html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/issues/4571#issuecomment-725455918,https://api.github.com/repos/pydata/xarray/issues/4571,725455918,MDEyOklzc3VlQ29tbWVudDcyNTQ1NTkxOA==,10194086,2020-11-11T14:32:08Z,2020-11-11T14:32:08Z,MEMBER,"Jup, given the effort required for 1. I also vote for 2. I learned a lot about typing/ mypy, though...","{""total_count"": 1, ""+1"": 0, ""-1"": 0, ""laugh"": 1, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,739281259
https://github.com/pydata/xarray/issues/4571#issuecomment-724768430,https://api.github.com/repos/pydata/xarray/issues/4571,724768430,MDEyOklzc3VlQ29tbWVudDcyNDc2ODQzMA==,10194086,2020-11-10T15:16:49Z,2020-11-11T14:30:51Z,MEMBER,"For context, mypy 0.790 now requires `__lt__` if something should be sorted. Thus the following fails (used to work):

```python

from typing import TypeVar
T = TypeVar(""T"")

def sort(lst: List[T]) -> List[T]):
    return sorted(lst)
```
which returns the error message above (`_LT` stands for `_LowerThan`). The workaround is to define a `TypeVar` that is `Sortable`:

<details>

```python
from typing import TypeVar, List, Protocol

class Sortable(Protocol):
    def __lt__(self, __other: Any) -> bool: ...

SortableT = TypeVar(""SortableT"", bound=Sortable)

def sort(lst: List[SortableT]) -> List[SortableT]):
    return sorted(lst)
```

</details>

What we want is a [sortable dict key](https://github.com/pydata/xarray/blob/7ce0110f727b37a776d509174365cf0905163234/xarray/core/utils.py#L465-L466). However, we define dict keys as `Hashable` (e.g. [dims](https://github.com/pydata/xarray/blob/f10a6fe273402015a7caab26bf66d6923b35169b/xarray/core/dataset.py#L688-L697)) which is not sortable (does not have `__lt__`). I see two possibilities:

1. type all keys of a `SortedKeysDict` as `SortableHashable` instead of `Hashable` (that would be a lot)
2. ignore this and add `# type: ignore` to [core/utils.py#L466](https://github.com/pydata/xarray/blob/7ce0110f727b37a776d509174365cf0905163234/xarray/core/utils.py#L466)

But maybe there is a better way?

Here is how 1. could look like (simplified):

<details>

```python
import collections
from typing import Any, Dict, Iterator, Mapping, Protocol, TypeVar


class HashableSortable(Protocol):
    def __lt__(self, __other: Any) -> bool:
        ...

    def __hash__(self) -> int:
        ...


HashableSortableK = TypeVar(""HashableSortableK"", bound=HashableSortable)
V = TypeVar(""V"")

# use UserDict for brevity
class SortedKeysDict(collections.UserDict[HashableSortableK, V]): # used to be [K, V]
    def __iter__(self) -> Iterator[HashableSortableK]:
        return iter(sorted(self.data))

# now we need to update the typing of all uses of `SortedKeysDict`, e.g. dims

class Dataset:

    _dims: Dict[HashableSortable, int] # used to be [Hashable, int]

    def __init__(self, dims) -> None:

        self._dims = dims

    @property
    def dims(self) -> Mapping[HashableSortable, int]:
        return SortedKeysDict(self._dims)
```

</details>









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