mirror of
https://github.com/kennethreitz/pydantic.git
synced 2026-06-05 23:00:18 +00:00
Samuel Colvin
67 lines
2.4 KiB
Markdown
67 lines
2.4 KiB
Markdown
!!! note
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Both postponed annotations via the future import and `ForwardRef` require python 3.7+.
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Postponed annotations (as described in [PEP563](https://www.python.org/dev/peps/pep-0563/))
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"just work".
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```py
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{!.tmp_examples/postponed_annotations_main.py!}
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```
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_(This script is complete, it should run "as is")_
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Internally, *pydantic* will call a method similar to `typing.get_type_hints` to resolve annotations.
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In cases where the referenced type is not yet defined, `ForwardRef` can be used (although referencing the
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type directly or by its string is a simpler solution in the case of
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[self-referencing models](#self-referencing-models)).
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In some cases, a `ForwardRef` won't be able to be resolved during model creation.
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For example, this happens whenever a model references itself as a field type.
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When this happens, you'll need to call `update_forward_refs` after the model has been created before it can be used:
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```py
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{!.tmp_examples/postponed_annotations_forward_ref.py!}
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```
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_(This script is complete, it should run "as is")_
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!!! warning
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To resolve strings (type names) into annotations (types), *pydantic* needs a namespace dict in which to
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perform the lookup. For this it uses `module.__dict__`, just like `get_type_hints`.
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This means *pydantic* may not play well with types not defined in the global scope of a module.
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For example, this works fine:
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```py
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{!.tmp_examples/postponed_annotations_works.py!}
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```
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While this will break:
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```py
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{!.tmp_examples/postponed_annotations_broken.py!}
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```
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Resolving this is beyond the call for *pydantic*: either remove the future import or declare the types globally.
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## Self-referencing Models
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Data structures with self-referencing models are also supported, provided the function
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`update_forward_refs()` is called once the model is created (you will be reminded
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with a friendly error message if you forget).
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Within the model, you can refer to the not-yet-constructed model using a string:
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```py
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{!.tmp_examples/postponed_annotations_self_referencing_string.py!}
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```
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_(This script is complete, it should run "as is")_
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Since `python 3.7`, you can also refer it by its type, provided you import `annotations` (see
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[above](postponed_annotations.md) for support depending on Python
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and *pydantic* versions).
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```py
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{!.tmp_examples/postponed_annotations_self_referencing_annotations.py!}
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```
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_(This script is complete, it should run "as is")_
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