[Python-Dev] PEP 563: Postponed Evaluation of Annotations

[Stéfane Fermigier]

On Thu, Nov 2, 2017 at 7:39 PM, Jukka Lehtosalo <jlehtosalo at gmail.com>
wrote:

>
> As type checking has become the main use case for annotations, using
> annotations without a type checker is fast becoming a marginal use case.
> Type checkers can easily and reliably validate that names in annotations aren't
> misspelled.
>

Another common use case is dependency injection / IoC:

Examples include:

- Injector (https://github.com/alecthomas/injector):

>>> class Outer:
...     @inject
...     def __init__(self, inner: Inner):
...         self.inner = inner


- Flsk-Injector (ok it's the same underlying injector):

# Route with injection
@app.route("/foo")
def foo(db: sqlite3.Connection):
    users = db.execute('SELECT * FROM users').all()
    return render("foo.html")

- Apistar components (https://github.com/encode/apistar#components ):

def say_hello(user: User):
    return {'hello': user.username}

=> In each of the examples, the type annotation are used at runtime by the
IoC container to inject an object of the appropriate type, based on some
specifications.

They may or may not be used by a typechecker too, but that's secondary.

  S.

-- 
Stefane Fermigier - http://fermigier.com/ - http://twitter.com/sfermigier -
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---
“You never change things by fighting the existing reality. To change
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