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延迟注解

Note

Both postponed annotations via the future import and ForwardRef require Python 3.7+.

Postponed annotations (as described in PEP563) "just work".

Internally, pydantic will call a method similar to typing.get_type_hints to resolve annotations.

In cases where the referenced type is not yet defined, ForwardRef can be used (although referencing the type directly or by its string is a simpler solution in the case of self-referencing models).

In some cases, a ForwardRef won't be able to be resolved during model creation. For example, this happens whenever a model references itself as a field type. When this happens, you'll need to call update_forward_refs after the model has been created before it can be used:

Warning

To resolve strings (type names) into annotations (types), pydantic needs a namespace dict in which to perform the lookup. For this it uses module.__dict__, just like get_type_hints. This means pydantic may not play well with types not defined in the global scope of a module.

For example, this works fine:

While this will break:

Resolving this is beyond the call for pydantic: either remove the future import or declare the types globally.

Self-referencing Models

Data structures with self-referencing models are also supported. Self-referencing fields will be automatically resolved after model creation.

Within the model, you can refer to the not-yet-constructed model using a string:

Since Python 3.7, you can also refer it by its type, provided you import annotations (see above for support depending on Python and pydantic versions).