What happened?

If you try to type check a project that is using xarray there will be a failure when checking open_zarr. It is a best practice not to ignore this since if you do you generally have to ignore all type checking info on the open call which is otherwise useful.

This error I believe is since the default argument is of type Default which is a dummy type and thus causes issues when checking it. This essentially narrows the type more strict than just Unknown to my understanding.

Log message:

error: Argument of type "Literal['auto']" cannot be assigned to parameter "chunks" of type "Default" in function "open_zarr"

What did you expect to happen?

Open zarr should be typed in such a way that match the input arguments or not have any type information and be specified as the Unknown type

Minimal Complete Verifiable Example

# /// script
# requires-python = ">=3.11"
# dependencies = [
#   "xarray[complete]@git+https://github.com/pydata/xarray.git@main",
# ]
# ///
#
# This script automatically imports the development branch of xarray to check for issues.
# Please delete this header if you have _not_ tested this script with `uv run`!

import inspect
import xarray as xr

xr.show_versions()

from xarray import open_zarr

sig = inspect.signature(open_zarr)
store_argument = sig.parameters["store"]

# should show no type information; different type checkers will narrow 
# this differently but many including pyright / pylance will fail on it
print(store_argument.annotation)

Steps to reproduce

Type check any xarray project with pyright that uses open_zarr and passes in "auto" for the chunks arg.

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

error: Argument of type "Literal['auto']" cannot be assigned to parameter "chunks" of type "Default" in function "open_zarr"

Anything else we need to know?

From the type info in the docstring it seems like the type should be

Literal["auto"] | None | Literal[-1] | dict

I can create a PR for this if needed.

I was using pyright

pyright 1.1.402

but I don't think this is specific to pyright

Environment