mirror of
https://github.com/kennethreitz/pydantic.git
synced 2026-06-05 23:00:18 +00:00
Jochen Kupperschmidt
66 lines
2.3 KiB
Markdown
66 lines
2.3 KiB
Markdown
!!! note
|
|
Both postponed annotations via the future import and `ForwardRef` require Python 3.7+.
|
|
|
|
Postponed annotations (as described in [PEP563](https://www.python.org/dev/peps/pep-0563/))
|
|
"just work".
|
|
|
|
```py
|
|
{!.tmp_examples/postponed_annotations_main.py!}
|
|
```
|
|
_(This script is complete, it should run "as is")_
|
|
|
|
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](#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:
|
|
|
|
```py
|
|
{!.tmp_examples/postponed_annotations_forward_ref.py!}
|
|
```
|
|
_(This script is complete, it should run "as is")_
|
|
|
|
!!! 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:
|
|
|
|
```py
|
|
{!.tmp_examples/postponed_annotations_works.py!}
|
|
```
|
|
|
|
While this will break:
|
|
|
|
```py
|
|
{!.tmp_examples/postponed_annotations_broken.py!}
|
|
```
|
|
|
|
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:
|
|
|
|
```py
|
|
{!.tmp_examples/postponed_annotations_self_referencing_string.py!}
|
|
```
|
|
_(This script is complete, it should run "as is")_
|
|
|
|
Since Python 3.7, you can also refer it by its type, provided you import `annotations` (see
|
|
[above](postponed_annotations.md) for support depending on Python
|
|
and *pydantic* versions).
|
|
|
|
```py
|
|
{!.tmp_examples/postponed_annotations_self_referencing_annotations.py!}
|
|
```
|
|
_(This script is complete, it should run "as is")_
|