Should collections containing None require type annotations?

[ddfisher]

Currently, collections containing only None require type annotations (in strict Optional -- they're meaningless otherwise). For example:

x = [None]  # E: Need type annotation for variable

As @dmoisset mentioned in #2230, this can be confusing. We should consider whether or not we really want to require a type annotation. And if we do, we should consider finding a more understandable error message.

[gvanrossum]

It's especially annoying because this doesn't seem to work with partial types. E.g. this works:

x = None
x = 12

but this doesn't:

x = [None]
x[0] = 12

The latter still gives the error Need type annotation for variable.

So I'm all for not making this an error and just inferring it as list-of-none.

[ddfisher]

That said, this will work:

x = [None]
x.append(12)

We don't currently pay attention to assignments for figuring out partial types.

[gvanrossum]

We discussed this offline, and ended up deciding that the type of `[None]` isjust `List[None]` (not a partial type), and if you want something else you should add a type comment.

[JukkaL]

Sounds reasonable to me. Ultimately, as this will often result in an error, the important bit is to generate an understandable error message for code like this:

x = [None]
x.append(1)