fix(python): require explicit region for S3 buckets with dots (#2892)

When region is not specific in the s3 path, `resolve_s3_region` from
"lance-format" project (see [here][1]) will resolve the region by
calling `resolve_bucket_region`, which is a function from the
"arrow-rs-object-store" project expecting [virtual-hosted-style
URLs][1]. When there are dot (".") in the virtual-hosted-style URLs, it
breaks automatic region detection. See more details in the issue
description:
https://github.com/lancedb/lancedb/issues/1898#issuecomment-3690142427

This PR add early validation in connect() and connect_async() to raise a
clear error with instructions when the region is not specified for such
buckets.


[1]:
https://github.com/lance-format/lance/blob/v2.0.0-beta.4/rust/lance-io/src/object_store/providers/aws.rs#L197
[2]:
eedbf3d7d8/src/aws/resolve.rs (L52C5-L52C65)
[3]:
https://docs.aws.amazon.com/AmazonS3/latest/userguide/VirtualHosting.html#virtual-hosted-style-access

Fixes #1898

python/python/lancedb/__init__.py

@@ -13,6 +13,7 @@ __version__ = importlib.metadata.version("lancedb")
 
 from ._lancedb import connect as lancedb_connect
 from .common import URI, sanitize_uri
+from urllib.parse import urlparse
 from .db import AsyncConnection, DBConnection, LanceDBConnection
 from .io import StorageOptionsProvider
 from .remote import ClientConfig
@@ -28,6 +29,39 @@ from .namespace import (
 )
 
 
+def _check_s3_bucket_with_dots(
+    uri: str, storage_options: Optional[Dict[str, str]]
+) -> None:
+    """
+    Check if an S3 URI has a bucket name containing dots and warn if no region
+    is specified. S3 buckets with dots cannot use virtual-hosted-style URLs,
+    which breaks automatic region detection.
+
+    See: https://github.com/lancedb/lancedb/issues/1898
+    """
+    if not isinstance(uri, str) or not uri.startswith("s3://"):
+        return
+
+    parsed = urlparse(uri)
+    bucket = parsed.netloc
+
+    if "." not in bucket:
+        return
+
+    # Check if region is provided in storage_options
+    region_keys = {"region", "aws_region"}
+    has_region = storage_options and any(k in storage_options for k in region_keys)
+
+    if not has_region:
+        raise ValueError(
+            f"S3 bucket name '{bucket}' contains dots, which prevents automatic "
+            f"region detection. Please specify the region explicitly via "
+            f"storage_options={{'region': '<your-region>'}} or "
+            f"storage_options={{'aws_region': '<your-region>'}}. "
+            f"See https://github.com/lancedb/lancedb/issues/1898 for details."
+        )
+
+
 def connect(
     uri: URI,
     *,
@@ -121,9 +155,11 @@ def connect(
             storage_options=storage_options,
             **kwargs,
         )
+    _check_s3_bucket_with_dots(str(uri), storage_options)
 
     if kwargs:
         raise ValueError(f"Unknown keyword arguments: {kwargs}")
+
     return LanceDBConnection(
         uri,
         read_consistency_interval=read_consistency_interval,
@@ -211,6 +247,8 @@ async def connect_async(
     if isinstance(client_config, dict):
         client_config = ClientConfig(**client_config)
 
+    _check_s3_bucket_with_dots(str(uri), storage_options)
+
     return AsyncConnection(
         await lancedb_connect(
             sanitize_uri(uri),

python/python/tests/test_s3_bucket_dots.py

@@ -0,0 +1,68 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+"""
+Tests for S3 bucket names containing dots.
+
+Related issue: https://github.com/lancedb/lancedb/issues/1898
+
+These tests validate the early error checking for S3 bucket names with dots.
+No actual S3 connection is made - validation happens before connection.
+"""
+
+import pytest
+import lancedb
+
+# Test URIs
+BUCKET_WITH_DOTS = "s3://my.bucket.name/path"
+BUCKET_WITH_DOTS_AND_REGION = ("s3://my.bucket.name", {"region": "us-east-1"})
+BUCKET_WITH_DOTS_AND_AWS_REGION = ("s3://my.bucket.name", {"aws_region": "us-east-1"})
+BUCKET_WITHOUT_DOTS = "s3://my-bucket/path"
+
+
+class TestS3BucketWithDotsSync:
+    """Tests for connect()."""
+
+    def test_bucket_with_dots_requires_region(self):
+        with pytest.raises(ValueError, match="contains dots"):
+            lancedb.connect(BUCKET_WITH_DOTS)
+
+    def test_bucket_with_dots_and_region_passes(self):
+        uri, opts = BUCKET_WITH_DOTS_AND_REGION
+        db = lancedb.connect(uri, storage_options=opts)
+        assert db is not None
+
+    def test_bucket_with_dots_and_aws_region_passes(self):
+        uri, opts = BUCKET_WITH_DOTS_AND_AWS_REGION
+        db = lancedb.connect(uri, storage_options=opts)
+        assert db is not None
+
+    def test_bucket_without_dots_passes(self):
+        db = lancedb.connect(BUCKET_WITHOUT_DOTS)
+        assert db is not None
+
+
+class TestS3BucketWithDotsAsync:
+    """Tests for connect_async()."""
+
+    @pytest.mark.asyncio
+    async def test_bucket_with_dots_requires_region(self):
+        with pytest.raises(ValueError, match="contains dots"):
+            await lancedb.connect_async(BUCKET_WITH_DOTS)
+
+    @pytest.mark.asyncio
+    async def test_bucket_with_dots_and_region_passes(self):
+        uri, opts = BUCKET_WITH_DOTS_AND_REGION
+        db = await lancedb.connect_async(uri, storage_options=opts)
+        assert db is not None
+
+    @pytest.mark.asyncio
+    async def test_bucket_with_dots_and_aws_region_passes(self):
+        uri, opts = BUCKET_WITH_DOTS_AND_AWS_REGION
+        db = await lancedb.connect_async(uri, storage_options=opts)
+        assert db is not None
+
+    @pytest.mark.asyncio
+    async def test_bucket_without_dots_passes(self):
+        db = await lancedb.connect_async(BUCKET_WITHOUT_DOTS)
+        assert db is not None