Pydantic

Pydantic v2 Framework Skill

Pydantic is a data validation library that uses Python type annotations to define data schemas, offering fast and extensible validation with automatic type coercion.

Quick Start

Basic Model Definition

from pydantic import BaseModel
from datetime import datetime
from typing import Optional

class User(BaseModel):
    id: int
    name: str
    email: str
    signup_ts: Optional[datetime] = None
    is_active: bool = True

# Automatic type coercion
user = User(
    id='123',  # String → int
    name='John Doe',
    email='john@example.com',
    signup_ts='2017-06-01 12:22'  # String → datetime
)

Validation from Data Sources

# From dict
user = User.model_validate({'id': 1, 'name': 'Alice', 'email': 'alice@test.com'})

# From JSON
user = User.model_validate_json('{"id": 1, "name": "Alice", "email": "alice@test.com"}')

# Serialization
print(user.model_dump())  # Python dict
print(user.model_dump_json())  # JSON string

Common Patterns

Field Configuration

from pydantic import BaseModel, Field, EmailStr, HttpUrl
from typing import Annotated

class Product(BaseModel):
    product_id: int = Field(alias='id', ge=1, description='Unique product identifier')
    name: str = Field(min_length=1, max_length=200)
    price: float = Field(gt=0, le=1000000)
    email: EmailStr
    website: HttpUrl
    tags: list[str] = Field(default_factory=list, max_length=10)
    internal_code: str = Field(exclude=True, default='N/A')

class User(BaseModel):
    username: Annotated[str, Field(min_length=3, pattern=r'^[a-zA-Z0-9_]+$')]
    age: int = Field(ge=0, le=150)

Model Configuration

from pydantic import BaseModel, ConfigDict

class StrictModel(BaseModel):
    model_config = ConfigDict(
        strict=True,              # No type coercion
        frozen=True,              # Immutable instances
        validate_assignment=True, # Validate on attribute assignment
        extra='forbid',           # Reject extra fields
        str_strip_whitespace=True,
        populate_by_name=True,    # Accept both alias and field name
        use_enum_values=True,     # Serialize enums as values
    )

    id: int
    name: str

Custom Validation

from pydantic import BaseModel, model_validator, field_validator, ValidationError
from typing import Any

class DateRange(BaseModel):
    start_date: str
    end_date: str

    @field_validator('start_date', 'end_date')
    @classmethod
    def validate_date_format(cls, v: str) -> str:
        # Custom validation logic
        if not v:
            raise ValueError('Date cannot be empty')
        return v

    @model_validator(mode='after')
    def check_dates_order(self) -> 'DateRange':
        # Cross-field validation
        if self.start_date > self.end_date:
            raise ValueError('start_date must be before end_date')
        return self

# Using the model
try:
    date_range = DateRange(start_date='2024-01-01', end_date='2024-01-31')
except ValidationError as e:
    for error in e.errors():
        print(f"{error['loc']}: {error['msg']}")

Serialization Control

from pydantic import BaseModel, Field, SecretStr
from datetime import datetime

class User(BaseModel):
    id: int
    username: str
    password: SecretStr
    created_at: datetime
    internal_data: dict = Field(exclude=True, default_factory=dict)

# Serialization options
user = User(
    id=1,
    username='john',
    password='secret',
    created_at=datetime.now()
)

# Basic serialization
print(user.model_dump())  # Python dict
print(user.model_dump_json())  # JSON string

# Excluding fields
print(user.model_dump(exclude={'password'}))
print(user.model_dump(exclude={'username', 'created_at'}))

# Include only specific fields
print(user.model_dump(include={'id', 'username'}))

# JSON-compatible serialization
print(user.model_dump(mode='json'))  # datetime → string
print(user.model_dump(by_alias=True))  # Use field aliases

Custom Serialization

from typing import Annotated, Any
from pydantic import BaseModel, field_serializer, PlainSerializer

class Model(BaseModel):
    number: int
    created_at: datetime

    @field_serializer('number')
    def serialize_number(self, value: int) -> str:
        return f"{value:,}"  # Format with commas

    # Using Annotated with PlainSerializer
    custom_field: Annotated[
        float,
        PlainSerializer(lambda x: round(x, 2), return_type=float)
    ]

Nested Models and Relationships

from pydantic import BaseModel
from typing import Optional, List

class Address(BaseModel):
    street: str
    city: str
    country: str = 'USA'
    zip_code: str

class User(BaseModel):
    id: int
    name: str
    addresses: List[Address]
    primary_address: Optional[Address] = None

# Usage
user = User(
    id=1,
    name='John Doe',
    addresses=[
        {'street': '123 Main St', 'city': 'New York', 'zip_code': '10001'},
        {'street': '456 Oak Ave', 'city': 'Boston', 'zip_code': '02101'}
    ],
    primary_address={'street': '123 Main St', 'city': 'New York', 'zip_code': '10001'}
)

Enum Integration

from enum import Enum, IntEnum
from pydantic import BaseModel

class Status(str, Enum):
    PENDING = 'pending'
    ACTIVE = 'active'
    COMPLETED = 'completed'

class Priority(IntEnum):
    LOW = 1
    MEDIUM = 2
    HIGH = 3

class Task(BaseModel):
    title: str
    status: Status = Status.PENDING
    priority: Priority = Priority.MEDIUM

    model_config = ConfigDict(use_enum_values=True)

# Can use enum values or names
task1 = Task(title='Task 1', status='active', priority=3)
task2 = Task(title='Task 2', status=Status.ACTIVE, priority=Priority.HIGH)

TypeAdapter for Standalone Validation

from pydantic import TypeAdapter
from typing import List, Optional

# Validate individual types without full models
int_adapter = TypeAdapter(int)
print(int_adapter.validate_python('123'))  # 123

list_adapter = TypeAdapter(List[int])
print(list_adapter.validate_python(['1', '2', '3']))  # [1, 2, 3]

# Generate JSON schemas
print(int_adapter.json_schema())
print(list_adapter.json_schema())

Data Validation Patterns

from pydantic import BaseModel, ValidationError
from typing import Union

class EmailValidator(BaseModel):
    email: str

    @field_validator('email')
    @classmethod
    def validate_email(cls, v: str) -> str:
        if '@' not in v:
            raise ValueError('Invalid email format')
        return v.lower()

# Validation error handling
try:
    user = User(id='invalid', name='', email='test')
except ValidationError as e:
    print(f"Errors: {e.error_count()}")
    for error in e.errors():
        print(f"  {error['loc']}: {error['msg']} ({error['type']})")

Requirements

• Python 3.8+
• Pydantic v2.x: uv add pydantic
• Optional dependencies for enhanced types:
  • uv add pydantic[email] for EmailStr
  • uv add pydantic[url] for HttpUrl
  • uv add pydantic[typing-extensions] for extended type support

Best Practices

1. Use specific types: Prefer conint(gt=0) over int for positive numbers
2. Configure models: Use ConfigDict to set global model behavior
3. Handle validation errors: Always wrap model creation in try/catch blocks
4. Use field validators: Implement custom validation logic with @field_validator
5. Control serialization: Use model_dump() parameters to control output format
6. Leverage type coercion: Pydantic automatically converts compatible types
7. Use nested models: Break complex data into smaller, reusable models