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참고/instructor-main/examples/validators/field_validator.py

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+from pydantic import BaseModel, ValidationError, field_validator
+
+
+class UserDetail(BaseModel):
+    age: int
+    name: str
+
+    @field_validator("name", mode="before")
+    def name_must_contain_space(cls, v):
+        """
+        This validator will be called after the default validator,
+        and will raise a validation error if the name does not contain a space.
+        then it will set the name to be lower case
+        """
+        if " " not in v:
+            raise ValueError("name be a first and last name separated by a space")
+        return v.lower()
+
+
+# Example 1) Valid input, notice that the name is lowercased
+person = UserDetail(age=29, name="Jason Liu")
+print(person.model_dump_json(indent=2))
+"""
+{
+    "age": 29,
+    "name": "jason liu"
+}
+"""
+
+# Example 2) Invalid input, we'll get a validation error
+# In the future this validation error will be raised by the API and
+# used by the LLM to generate a better response
+try:
+    person = UserDetail(age=29, name="Jason")
+except ValidationError as e:
+    print(e)
+    """
+    1 validation error for UserDetail 
+        name
+    Value error, must contain a space [type=value_error, input_value='Jason', input_type=str]
+        For further information visit https://errors.pydantic.dev/2.3/v/value_error
+    """