Throw an error in sequence_metadata getter if there's no URL

If CladeTime doesn't have a value for url_sequence_metadata,
there's no point in proceeding.

pyproject.toml

@@ -59,6 +59,9 @@ filterwarnings = [
     "ignore::DeprecationWarning",
     'ignore:polars found a filename',
 ]
+testpaths = [
+    "tests",
+]
 
 [tool.ruff]
 line-length = 120

src/virus_clade_utils/_typing.py

@@ -0,0 +1,10 @@
+"""Type aliases for this package."""
+
+from pathlib import Path
+from typing import TypeAlias, Union
+
+from cloudpathlib import AnyPath, CloudPath
+
+# Data types
+# Pathlike: TypeAlias = Path | AnyPath | CloudPath
+Pathlike: TypeAlias = Union["Path", "AnyPath", "CloudPath"]

src/virus_clade_utils/cladetime.py

@@ -5,7 +5,7 @@
 import polars as pl
 import structlog
 
-from virus_clade_utils.exceptions import CladeTimeInvalidDateError
+from virus_clade_utils.exceptions import CladeTimeInvalidDateError, CladeTimeInvalidURLError
 from virus_clade_utils.util.config import Config
 from virus_clade_utils.util.reference import _get_s3_object_url
 from virus_clade_utils.util.sequence import _get_ncov_metadata, get_covid_genome_metadata
@@ -103,7 +103,7 @@ def sequence_metadata(self) -> pl.LazyFrame:
             sequence_metadata = get_covid_genome_metadata(metadata_url=self.url_sequence_metadata)
             return sequence_metadata
         else:
-            sequence_metadata = pl.LazyFrame()
+            raise CladeTimeInvalidURLError("CladeTime is missing url_sequence_metadata")
         return sequence_metadata
 
     def __repr__(self):

src/virus_clade_utils/exceptions.py

@@ -7,3 +7,7 @@ class Error(Exception):
 
 class CladeTimeInvalidDateError(Error):
     """Raised when an invalid date string is passed to CladeTime."""
+
+
+class CladeTimeInvalidURLError(Error):
+    """Raised when CladeTime encounters an invalid URL."""

tests/unit/test_cladetime.py

@@ -6,7 +6,7 @@
 import pytest
 from freezegun import freeze_time
 from virus_clade_utils.cladetime import CladeTime
-from virus_clade_utils.exceptions import CladeTimeInvalidDateError
+from virus_clade_utils.exceptions import CladeTimeInvalidDateError, CladeTimeInvalidURLError
 
 
 def test_cladetime_no_args():
@@ -124,5 +124,6 @@ def test_cladetime_sequence_metadata_no_url(test_config):
     with patch("virus_clade_utils.cladetime.CladeTime._get_config", mock):
         ct = CladeTime()
     ct.url_sequence_metadata = None
-    # if there's no metadata url, sequence metadata should be an empty LazyFrame
-    assert ct.sequence_metadata.collect().shape == (0, 0)
+
+    with pytest.raises(CladeTimeInvalidURLError):
+        ct.sequence_metadata