fix and add test for sequence_to_str (#5213)

* fix and add test for sequence_to_str

* remove manual ids

Author: pmeier

test/test_prototype_utils.py

@@ -0,0 +1,17 @@
+import pytest
+from torchvision.prototype.utils._internal import sequence_to_str
+
+
+@pytest.mark.parametrize(
+    ("seq", "separate_last", "expected"),
+    [
+        ([], "", ""),
+        (["foo"], "", "'foo'"),
+        (["foo", "bar"], "", "'foo', 'bar'"),
+        (["foo", "bar"], "and ", "'foo' and 'bar'"),
+        (["foo", "bar", "baz"], "", "'foo', 'bar', 'baz'"),
+        (["foo", "bar", "baz"], "and ", "'foo', 'bar', and 'baz'"),
+    ],
+)
+def test_sequence_to_str(seq, separate_last, expected):
+    assert sequence_to_str(seq, separate_last=separate_last) == expected

torchvision/prototype/utils/_internal.py

@@ -30,10 +30,15 @@ class StrEnum(enum.Enum, metaclass=StrEnumMeta):
 
 
 def sequence_to_str(seq: Sequence, separate_last: str = "") -> str:
+    if not seq:
+        return ""
     if len(seq) == 1:
         return f"'{seq[0]}'"
 
-    return f"""'{"', '".join([str(item) for item in seq[:-1]])}', {separate_last}'{seq[-1]}'"""
+    head = "'" + "', '".join([str(item) for item in seq[:-1]]) + "'"
+    tail = f"{'' if separate_last and len(seq) == 2 else ','} {separate_last}'{seq[-1]}'"
+
+    return head + tail
 
 
 def add_suggestion(