Typing information lost while calling __iter__

[JulienPalard]

With the following example:

from typing import Literal

dest: Literal["tata", "tutu"]

for src in "tata", "tutu":
    dest = src

I'm getting:

error: Incompatible types in assignment (expression has type "str", variable has type "Literal['http', 'https']")

Trying to dig with reveal_type:

t = "tata", "tutu"
reveal_type(t)
reveal_type(t.__iter__())

I get:

test.py:2: note: Revealed type is "Tuple[builtins.str, builtins.str]"
test.py:3: note: Revealed type is "typing.Iterator[builtins.str*]"

So it looks OK, but digging with pdb, in mypy/checker.py:analyze_iterable_item_type(), after calling:

iterator = echk.check_method_call_by_name('__iter__', iterable, [], [], expr)[0]

I see:

iterable == Tuple[Literal['foo']?, Literal['bar']?]
iterator == typing.Iterator[builtins.str*])

I think some information is lost here, should iterator get: typing.Iterator[Literal['foo', 'bar']] instead?

[JulienPalard]

Hum I played with join_instances so joining two Literal of strings give an union of the two string literals to avoid loosing information, instead of returning str.

But later during the assignement of the for loop variable, in checker.py, in check_assignment(), even if the value from the right is precise (like Union[Literal['tutu']?, Literal['tata']?], the info get lost at:

rvalue_type = remove_instance_last_known_values(rvalue_type)

which falls back again to:

Union[builtins.str, builtins.str]

So no luck, instead of getting:

error: Incompatible types in assignment (expression has type "str", variable has type "Union[Literal['tata'], Literal['tutu']]")

I'm getting:

error: Incompatible types in assignment (expression has type "Union[str, str]", variable has type "Union[Literal['tata'], Literal['tutu']]")

(I'm not sure I'm following the right path...)

[chrisyeh96]

I'm having the same issue. My code looks like:

from typing import Literal

dest: Literal["tata", "tutu"]

for src in ["tata", "tutu"]:
    dest = src

and mypy is complaining

[hauntsaninja]

Fixed by #18014