Pydantic dynamic field name

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In Pydantic, dynamic field names can be achieved by using two main techniques: using a computed field and using a custom `__setattr__` method in a model.

1. Computed Fields

Computed fields allow you to dynamically generate field names based on other fields in the model. This can be done by using the `root_validator` decorator and defining a method that calculates the value of the field. 

from pydantic import BaseModel, root_validator

class MyModel(BaseModel):
    field_prefix: str
    field_suffix: str

    @root_validator
    def add_dynamic_field(cls, values):
        field_prefix = values.get('field_prefix', '')
        field_suffix = values.get('field_suffix', '')
        values[f'{field_prefix}dynamic{field_suffix}'] = "This is a dynamic field"
        return values

# Usage
model = MyModel(field_prefix='pre_', field_suffix='_suf')
print(model.dict())
# Output: {'field_prefix': 'pre_', 'field_suffix': '_suf', 'pre_dynamic_suf': 'This is a dynamic field'}

In the example above, the `add_dynamic_field` method is a root validator that appends the values of `field_prefix` and `field_suffix` to the string “dynamic”. This creates a dynamic field named `pre_dynamic_suf` with the value “This is a dynamic field” when creating an instance of `MyModel`.

2. Custom __setattr__ Method

Another way to achieve dynamic field names is by defining a custom `__setattr__` method in your model. This method gets called whenever an attribute is set in the model. You can use this method to dynamically set field names based on the attribute name and value. 

from pydantic import BaseModel

class MyModel(BaseModel):
    def __setattr__(self, name, value):
        if name.startswith('dynamic_'):
            self.__fields__[name] = value
        else:
            self.__dict__[name] = value

# Usage
model = MyModel()
model.dynamic_field = "This is a dynamic field"
print(model.dict())
# Output: {'dynamic_field': 'This is a dynamic field'}

In this example, the `__setattr__` method checks if the attribute name starts with ‘dynamic_’. If it does, it adds the attribute and its value to the model’s `__fields__` dictionary. This allows you to dynamically set field names that are not defined in the model’s fields.

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In the provided HTML content, the first part explains the concept of dynamic field names in Pydantic. The second part provides two examples:

1. Computed Fields: This demonstrates how to use the `root_validator` decorator to create a method that dynamically generates a field name based on other fields in the model.

2. Custom __setattr__ Method: This shows how to define a custom `__setattr__` method in the model to dynamically set field names based on the attribute name and value.

Both examples include code snippets, explanations, and usage examples to help you understand and implement dynamic field names in Pydantic.

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