How to use typing.Annotated
Apr 17
I'm having a hard time understanding what typing.Annotated is good for from the documentation and an even harder time finding explanations/examples outside the documentation. In what context would you use Annotated? Does it depend on third-party libraries?
python1 answer
Accepted answer · original discussion
Jun 4
Annotated in python allows developers to declare the type of a reference and provide additional information related to it.
name: Annotated[str, "first letter is capital"]
This tells that name is of type str and that name[0] is a capital letter.
On its own Annotated does not do anything other than assigning extra information (metadata) to a reference. It is up to another code, which can be a library, framework or your own code, to interpret the metadata and make use of it.
For example, FastAPI uses Annotated for data validation:
def read_items(q: Annotated[str, Query(max_length=50)])
Here the parameter q is of type str with a maximum length of 50. This information was communicated to FastAPI (or any other underlying library) using the Annotated keyword.
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May 16
pydantic that uses Annotated to impose additional validators.Mar 19
Annotated lets you attach arbitrary metadata (though still with an eye towards using that for further type checking) to the type itself in a function or variable annotation.