BUG: Inconsistent conversion of missing column names (#44878)

Author: johnzangwill

doc/source/whatsnew/v1.4.0.rst

@@ -856,6 +856,7 @@ Other
 - Bug in :meth:`DataFrame.shift` with ``axis=1`` and ``ExtensionDtype`` columns incorrectly raising when an incompatible ``fill_value`` is passed (:issue:`44564`)
 - Bug in :meth:`DataFrame.diff` when passing a NumPy integer object instead of an ``int`` object (:issue:`44572`)
 - Bug in :meth:`Series.replace` raising ``ValueError`` when using ``regex=True`` with a :class:`Series` containing ``np.nan`` values (:issue:`43344`)
+- Bug in :meth:`DataFrame.to_records` where an incorrect ``n`` was used when missing names were replaced by ``level_n`` (:issue:`44818`)
 
 .. ***DO NOT USE THIS SECTION***
 

pandas/core/common.py

@@ -18,8 +18,10 @@
     Any,
     Callable,
     Collection,
+    Hashable,
     Iterable,
     Iterator,
+    Sequence,
     cast,
     overload,
 )
@@ -604,3 +606,22 @@ def is_builtin_func(arg):
     otherwise return the arg
     """
     return _builtin_table.get(arg, arg)
+
+
+def fill_missing_names(names: Sequence[Hashable | None]) -> list[Hashable]:
+    """
+    If a name is missing then replace it by level_n, where n is the count
+
+    .. versionadded:: 1.4.0
+
+    Parameters
+    ----------
+    names : list-like
+        list of column names or None values.
+
+    Returns
+    -------
+    list
+        list of column names with the None values replaced.
+    """
+    return [f"level_{i}" if name is None else name for i, name in enumerate(names)]

pandas/core/frame.py

@@ -2370,11 +2370,7 @@ def to_records(
             index_names = list(self.index.names)
 
             if isinstance(self.index, MultiIndex):
-                count = 0
-                for i, n in enumerate(index_names):
-                    if n is None:
-                        index_names[i] = f"level_{count}"
-                        count += 1
+                index_names = com.fill_missing_names(index_names)
             elif index_names[0] is None:
                 index_names = ["index"]
 
@@ -5796,10 +5792,7 @@ class    max    type
         if not drop:
             to_insert: Iterable[tuple[Any, Any | None]]
             if isinstance(self.index, MultiIndex):
-                names = [
-                    (n if n is not None else f"level_{i}")
-                    for i, n in enumerate(self.index.names)
-                ]
+                names = com.fill_missing_names(self.index.names)
                 to_insert = zip(self.index.levels, self.index.codes)
             else:
                 default = "index" if "index" not in self else "level_0"

pandas/core/indexes/multi.py

@@ -736,11 +736,9 @@ def dtypes(self) -> Series:
         """
         from pandas import Series
 
+        names = com.fill_missing_names([level.name for level in self.levels])
         return Series(
-            {
-                f"level_{idx}" if level.name is None else level.name: level.dtype
-                for idx, level in enumerate(self.levels)
-            }
+            {names[idx]: level.dtype for idx, level in enumerate(self.levels)}
         )
 
     def __len__(self) -> int:

pandas/io/json/_table_schema.py

@@ -103,11 +103,7 @@ def set_default_names(data):
 
     data = data.copy()
     if data.index.nlevels > 1:
-        names = [
-            name if name is not None else f"level_{i}"
-            for i, name in enumerate(data.index.names)
-        ]
-        data.index.names = names
+        data.index.names = com.fill_missing_names(data.index.names)
     else:
         data.index.name = data.index.name or "index"
     return data

pandas/io/pytables.py

@@ -3377,9 +3377,7 @@ def validate_multiindex(
         validate that we can store the multi-index; reset and return the
         new object
         """
-        levels = [
-            l if l is not None else f"level_{i}" for i, l in enumerate(obj.index.names)
-        ]
+        levels = com.fill_missing_names(obj.index.names)
         try:
             reset_obj = obj.reset_index()
         except ValueError as err:

pandas/io/sql.py

@@ -44,6 +44,7 @@
     Series,
 )
 from pandas.core.base import PandasObject
+import pandas.core.common as com
 from pandas.core.tools.datetimes import to_datetime
 from pandas.util.version import Version
 
@@ -1010,10 +1011,7 @@ def _index_name(self, index, index_label):
             ):
                 return ["index"]
             else:
-                return [
-                    l if l is not None else f"level_{i}"
-                    for i, l in enumerate(self.frame.index.names)
-                ]
+                return com.fill_missing_names(self.frame.index.names)
 
         # for reading: index=(list of) string to specify column to set as index
         elif isinstance(index, str):

pandas/tests/frame/methods/test_to_records.py

@@ -90,8 +90,16 @@ def test_to_records_index_name(self):
 
         df.index = MultiIndex.from_tuples([("a", "x"), ("a", "y"), ("b", "z")])
         df.index.names = ["A", None]
-        rs = df.to_records()
-        assert "level_0" in rs.dtype.fields
+        result = df.to_records()
+        expected = np.rec.fromarrays(
+            [np.array(["a", "a", "b"]), np.array(["x", "y", "z"])]
+            + [np.asarray(df.iloc[:, i]) for i in range(3)],
+            dtype={
+                "names": ["A", "level_1", "0", "1", "2"],
+                "formats": ["<U1", "<U1", "<f8", "<f8", "<f8"],
+            },
+        )
+        tm.assert_numpy_array_equal(result, expected)
 
     def test_to_records_with_unicode_index(self):
         # GH#13172