fix: Use correct file protocol prefixing

dataframely/_storage/__init__.py

@@ -2,7 +2,6 @@
 # SPDX-License-Identifier: BSD-3-Clause
 
 from ._base import StorageBackend
+from ._fsspec import get_file_prefix
 
-__all__ = [
-    "StorageBackend",
-]
+__all__ = ["StorageBackend", "get_file_prefix"]

dataframely/_storage/_fsspec.py

@@ -0,0 +1,18 @@
+# Copyright (c) QuantCo 2025-2026
+# SPDX-License-Identifier: BSD-3-Clause
+
+from fsspec import AbstractFileSystem
+
+
+def get_file_prefix(fs: AbstractFileSystem) -> str:
+    match fs.protocol:
+        case "file":
+            return ""
+        case str():
+            return f"{fs.protocol}://"
+        case ["file", *_]:
+            return ""
+        case [proto, *_]:
+            return f"{proto}://"
+        case _:
+            raise ValueError(f"Unexpected fs.protocol: {fs.protocol}")

dataframely/_storage/parquet.py

@@ -7,6 +7,8 @@
 import polars as pl
 from fsspec import AbstractFileSystem, url_to_fs
 
+from dataframely._storage import get_file_prefix
+
 from ._base import (
     SerializedCollection,
     SerializedRules,
@@ -155,15 +157,7 @@ def _collection_from_parquet(
                 if is_file:
                     collection_types.append(_read_serialized_collection(source_path))
                 else:
-                    prefix = (
-                        ""
-                        if fs.protocol == "file"
-                        else (
-                            f"{fs.protocol}://"
-                            if isinstance(fs.protocol, str)
-                            else f"{fs.protocol[0]}://"
-                        )
-                    )
+                    prefix = get_file_prefix(fs)
                     for file in fs.glob(fs.sep.join([source_path, "**", "*.parquet"])):
                         collection_types.append(
                             _read_serialized_collection(f"{prefix}{file}")

dataframely/testing/storage.py

@@ -10,6 +10,7 @@
 import dataframely as dy
 from dataframely import FailureInfo, Validation
 from dataframely._compat import deltalake
+from dataframely._storage import get_file_prefix
 from dataframely._storage.delta import _to_delta_table
 
 # ----------------------------------- Schema -------------------------------------------
@@ -190,21 +191,7 @@ def set_metadata(self, path: str, metadata: dict[str, Any]) -> None:
         metadata."""
 
     def _prefix_path(self, path: str, fs: AbstractFileSystem) -> str:
-        return f"{self._get_prefix(fs)}{path}"
-
-    @staticmethod
-    def _get_prefix(fs: AbstractFileSystem) -> str:
-        match fs.protocol:
-            case "file":
-                return ""
-            case str():
-                return f"{fs.protocol}://"
-            case ["file", *_]:
-                return ""
-            case [proto, *_]:
-                return f"{proto}://"
-            case _:
-                raise ValueError(f"Unexpected fs.protocol: {fs.protocol}")
+        return f"{get_file_prefix(fs)}{path}"
 
 
 class ParquetCollectionStorageTester(CollectionStorageTester):