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On 21 December 2017 at 11:22, Terry Reedy <tjreedy@udel.edu> wrote:

On 12/21/2017 4:22 AM, Eric V. Smith wrote:

On 12/21/2017 1:46 AM, Chris Barker wrote:

I suggest that it be clear in the docs, and ideally in the PEP, that the dataclass decorator is using the \*annotation" syntax, and that the the only relevant part it uses is that an annotation exists, but the value of the annotation is essentially (completely?) ignored.

I think the PEP is very clear about this: "The dataclass decorator examines the class to find fields. A field is defined as any variable identified in \_\_annotations\_\_. That is, a variable that has a type annotation. With two exceptions described below, none of the Data Class machinery examines the type specified in the annotation."

This seems clear enough. It could come after describing what a dataclass \*is\*.

I agree the docs should also be clear about this.

So we should have examples like:

@dataclass
class C:
a: ... # field with no default
b: ... = 0 # filed with a default value

Then maybe:

@dataclass
class C:
a: "the a parameter" # field with no default
b: "another, different parameter" = 0.0 # field with a default

Then the docs can go to say that if the user wants to specify a type for use with a static type checking pre-processor, they can do it like so:

@dataclass
class C:
a: int # integer field with no default
b: float = 0.0 # float field with a default

And the types will be recognized by type checkers such as mypy.

And I think the non-typed examples should go first in the docs.

Module some bike-shedding, the above seems pretty good to me.

For me, the three options for "don't care" have a bit different meaning:

\* typing.Any: this class is supposed to be used with static type checkers, but this field is too dynamic

\* ... (ellipsis): this class may or may not be used with static type checkers, use the inferred type in the latter case

\* "field docstring": this class should not be used with static type checkers

Assuming this, the second option would be the "real" "don't care". If this makes sense,

then we can go the way proposed in https://github.com/python/typing/issues/276 and make ellipsis semantics "official" in PEP 484.

(pending Guido's approval)

--

Ivan

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