fix(memtable): ensure column names match provided data

Previously, the `pandas` memtable constructor was performing two tasks
inconsistently, one was to subselect columns out of the provided
dataframe, the other was to rename those columns.
This led to some weird behavior where a mismatch in provided names could
lead to a dataframe consisting of NaNs.

Now, `columns` can only be provided to rename existing columns and there
is no subselection behavior.  If the length of the `columns` iterable
does not match the number of columns in the provided data, we error.

ibis/backends/tests/test_generic.py

@@ -947,6 +947,39 @@ def test_memtable_construct(backend, con, monkeypatch):
     )
 
 
+@pytest.mark.parametrize(
+    "df, columns, expected",
+    [
+        (pd.DataFrame([("a", 1.0)], columns=["d", "f"]), ["a", "b"], ["a", "b"]),
+        (pd.DataFrame([("a", 1.0)]), ["A", "B"], ["A", "B"]),
+        (pd.DataFrame([("a", 1.0)], columns=["c", "d"]), None, ["c", "d"]),
+        ([("a", "1.0")], None, ["col0", "col1"]),
+        ([("a", "1.0")], ["d", "e"], ["d", "e"]),
+    ],
+)
+def test_memtable_column_naming(backend, con, monkeypatch, df, columns, expected):
+    monkeypatch.setattr(ibis.options, "default_backend", con)
+
+    t = ibis.memtable(df, columns=columns)
+    assert all(t.to_pandas().columns == expected)
+
+
+@pytest.mark.parametrize(
+    "df, columns",
+    [
+        (pd.DataFrame([("a", 1.0)], columns=["d", "f"]), ["a"]),
+        (pd.DataFrame([("a", 1.0)]), ["A", "B", "C"]),
+        ([("a", "1.0")], ["col0", "col1", "col2"]),
+        ([("a", "1.0")], ["d"]),
+    ],
+)
+def test_memtable_column_naming_mismatch(backend, con, monkeypatch, df, columns):
+    monkeypatch.setattr(ibis.options, "default_backend", con)
+
+    with pytest.raises(ValueError):
+        ibis.memtable(df, columns=columns)
+
+
 @pytest.mark.notimpl(
     ["dask", "datafusion", "pandas", "polars"],
     raises=NotImplementedError,

ibis/expr/api.py

@@ -371,7 +371,8 @@ def memtable(
         Do not depend on the underlying storage type (e.g., pyarrow.Table), it's subject
         to change across non-major releases.
     columns
-        Optional [](`typing.Iterable`) of [](`str`) column names.
+        Optional [](`typing.Iterable`) of [](`str`) column names. If provided,
+        must match the number of columns in `data`.
     schema
         Optional [`Schema`](./schemas.qmd#ibis.expr.schema.Schema).
         The functions use `data` to infer a schema if not passed.
@@ -468,7 +469,11 @@ def _memtable_from_dataframe(
 
     from ibis.expr.operations.relations import PandasDataFrameProxy
 
-    df = pd.DataFrame(data, columns=columns)
+    if not isinstance(data, pd.DataFrame):
+        df = pd.DataFrame(data, columns=columns)
+    else:
+        df = data
+
     if df.columns.inferred_type != "string":
         cols = df.columns
         newcols = getattr(
@@ -478,6 +483,15 @@ def _memtable_from_dataframe(
         )
         df = df.rename(columns=dict(zip(cols, newcols)))
 
+    if columns is not None:
+        if (provided_col := len(columns)) != (exist_col := len(df.columns)):
+            raise ValueError(
+                "Provided `columns` must have an entry for each column in `data`.\n"
+                f"`columns` has {provided_col} elements but `data` has {exist_col} columns."
+            )
+
+        df = df.rename(columns=dict(zip(df.columns, columns)))
+
     # verify that the DataFrame has no duplicate column names because ibis
     # doesn't allow that
     cols = df.columns