first chapter

models/user.py

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+from __future__ import annotations
+
+from typing import Union, Optional
+from pydantic import BaseModel, PositiveInt, field_validator, Field, ValidationError
+
+
+class User(BaseModel):
+    name: str
+    surname: str
+    age: int
+    registered: bool
+
+class UserNone(BaseModel):
+    # preferred variant
+    name: str
+    surname: str
+    age: int | None
+    registered: bool = False
+
+class UserUnion(BaseModel):
+    name: str
+    surname: str
+    age: Union[int, None] = None
+    registered: bool = False
+
+class UserOptional(BaseModel):
+    name: str
+    surname: str
+    age: Optional[int] # default value is None
+    registered: bool = False
+
+class UserPositiveInt(BaseModel):
+    name: str
+    surname: str
+    age: Union[PositiveInt, None] = None
+    registered: bool = False
+
+class UserFieldValidator(BaseModel):
+    name: str
+    surname: str
+    age: int | None
+    registered: bool = False
+
+    @field_validator("age")
+    def validate_age(cls, value):
+        if value < 0:
+            raise ValidationError("age must be positive")
+        if value < 18:
+            raise ValidationError("age must be at least 18")
+        return value
+
+
+class UserMaxLength(BaseModel):
+    name: str = Field(..., max_length=10)
+    surname: str = Field(..., max_length=12)
+    age: int | None
+    registered: bool = False
+
+    @field_validator("age")
+    def check_age(cls, value):
+        if value < 0:
+            raise ValidationError("age must be positive")
+        if value < 18:
+            raise ValueError("You are too young to register")
+        return value