TypeVar single constraint not allowed, why?

[oliver]

The documentation for typing.TypeVar gives these two examples:

T = TypeVar('T') # Can be anything
A = TypeVar('A', str, bytes) # Must be str or bytes

I was suprised to find out that the following does not work, exception says
that TypeVar is not definable with only one constraint:

A = TypeVar('A', type) # Must be a type, like class Foo, etc (rather
than an instance of a type)

The TypeVar source code explicitely forbids only one type constraint, what
is the rationale behind this?

For those who wonder, my function parameter expects a type rather than an
instance of a type, and returns a type.

--
Oliver
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[Ned Batchelder]

On Tuesday, May 2, 2017 at 11:45:39 AM UTC-4, oliver wrote:
> The documentation for typing.TypeVar gives these two examples:
>
> T = TypeVar('T') # Can be anything
> A = TypeVar('A', str, bytes) # Must be str or bytes
>
> I was suprised to find out that the following does not work, exception says
> that TypeVar is not definable with only one constraint:
>
> A = TypeVar('A', type) # Must be a type, like class Foo, etc (rather
> than an instance of a type)
>
> The TypeVar source code explicitely forbids only one type constraint, what
> is the rationale behind this?
>
> For those who wonder, my function parameter expects a type rather than an
> instance of a type, and returns a type.

Couldn't you simply annotate it as ": type", without using a TypeVar?

--Ned.

[Gregory Ewing]

Ned Batchelder wrote:
> Couldn't you simply annotate it as ": type", without using a TypeVar?

Not if you want to constrain two types to be the same type.

--
Greg

[oliver]

On Wed, 3 May 2017 at 18:36 Gregory Ewing <greg....@canterbury.ac.nz>
wrote:

> Ned Batchelder wrote:
> > Couldn't you simply annotate it as ": type", without using a TypeVar?
>
> Not if you want to constrain two types to be the same type.
>
>

Exactly! It would be a lot more expressive to write something like (with
Type assumed imported from typing module):

def func(a: type, b: type) -> Tuple[Type(a), Type(b)]:
...

This will not work because a and b are not globals. I could use strings but
this is a hack / language wart as far as I'm concerned (the language should
be extended so we don't have to resort to a lowly string). I suppose I
could do:

def func(a: type, b: type) -> Tuple[TypeVar['a'], TypeVar['b']]:
...

but this is not pythonic (not clean, simple), and too easy to have typo.
Same with forward declarations, where a class references itself in an
annotation: we should not have to resort to a string to express this, very
clunky and hacky.



> --
> Greg
> --
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