zzstoatzz.io
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languages/python/ecosystem/pydantic.md
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languages/python/ecosystem/pydantic.md
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from [pdsx/_internal/config.py](https://github.com/zzstoatzz/pdsx/blob/main/src/pdsx/_internal/config.py)
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## annotated types for reusable validation
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when multiple schemas share the same validation logic, bind it to the type itself instead of repeating `@field_validator` on each schema:
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```python
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from datetime import timedelta
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from typing import Annotated
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from pydantic import AfterValidator, BaseModel
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def _validate_non_negative_timedelta(v: timedelta) -> timedelta:
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if v < timedelta(seconds=0):
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raise ValueError("timedelta must be non-negative")
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return v
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NonNegativeTimedelta = Annotated[
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timedelta,
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AfterValidator(_validate_non_negative_timedelta)
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]
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class RunDeployment(BaseModel):
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schedule_after: NonNegativeTimedelta
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```
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benefits:
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- write validation once
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- field types become swappable interfaces
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- types are self-documenting
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from [coping with python's type system](https://blog.zzstoatzz.io/coping-with-python-type-system/)
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## when to use what
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pydantic models are heavier than they look - they do a lot of work on instantiation. for internal data you control, python's `dataclasses` are simpler:
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sources:
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- [how to use pydantic-settings](https://blog.zzstoatzz.io/how-to-use-pydantic-settings/)
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- [coping with python's type system](https://blog.zzstoatzz.io/coping-with-python-type-system/)