",
        env_nested_delimiter="__",
        extra="ignore",
    )

    debug: bool = False
    secret_key: SecretStr
    database: DatabaseSettings = DatabaseSettings()
    allowed_hosts: list[str] = ["localhost"]


# --- Usage ---
# Validate untrusted input
registration = UserRegistration.model_validate({
    "username": "alice",
    "email": "alice@example.com",
    "password": "s3cure!pw",
    "confirm_password": "s3cure!pw",
})

# Load settings once at startup
# DATABASE__HOST=db.prod DATABASE__PORT=5433 SECRET_KEY=xyz python app.py
settings = AppSettings()
db_url = settings.database.url

---

References Index

For detailed guides and configuration examples, refer to the following documents in references/:

• BaseSettings Deep Reference -- SettingsConfigDict options, nested models, secrets directory, multiple env files, custom sources, and full working example.
• V1→V2 Migration Guide -- Full API mapping table, breaking changes, gotchas, and coexistence patterns during incremental migration.

---

Official References

• https://docs.pydantic.dev/latest/
• https://docs.pydantic.dev/latest/concepts/models/
• https://docs.pydantic.dev/latest/concepts/validators/
• https://docs.pydantic.dev/latest/concepts/serialization/
• https://docs.pydantic.dev/latest/concepts/pydantic_settings/
• https://docs.pydantic.dev/latest/migration/
• https://github.com/pydantic/pydantic
• https://github.com/pydantic/pydantic-settings

Shared Styleguide Baseline

• Use shared styleguides for generic language/framework rules to reduce duplication in this skill.
• General Principles
• Python
• Keep this skill focused on tool-specific workflows, edge cases, and integration details.