PEP 747: TypeExpr: Type Hint for a Type Expression

[davidfstr (David Foster)]

It seems the opinion of the typing experts vocal in this thread is mainly in favor of the name TypeForm. To paraphrase:

A TypeForm is any of the forms of a type, both common forms (i.e. class objects) and special forms.

• The reasoning cited is consistent with the reasoning I used originally when suggesting the name TypeForm back in 2020: showing an alignment with the existing term “special form”.
• Typing experts reasonably argue that “type form” is what the concept is rather than merely how it is spelled, aligning with a preference toward naming the concept for what it is.
• Several folks don’t seem to find the name collision with the Typeform survey product to be a problem in the long term.

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@xmw

How about TypeValue to indicate “a type used in a value context”? “Value expression context” or “value context” seems to be how it’s most widely described in this thread.

TypeValue is an interesting name, well-contrasted with TypeAlias. However:

• I don’t think TypeValue is sufficiently distinct from Type. Similarly if I had to choose between List and ListValue, I’d prefer List because it’s shorter and more direct. Sadly the name Type itself is taken.
• The contrast with TypeAlias isn’t really that important, vs showing a contrast with Type. Additionally TypeAlias is soft-deprecated in favor of the type statement, so showing contrast with a less-and-less used construct will be increasingly less informative to users.

@xmw

TypeForm does not declare a special form, it is a special form.

At runtime a variable of TypeForm type would indeed contain a “special form” object if it did not contain a class object. So it is a “form” in that sense. Picking a name that aligns with what the concept is seems desirable.

@rusty-snake

The way I read TypeExpr is that it’s a special kind of expression that evaluates to a type. Seems pretty natural to me. Is this interpretation inaccurate?

That’s accurate. However if it’s possible to use a term that better aligns with what the concept is rather than how it is spelled, such a term would be preferred.

Regarding what the concept is:

• From the perspective of a user, the concept is a “type”.
• From the perspective of a static type checker, the concept is a “type” (or a “type form”).
• From the perspective of a runtime type checker the concept is a “type expression” (or a “type form”), whose parts are individually introspected and manipulated.

I prefer the user’s perspective over all others where I’m able to.