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3.1 KiB
3.1 KiB
assert_never
Basic functionality
assert_never makes sure that the type of the argument is Never.
Correct usage
from typing_extensions import assert_never, Never, Any
from ty_extensions import Unknown
def _(never: Never):
assert_never(never) # fine
Diagnostics
If it is not, a type-assertion-failure diagnostic is emitted.
from typing_extensions import assert_never, Never, Any
from ty_extensions import Unknown
def _():
assert_never(0) # error: [type-assertion-failure]
def _():
assert_never("") # error: [type-assertion-failure]
def _():
assert_never(None) # error: [type-assertion-failure]
def _():
assert_never(()) # error: [type-assertion-failure]
def _(flag: bool, never: Never):
assert_never(1 if flag else never) # error: [type-assertion-failure]
def _(any_: Any):
assert_never(any_) # error: [type-assertion-failure]
def _(unknown: Unknown):
assert_never(unknown) # error: [type-assertion-failure]
Use case: Type narrowing and exhaustiveness checking
[environment]
python-version = "3.10"
assert_never can be used in combination with type narrowing as a way to make sure that all cases
are handled in a series of isinstance checks or other narrowing patterns that are supported.
from typing_extensions import assert_never, Literal
class A: ...
class B: ...
class C: ...
def if_else_isinstance_success(obj: A | B):
if isinstance(obj, A):
pass
elif isinstance(obj, B):
pass
elif isinstance(obj, C):
pass
else:
assert_never(obj)
def if_else_isinstance_error(obj: A | B):
if isinstance(obj, A):
pass
# B is missing
elif isinstance(obj, C):
pass
else:
# error: [type-assertion-failure] "Argument does not have asserted type `Never`"
assert_never(obj)
def if_else_singletons_success(obj: Literal[1, "a"] | None):
if obj == 1:
pass
elif obj == "a":
pass
elif obj is None:
pass
else:
assert_never(obj)
def if_else_singletons_error(obj: Literal[1, "a"] | None):
if obj == 1:
pass
elif obj is "A": # "A" instead of "a"
pass
elif obj is None:
pass
else:
# error: [type-assertion-failure] "Argument does not have asserted type `Never`"
assert_never(obj)
def match_singletons_success(obj: Literal[1, "a"] | None):
match obj:
case 1:
pass
case "a":
pass
case None:
pass
case _ as obj:
assert_never(obj)
def match_singletons_error(obj: Literal[1, "a"] | None):
match obj:
case 1:
pass
case "A": # "A" instead of "a"
pass
case None:
pass
case _ as obj:
# TODO: We should emit an error here, but the message should
# show the type `Literal["a"]` instead of `@Todo(…)`.
# error: [type-assertion-failure] "Argument does not have asserted type `Never`"
assert_never(obj)