allow single extension as str in make_dataset (#5229)

* allow single extension as str in make_dataset

* remove test class

* remove regex

* revert collection to tuple

* cleanup

test/test_datasets_utils.py

 import contextlib
 import gzip
 import os
+import pathlib
+import re
 import tarfile
 import zipfile
 
 import pytest
 import torchvision.datasets.utils as utils
 from torch._utils_internal import get_file_path_2
+from torchvision.datasets.folder import make_dataset
 from torchvision.datasets.utils import _COMPRESSED_FILE_OPENERS
 
-
 TEST_FILE = get_file_path_2(
     os.path.dirname(os.path.abspath(__file__)), "assets", "encode_jpeg", "grace_hopper_517x606.jpg"
 )
@@ -214,5 +216,29 @@ class TestDatasetsUtils:
         pytest.raises(ValueError, utils.verify_str_arg, "b", ("a",), "arg")
 
 
+@pytest.mark.parametrize(
+    ("kwargs", "expected_error_msg"),
+    [
+        (dict(is_valid_file=lambda path: pathlib.Path(path).suffix in {".png", ".jpeg"}), "classes c"),
+        (dict(extensions=".png"), re.escape("classes b, c. Supported extensions are: .png")),
+        (dict(extensions=(".png", ".jpeg")), re.escape("classes c. Supported extensions are: .png, .jpeg")),
+    ],
+)
+def test_make_dataset_no_valid_files(tmpdir, kwargs, expected_error_msg):
+    tmpdir = pathlib.Path(tmpdir)
+
+    (tmpdir / "a").mkdir()
+    (tmpdir / "a" / "a.png").touch()
+
+    (tmpdir / "b").mkdir()
+    (tmpdir / "b" / "b.jpeg").touch()
+
+    (tmpdir / "c").mkdir()
+    (tmpdir / "c" / "c.unknown").touch()
+
+    with pytest.raises(FileNotFoundError, match=expected_error_msg):
+        make_dataset(str(tmpdir), **kwargs)
+
+
 if __name__ == "__main__":
     pytest.main([__file__])

torchvision/datasets/folder.py

 import os
 import os.path
 from typing import Any, Callable, cast, Dict, List, Optional, Tuple
+from typing import Union
 
 from PIL import Image
 
 from .vision import VisionDataset
 
 
-def has_file_allowed_extension(filename: str, extensions: Tuple[str, ...]) -> bool:
+def has_file_allowed_extension(filename: str, extensions: Union[str, Tuple[str, ...]]) -> bool:
     """Checks if a file is an allowed extension.
 
     Args:
@@ -17,7 +18,7 @@ def has_file_allowed_extension(filename: str, extensions: Tuple[str, ...]) -> bo
     Returns:
         bool: True if the filename ends with one of given extensions
     """
-    return filename.lower().endswith(extensions)
+    return filename.lower().endswith(extensions if isinstance(extensions, str) else tuple(extensions))
 
 
 def is_image_file(filename: str) -> bool:
@@ -48,7 +49,7 @@ def find_classes(directory: str) -> Tuple[List[str], Dict[str, int]]:
 def make_dataset(
     directory: str,
     class_to_idx: Optional[Dict[str, int]] = None,
-    extensions: Optional[Tuple[str, ...]] = None,
+    extensions: Optional[Union[str, Tuple[str, ...]]] = None,
     is_valid_file: Optional[Callable[[str], bool]] = None,
 ) -> List[Tuple[str, int]]:
     """Generates a list of samples of a form (path_to_sample, class).
@@ -73,7 +74,7 @@ def make_dataset(
     if extensions is not None:
 
         def is_valid_file(x: str) -> bool:
-            return has_file_allowed_extension(x, cast(Tuple[str, ...], extensions))
+            return has_file_allowed_extension(x, extensions)  # type: ignore[arg-type]
 
     is_valid_file = cast(Callable[[str], bool], is_valid_file)
 
@@ -98,7 +99,7 @@ def make_dataset(
     if empty_classes:
         msg = f"Found no valid file for the classes {', '.join(sorted(empty_classes))}. "
         if extensions is not None:
-            msg += f"Supported extensions are: {', '.join(extensions)}"
+            msg += f"Supported extensions are: {extensions if isinstance(extensions, str) else ', '.join(extensions)}"
         raise FileNotFoundError(msg)
 
     return instances