GH-41365: [Python] Fix S3 URI with whitespace silently falling back to LocalFileSystem

python/pyarrow/fs.py

@@ -82,6 +82,32 @@ def __getattr__(name):
     )
 
 
+def _is_likely_uri(path):
+    """
+    Check if a string looks like a URI (has a valid scheme followed by ':').
+
+    This is a Python port of the C++ ``IsLikelyUri()`` heuristic in
+    ``cpp/src/arrow/filesystem/path_util.cc``.  It is intentionally
+    conservative: single-letter schemes are excluded (could be a Windows
+    drive letter) and the scheme must conform to RFC 3986.
+    """
+    if not path or path[0] == '/':
+        return False
+    colon_pos = path.find(':')
+    if colon_pos < 0:
+        return False
+    # One-letter URI schemes don't officially exist; may be a Windows drive
+    if colon_pos < 2:
+        return False
+    # The largest IANA-registered scheme is 36 characters
+    if colon_pos > 36:
+        return False
+    scheme = path[:colon_pos]
+    if not scheme[0].isalpha():
+        return False
+    return all(c.isalnum() or c in ('+', '-', '.') for c in scheme[1:])
+
+
 def _ensure_filesystem(filesystem, *, use_mmap=False):
     if isinstance(filesystem, FileSystem):
         return filesystem
@@ -174,13 +200,17 @@ def _resolve_filesystem_and_path(path, filesystem=None, *, memory_map=False):
             filesystem, path = FileSystem.from_uri(path)
         except ValueError as e:
             msg = str(e)
-            if "empty scheme" in msg or "Cannot parse URI" in msg:
-                # neither an URI nor a locally existing path, so assume that
-                # local path was given and propagate a nicer file not found
-                # error instead of a more confusing scheme parsing error
+            if "empty scheme" in msg:
+                # No scheme at all — treat as a local path and propagate
+                # a nicer "file not found" error later.
+                pass
+            elif "Cannot parse URI" in msg and not _is_likely_uri(path):
+                # Path doesn't look like a URI (no valid scheme prefix),
+                # so treat it as a local path rather than surfacing a
+                # confusing URI-parsing error.
                 pass
             else:
-                raise e
+                raise
     else:
         path = filesystem.normalize_path(path)
 

python/pyarrow/tests/test_fs.py

@@ -1695,6 +1695,67 @@ def test_filesystem_from_path_object(path):
     assert path == p.resolve().absolute().as_posix()
 
 
+def test_is_likely_uri():
+    """Unit tests for the _is_likely_uri() heuristic."""
+    from pyarrow.fs import _is_likely_uri
+
+    # Valid URI schemes
+    assert _is_likely_uri("s3://bucket/key")
+    assert _is_likely_uri("gs://bucket/key")
+    assert _is_likely_uri("hdfs://host/path")
+    assert _is_likely_uri("file:///local/path")
+    assert _is_likely_uri("abfss://container@account/path")
+    assert _is_likely_uri("grpc+https://host:443")
+
+    # Not URIs — local paths, Windows drives, empty, etc.
+    assert not _is_likely_uri("")
+    assert not _is_likely_uri("/absolute/path")
+    assert not _is_likely_uri("relative/path")
+    assert not _is_likely_uri("C:\\Users\\foo")      # single-letter → drive
+    assert not _is_likely_uri("C:/Users/foo")
+    assert not _is_likely_uri("3bucket://key")       # scheme starts with digit
+    assert not _is_likely_uri("-scheme://key")        # scheme starts with dash
+
+
+def test_resolve_filesystem_and_path_uri_with_spaces():
+    """
+    URIs with a recognised scheme but un-encoded spaces must raise
+    ValueError — NOT silently fall back to LocalFileSystem.
+    (GH-41365)
+    """
+    from pyarrow.fs import _resolve_filesystem_and_path
+
+    # S3 URI with spaces should raise, not return LocalFileSystem
+    with pytest.raises(ValueError, match="Cannot parse URI"):
+        _resolve_filesystem_and_path("s3://bucket/path with space/file.parquet")
+
+    # GCS URI with spaces should also raise
+    with pytest.raises(ValueError, match="Cannot parse URI"):
+        _resolve_filesystem_and_path("gs://bucket/path with space/file.csv")
+
+    # abfss URI with spaces
+    with pytest.raises(ValueError, match="Cannot parse URI"):
+        _resolve_filesystem_and_path(
+            "abfss://container@account/dir with space/file"
+        )
+
+
+def test_resolve_filesystem_and_path_local_with_spaces():
+    """
+    Local paths (no scheme) with spaces should still resolve to
+    LocalFileSystem — they must NOT be confused with malformed URIs.
+    """
+    from pyarrow.fs import _resolve_filesystem_and_path
+
+    # Absolute local path with spaces → LocalFileSystem
+    fs, path = _resolve_filesystem_and_path("/tmp/path with spaces/data")
+    assert isinstance(fs, LocalFileSystem)
+
+    # Non-existent absolute path → still LocalFileSystem
+    fs, path = _resolve_filesystem_and_path("/nonexistent/path")
+    assert isinstance(fs, LocalFileSystem)
+
+
 @pytest.mark.s3
 def test_filesystem_from_uri_s3(s3_server):
     from pyarrow.fs import S3FileSystem