Assertion and checking functions #703

srittau · 2020-02-19T11:36:13Z

I'll just leave this here for potential future consideration as it came up on gitter's typing-dev channel. Typescript has something called assertion functions, which enables the type checker to understand that a function does not return if a certain condition is false. I think something similar, but more generalized, would be interesting, especially for unit test functions like assert_is_instance(), assert_true() etc.

def assert_positive(x: object) -> AssertsInstance["x", int]:
    assert isinstance(x, int) and x > 0

Related, checking functions could work like this:

def is_positive(x: object) -> ChecksInstance["x", int]:
    return isinstance(x, int) and x > 0

def my_func(x: Union[int, str]) -> None:
    if is_positive(x):
        # x must be an int
        ...
    else:
        # x can be an int or str
        ...

TeamSpen210 · 2020-02-20T09:14:25Z

This is already possible with overload and Literal, though I don't think any type checkers can take advantage of this information yet.

@overload
def assert_positive(x: int) -> bool: ...
@overload
def assert_positive(x: object) -> NoReturn: ...

@overload
def is_positive(x: int) -> Literal[True]: ...
@overload
def is_positive(x: object) -> Literal[False]: ...

srittau · 2020-02-20T10:44:21Z

That is a clever trick that works for some functions like assert_is_instance(), but in the examples given it does not work, since it ignores the non-type check part. Another example is type narrowing, based on object fields:

class Proto(Protocol): ...
class SubProto(Proto):
    sub_field: int

class MyClass: # implements protocol SubProto
    sub_field = 42

def is_sub_proto(o: Proto) -> ChecksInstance["o", SubProto]:
    return hasattr(o, "sub_field")

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python / typing

Assertion and checking functions #703

Assertion and checking functions #703

srittau commented Feb 19, 2020

TeamSpen210 commented Feb 20, 2020

srittau commented Feb 20, 2020

python / typing

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Assertion and checking functions #703

Assertion and checking functions #703

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srittau commented Feb 19, 2020

TeamSpen210 commented Feb 20, 2020

srittau commented Feb 20, 2020

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