Move absolute path finder from open_mfdataset to own function (#7968)

* Move absolute path finder to own function

* Update common.py

* Workaround for strange \ behavior

* More workarounds

* Update common.py

* Update common.py

* Update common.py

* Update xarray/backends/common.py

Co-authored-by: Michael Niklas  <mick.niklas@gmail.com>

* Update common.py

---------

Co-authored-by: Michael Niklas  <mick.niklas@gmail.com>

xarray/backends/api.py

@@ -3,7 +3,6 @@
 import os
 from collections.abc import Hashable, Iterable, Mapping, MutableMapping, Sequence
 from functools import partial
-from glob import glob
 from io import BytesIO
 from numbers import Number
 from typing import (
@@ -21,7 +20,12 @@
 
 from xarray import backends, conventions
 from xarray.backends import plugins
-from xarray.backends.common import AbstractDataStore, ArrayWriter, _normalize_path
+from xarray.backends.common import (
+    AbstractDataStore,
+    ArrayWriter,
+    _find_absolute_paths,
+    _normalize_path,
+)
 from xarray.backends.locks import _get_scheduler
 from xarray.core import indexing
 from xarray.core.combine import (
@@ -967,37 +971,7 @@ def open_mfdataset(
     .. [1] https://docs.xarray.dev/en/stable/dask.html
     .. [2] https://docs.xarray.dev/en/stable/dask.html#chunking-and-performance
     """
-    if isinstance(paths, str):
-        if is_remote_uri(paths) and engine == "zarr":
-            try:
-                from fsspec.core import get_fs_token_paths
-            except ImportError as e:
-                raise ImportError(
-                    "The use of remote URLs for opening zarr requires the package fsspec"
-                ) from e
-
-            fs, _, _ = get_fs_token_paths(
-                paths,
-                mode="rb",
-                storage_options=kwargs.get("backend_kwargs", {}).get(
-                    "storage_options", {}
-                ),
-                expand=False,
-            )
-            tmp_paths = fs.glob(fs._strip_protocol(paths))  # finds directories
-            paths = [fs.get_mapper(path) for path in tmp_paths]
-        elif is_remote_uri(paths):
-            raise ValueError(
-                "cannot do wild-card matching for paths that are remote URLs "
-                f"unless engine='zarr' is specified. Got paths: {paths}. "
-                "Instead, supply paths as an explicit list of strings."
-            )
-        else:
-            paths = sorted(glob(_normalize_path(paths)))
-    elif isinstance(paths, os.PathLike):
-        paths = [os.fspath(paths)]
-    else:
-        paths = [os.fspath(p) if isinstance(p, os.PathLike) else p for p in paths]
+    paths = _find_absolute_paths(paths, engine=engine, **kwargs)
 
     if not paths:
         raise OSError("no files to open")

xarray/backends/common.py

@@ -5,6 +5,7 @@
 import time
 import traceback
 from collections.abc import Iterable
+from glob import glob
 from typing import TYPE_CHECKING, Any, ClassVar
 
 import numpy as np
@@ -19,6 +20,7 @@
     from io import BufferedIOBase
 
     from xarray.core.dataset import Dataset
+    from xarray.core.types import NestedSequence
 
 # Create a logger object, but don't add any handlers. Leave that to user code.
 logger = logging.getLogger(__name__)
@@ -28,6 +30,24 @@
 
 
 def _normalize_path(path):
+    """
+    Normalize pathlikes to string.
+
+    Parameters
+    ----------
+    path :
+        Path to file.
+
+    Examples
+    --------
+    >>> from pathlib import Path
+
+    >>> directory = Path(xr.backends.common.__file__).parent
+    >>> paths_path = Path(directory).joinpath("comm*n.py")
+    >>> paths_str = xr.backends.common._normalize_path(paths_path)
+    >>> print([type(p) for p in (paths_str,)])
+    [<class 'str'>]
+    """
     if isinstance(path, os.PathLike):
         path = os.fspath(path)
 
@@ -37,6 +57,64 @@ def _normalize_path(path):
     return path
 
 
+def _find_absolute_paths(
+    paths: str | os.PathLike | NestedSequence[str | os.PathLike], **kwargs
+) -> list[str]:
+    """
+    Find absolute paths from the pattern.
+
+    Parameters
+    ----------
+    paths :
+        Path(s) to file(s). Can include wildcards like * .
+    **kwargs :
+        Extra kwargs. Mainly for fsspec.
+
+    Examples
+    --------
+    >>> from pathlib import Path
+
+    >>> directory = Path(xr.backends.common.__file__).parent
+    >>> paths = str(Path(directory).joinpath("comm*n.py"))  # Find common with wildcard
+    >>> paths = xr.backends.common._find_absolute_paths(paths)
+    >>> [Path(p).name for p in paths]
+    ['common.py']
+    """
+    if isinstance(paths, str):
+        if is_remote_uri(paths) and kwargs.get("engine", None) == "zarr":
+            try:
+                from fsspec.core import get_fs_token_paths
+            except ImportError as e:
+                raise ImportError(
+                    "The use of remote URLs for opening zarr requires the package fsspec"
+                ) from e
+
+            fs, _, _ = get_fs_token_paths(
+                paths,
+                mode="rb",
+                storage_options=kwargs.get("backend_kwargs", {}).get(
+                    "storage_options", {}
+                ),
+                expand=False,
+            )
+            tmp_paths = fs.glob(fs._strip_protocol(paths))  # finds directories
+            paths = [fs.get_mapper(path) for path in tmp_paths]
+        elif is_remote_uri(paths):
+            raise ValueError(
+                "cannot do wild-card matching for paths that are remote URLs "
+                f"unless engine='zarr' is specified. Got paths: {paths}. "
+                "Instead, supply paths as an explicit list of strings."
+            )
+        else:
+            paths = sorted(glob(_normalize_path(paths)))
+    elif isinstance(paths, os.PathLike):
+        paths = [os.fspath(paths)]
+    else:
+        paths = [os.fspath(p) if isinstance(p, os.PathLike) else p for p in paths]
+
+    return paths
+
+
 def _encode_variable_name(name):
     if name is None:
         name = NONE_VAR_NAME