fix(base): TableRows.__len__ read a SELECT's rowcount (-1 on SQLite)

sqldol/base.py

@@ -99,8 +99,14 @@ def __iter__(self):
             yield from result
 
     def __len__(self):
-        with rows_iter(self.table, self.filt, engine=self.engine) as result:
-            return result.rowcount
+        # Note: Counted by the database, not read off a SELECT's ``rowcount``, which
+        # is -1 on drivers that don't pre-buffer results (SQLite, for one) -- so
+        # ``len()`` raised ``ValueError`` there instead of counting.
+        query = select(func.count()).select_from(self.table)
+        if self.filt is not None:
+            query = query.where(self.filt)
+        with self.engine.connect() as connection:
+            return connection.execute(query).scalar_one()
 
     @property
     def table_name(self):

sqldol/tests/test_table_rows_len.py

@@ -0,0 +1,36 @@
+"""Tests for ``TableRows.__len__``: it must count rows, not report a SELECT's rowcount.
+
+Uses in-memory SQLite only -- no external service, no credentials.
+"""
+
+import pytest
+from sqlalchemy import create_engine, text
+
+from sqldol import TableRows
+
+TABLE_NAME = "t"
+
+
+@pytest.fixture
+def engine():
+    """An in-memory SQLite engine holding ``t(k, v)`` with three rows."""
+    engine = create_engine("sqlite:///:memory:")
+    with engine.connect() as connection:
+        connection.execute(text(f"CREATE TABLE {TABLE_NAME} (k TEXT, v INTEGER)"))
+        connection.execute(
+            text(f"INSERT INTO {TABLE_NAME} VALUES ('a', 1), ('b', 2), ('c', 3)")
+        )
+        connection.commit()
+    return engine
+
+
+def test_len_counts_the_rows(engine):
+    """``rowcount`` of a SELECT is -1 on SQLite, so ``len`` used to raise ValueError."""
+    rows = TableRows(TABLE_NAME, engine=engine)
+    assert len(rows) == 3 == len(list(rows))
+
+
+def test_len_honours_the_filter(engine):
+    rows = TableRows(TABLE_NAME, engine=engine)
+    filtered = TableRows(rows.table, rows.table.c.v >= 2, engine=engine)
+    assert len(filtered) == 2 == len(list(filtered))