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Unable to remove Optional wrapper when using Mapping[str, str] instead of dict #11618
Description
Hello, I'm not sure if this is a bug, but perhaps it'll help to clarify the difference between using dict and Mapping[str, str] as annotations.
This is a common pattern in Python to avoid mutable default arguments:
def foo(x: Optional[dict] = None):
if x is None:
x = {}So I tried writing a function to capture this pattern.
from typing import Optional, TypeVar
T = TypeVar("T")
def assign_if_none(obj: Optional[T], assignment: T) -> T:
return assignment if obj is None else objNow you can use it like this:
def foo(x: Optional[dict] = None):
x = assign_to_none(x, {})But mypy raises errors when I use a type annotation with Mapping or Dict instead of dict.
Full example:
from typing import Mapping, Optional, TypeVar
T = TypeVar("T")
def assign_if_none(obj: Optional[T], assignment: T) -> T:
return assignment if obj is None else obj
def func_1(mapping: Optional[dict] = None) -> str:
mapping = assign_if_none(mapping, {})
if "key" in mapping:
return "Included"
return "Excluded"
def func_2(mapping: Optional[Mapping[str, str]] = None) -> str:
mapping = assign_if_none(mapping, {})
if "key" in mapping:
return "Included"
return "Excluded"func_1 has no errors, but func_2 does.
demo.py:23: error: Incompatible types in assignment (expression has type "object", variable has type "Optional[Mapping[str, str]]")
demo.py:25: error: Unsupported right operand type for in ("Optional[Mapping[str, str]]")
I tried putting reveal_type(mapping) before and after the call to assign_if_none in both functions. Here are the results:
func_1:
demo.py:13: note: Revealed type is "Union[builtins.dict[Any, Any], None]"
demo.py:15: note: Revealed type is "builtins.dict[Any, Any]"
func_2:
demo.py:25: note: Revealed type is "Union[typing.Mapping[builtins.str, builtins.str], None]"
demo.py:27: note: Revealed type is "Union[typing.Mapping[builtins.str, builtins.str], None]"
So in func_1, mypy correctly infers that the call to assign_if_none removes the Optional wrapper. But this doesn't happen in func_2.
If this isn't a bug with mypy, is there a better way to annotate the function assign_if_none so it works in func_2?
Environment
- Mypy version used: 0.910
- Python version used: 3.8.12
Activity
erictraut commentedon Nov 25, 2021
This does look like a bug in mypy, probably in the TypeVar constraint solver logic. This type checks fine in pyright.
Interestingly, if you add a
castor an explicit type annotation for{}, the errors go away.or
JelleZijlstra commentedon Nov 26, 2021
Does
--allow-redefinitionhelp here?hauntsaninja commentedon Nov 26, 2021
My guess would be that mypy infers
{}as adict[bottom, bottom], but dict is invariant in its key type (a common surprise for users) and so mypy joinsMapping[str, str]anddict[bottom, bottom]toobject. Evidence in favour of this guess is thatmapping = assign_if_none(mapping, cast(dict[str, str], {}))works as well.hauntsaninja commentedon Nov 26, 2021
Yeah, looks like that's it. A similar thing happens with List, but not with Sequence
hauntsaninja commentedon Jun 21, 2024
This example got fixed in #16994