fix(memtable): ensure that constructing an empty memtable from a dataframe works (#10945)

Closes #10940.

Author: cpcloud

ibis/backends/tests/test_generic.py

@@ -40,6 +40,7 @@
 np = pytest.importorskip("numpy")
 pd = pytest.importorskip("pandas")
 tm = pytest.importorskip("pandas.testing")
+pa = pytest.importorskip("pyarrow")
 
 NULL_BACKEND_TYPES = {
     "bigquery": "NULL",
@@ -2516,3 +2517,17 @@ def test_table_describe_with_multiple_decimal_columns(con):
     expr = t.describe()
     result = con.to_pyarrow(expr)
     assert len(result) == 2
+
+
+@pytest.mark.parametrize(
+    "input",
+    [[], pa.table([[]], pa.schema({"x": pa.int64()}))],
+    ids=["list", "pyarrow-table"],
+)
+@pytest.mark.notyet(["druid"], raises=PyDruidProgrammingError)
+@pytest.mark.notyet(
+    ["flink"], raises=ValueError, reason="flink doesn't support empty tables"
+)
+def test_empty_memtable(con, input):
+    t = ibis.memtable(input, schema={"x": "int64"})
+    assert not len(con.to_pyarrow(t))

ibis/expr/api.py

@@ -429,15 +429,24 @@ def _memtable(
     schema: SchemaLike | None = None,
     name: str | None = None,
 ) -> Table:
+    import ibis
+
     if hasattr(data, "__arrow_c_stream__"):
         # Support objects exposing arrow's PyCapsule interface
         import pyarrow as pa
 
-        data = pa.table(data)
+        data = pa.table(
+            data,
+            schema=ibis.schema(schema).to_pyarrow() if schema is not None else None,
+        )
     else:
         import pandas as pd
 
-        data = pd.DataFrame(data, columns=columns)
+        data = pd.DataFrame(
+            data,
+            columns=columns
+            or (ibis.schema(schema).names if schema is not None else None),
+        )
     return _memtable(data, columns=columns, schema=schema, name=name)