[Python-Dev] Is static typing still optional?

[Chris Barker]

On Wed, Dec 20, 2017 at 5:29 PM, Eric V. Smith <eric at trueblade.com> wrote:

> There is definitely a passive bias towards using types with dataclasses in
> that the Eric (the author) doesn't appear to want an example without them
> in the pep/docs.
>
>>
>> I'm not sure what such an example would look like. Do you mean without
>> annotations?
>
>
IIUC, there is not way to make a dataclass without annotations, yes? That
it, using annotations to determine the fields is the one and only way the
decorator works. So it's impossible to give an example without annotations,
yes?


> Or do you mean without specifying the "correct" type, like:
>>
>> @dataclass
>> class C:
>>     x: int = 'hello world'
>>
>
It may be a good idea to have an example like that in the docs (but
probably not the PEP) to make it clear that the type is not used in any way
at run time.

But I don't think that anyone is suggesting that would be  a recommended
practice.

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. 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.

This is completely analogous to how all the other parts of python are
taught. Would anyone suggest that the very first example of a function
definition that a newbie sees would be:

def func(a: int, b:float = 0.0):
    body_of_function

Then, _maybe_ way down on the page, you mention that oh, by the way, those
types are completely ignored by Python. And not even give any examples
without types?


>  Re-reading my post you referenced, is it just an example using
typing.Any?

I actually think that is exactly the wrong point -- typing.Any is still
using type hinting -- it's an explicit way to say, "any type will do", but
it's only relevant if you are using a type checker. We really need examples
for folks that don't know or care about type hinting at all.

typing.Any is for use by people that are explicitly adding type hinting,
and should be discussed in type hinting documentation.

>  I'm okay with that in the docs, I just didn't want to focus on it in the
PEP. I want the PEP to only
> have the one reference to typing, for typing.ClassVar. I figure the
people reading the PEP can
> extrapolate to all of the possible uses for annotations that they don't
need to see a typing.Any
> example.

no they don't, but they DO need to see examples without type hints at all.

-Chris

-- 

Christopher Barker, Ph.D.
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