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0.920 Regression: literal unions aren't inferred from conditionals anymore #11782
Description
test.py:
def foo(h: bytes) -> int:
if h[5] == 2:
byteorder = "big"
elif h[5] == 1:
byteorder = "little"
else:
raise AssertionError(
f"unexpected ELF endianness: {h[5]}"
)
return int.from_bytes(h[16:18], byteorder=byteorder, signed=False)Results in
$ mypy test.py
1.py:10:17: note: Revealed type is "builtins.str"
1.py:11:47: error: Argument "byteorder" to "from_bytes" of "int" has incompatible type "str"; expected "Union[Literal['little'], Literal['big']]"
With mypy-0.920 and later. 0.910 correctly inferred byteorder as Literal['little'] | Literal['big']
Activity
pranavrajpal commentedon Dec 18, 2021
I don't think mypy ever inferred the type of
byteorderasLiteral['little', 'big']. I think we always inferredstrbut 0.920 changed the type signature ofint.from_bytesto take aLiteralinstead of astr, turning this into an error. (That change was added in python/typeshed#5715 and added to mypy in #10862, which was included in 0.920).You can silence the error by adding an explicit
Literal["little", "big"]type forbyteorder.hauntsaninja commentedon Dec 18, 2021
TBH I find several of these typeshed Literal changes to be quite problematic, e.g. here's one I recently reverted python/typeshed#6566
elprans commentedon Dec 18, 2021
@pranavrajpal, you're right, 0.910 also doesn't infer
byteorderasLiteral. I somehow convinced myself that it did. Not an inference regression then. Thanks for the explanation!