pydantic

Cursor Rules for Pydantic Python (Strict Mode)

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Core Principles

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- Always use Pydantic v2 (latest version)

- Enable strict mode by default for type safety

- Prefer explicit over implicit type definitions

- Use type hints for all fields

- Validate data at boundaries (API, database, external services)

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Model Definition Rules

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from pydantic import BaseModel, Field, ConfigDict, field_validator, model_validator from typing import Optional, List, Dict, Any, Union, Literal from datetime import datetime, date from decimal import Decimal from enum import Enum

Always use ConfigDict for configuration (Pydantic v2)

class StrictModel(BaseModel): model_config = ConfigDict( # Strict mode - no type coercion strict=True,

    # Validate on assignment
    validate_assignment=True,
    
    # Use enum values instead of names
    use_enum_values=True,
    
    # Forbid extra fields
    extra='forbid',
    
    # Validate default values
    validate_default=True,
    
    # Enable arbitrary types if needed
    arbitrary_types_allowed=False,
    
    # Freeze model after creation (immutable)
    frozen=False,  # Set to True for immutable models
    
    # JSON schema extras
    json_schema_extra={
        "example": {
            "field1": "value1",
            "field2": 123
        }
    }
)

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Field Definition Rules

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class UserModel(BaseModel): model_config = ConfigDict(strict=True, extra='forbid')

# Required fields with no default
id: int
username: str

# Optional fields with explicit None default
email: Optional[str] = None

# Fields with defaults
is_active: bool = True

# Fields with Field() for additional validation
age: int = Field(gt=0, le=150, description="User's age")