Better error message for users trying to use abfss or s3 schemes with SDK DBUtils (#634)

## Changes
DBUtils in the SDK only supports the `dbfs` and `file` schemes. Within
DBFS, only paths in the root filesystem are supported, not mountpoints.
Other schemes are not supported. To minimize customer confusion, we can
check for this case during all DBUtils FS operations when constructing
the path.

## Tests
Added a unit test for this case.

- [ ] `make test` run locally
- [ ] `make fmt` applied
- [ ] relevant integration tests applied

Author: mgyucht

databricks/sdk/mixins/files.py

@@ -11,6 +11,7 @@
 from types import TracebackType
 from typing import (TYPE_CHECKING, AnyStr, BinaryIO, Generator, Iterable,
                     Iterator, Type, Union)
+from urllib import parse
 
 from .._property import _cached_property
 from ..errors import NotFound
@@ -570,15 +571,19 @@ def exists(self, path: str) -> bool:
         p = self._path(path)
         return p.exists()
 
+    __ALLOWED_SCHEMES = [None, 'file', 'dbfs']
+
     def _path(self, src):
-        src = str(src)
-        if src.startswith('file:'):
-            return _LocalPath(src)
-        if src.startswith('dbfs:'):
-            src = src[len('dbfs:'):]
-        if src.startswith('/Volumes'):
-            return _VolumesPath(self._files_api, src)
-        return _DbfsPath(self._dbfs_api, src)
+        src = parse.urlparse(str(src))
+        if src.scheme and src.scheme not in self.__ALLOWED_SCHEMES:
+            raise ValueError(
+                f'unsupported scheme "{src.scheme}". DBUtils in the SDK only supports local, root DBFS, and '
+                'UC Volumes paths, not external locations or DBFS mount points.')
+        if src.scheme == 'file':
+            return _LocalPath(src.geturl())
+        if src.path.startswith('/Volumes'):
+            return _VolumesPath(self._files_api, src.geturl())
+        return _DbfsPath(self._dbfs_api, src.geturl())
 
     def copy(self, src: str, dst: str, *, recursive=False, overwrite=False):
         """Copy files between DBFS and local filesystems"""

tests/test_dbfs_mixins.py

@@ -63,3 +63,10 @@ def test_moving_local_dir_to_dbfs(config, tmp_path, mocker):
 def test_fs_path(config, path, expected_type):
     dbfs_ext = DbfsExt(config)
     assert isinstance(dbfs_ext._path(path), expected_type)
+
+
+def test_fs_path_invalid(config):
+    dbfs_ext = DbfsExt(config)
+    with pytest.raises(ValueError) as e:
+        dbfs_ext._path('s3://path/to/file')
+    assert 'unsupported scheme "s3"' in str(e.value)