Python type annotations: Any way to annotate a property?

Question

I'm defining a Python class:

class Foo:
    bar = property(lambda self: Bar(self))

I want to annotate the bar property to say that it contains an item of class Bar. Is there an accepted way of doing that? I know that this is valid Python syntax:

bar: Bar = property(lambda self: Bar(self))

But is this an accepted way of annotating a property?

Answer 1

If you want to be safe, simply refactor it into using the decorator form and specify a return value:

class Foo:
    @property
    def bar(self) -> Bar:
        return Bar(self)

this is also the form used in typeshed which collects stubs annotating the standard library and also smoothly passes through mypy.

As for variable annotation syntax of Python 3.6, the form:

bar: Bar = property(lambda self: Bar(self))

is currently acceptable when checking with mypy.