Narrow types after 'in' operator.

Sometimes it is useful to narrow down a type after an in/not in comparison. For example:

class Option:
    ...
opt: Union[Option, str]
if opt in ('verbose', 'silent'):
    reveal_type(opt)  # we know this is 'str'
else:
    ...  # still a union here

presets: List[Option]
if opt not in presets:
    ...  # still a union
else:
    reveal_type(opt)  # we know this can be only 'Option'

This seems to be especially useful for --strict-optional, for example:

class Node:
    fullname: Optional[str]
n: Node

reverse_aliases: Dict[str, str]
if n.fullname in reverse_aliases:
    reveal_type(n.fullname)  # This should be 'str'

(related #2980)

A similar idea was discussed in #1749, but emitting an error on if 'a' in [1, 2, 3]: ... would require a larger change: emitting errors on unreachable code in binder.

I think this can also fall out of #4063 (just as a note; of course a dedicated PR is more practical)

Careful. For unconstrained x, after x in (1, 2), x may still be a float. (Or a fraction or decimal?)

…

On Oct 8, 2017 6:15 AM, "Elazar Gershuni" ***@***.***> wrote: I think this can also fall out of #4063 <#4063> (just as a note; of course a dedicated PR is more practical) — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#4071 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ACwrMifVTAK8FQdpXnhnADA9IBWzQ_7jks5sqMr8gaJpZM4PxpHz> .

@gvanrossum

@gvanrossum

Careful. For unconstrained x, after x in (1, 2), x may still be a float. (Or a fraction or decimal?)

OK, makes sense, we should not narrow down if we start from Any and should not use promotions (this is actually default in is_overlapping_types). I will update the PR now.

@gvanrossum

@gvanrossum OK, I updated PR #4072
Now it does not narrow type from Any and does not use promotions:

x: Any
if x in [1, 2, 3]:
    reveal_type(x)  # Revealed type is 'Any'
y: float
if y in [1, 2, 3]:
    reveal_type(x)  # Revealed type is 'builtins.float'

It is however not easy to prohibit this:

z: object
if z in [1, 2, 3]:
    reveal_type(z)  # Revealed type is 'builtins.int'

This is still formally speaking unsafe, but I think it is a reasonable price to pay.

Narrow types after 'in' operator (#4072)

Navigation Menu

Repository navigation

Narrow types after 'in' operator. #4071

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Narrow types after 'in' operator. #4071

Description

Activity

elazarg commented on Oct 8, 2017

gvanrossum commented on Oct 8, 2017

ilevkivskyi commented on Oct 8, 2017

ilevkivskyi commented on Oct 8, 2017

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions