remove unused dtype from MultiIndex constructor (#60963)

* remove unused dtype from MultiIndex constructor

* remove from docstring

Author: MarcoGorelli

Diff for: doc/source/whatsnew/v3.0.0.rst

@@ -358,6 +358,7 @@ Other API changes
 - Made ``dtype`` a required argument in :meth:`ExtensionArray._from_sequence_of_strings` (:issue:`56519`)
 - Passing a :class:`Series` input to :func:`json_normalize` will now retain the :class:`Series` :class:`Index`, previously output had a new :class:`RangeIndex` (:issue:`51452`)
 - Removed :meth:`Index.sort` which always raised a ``TypeError``. This attribute is not defined and will raise an ``AttributeError`` (:issue:`59283`)
+- Unused ``dtype`` argument has been removed from the :class:`MultiIndex` constructor (:issue:`60962`)
 - Updated :meth:`DataFrame.to_excel` so that the output spreadsheet has no styling. Custom styling can still be done using :meth:`Styler.to_excel` (:issue:`54154`)
 - pickle and HDF (``.h5``) files created with Python 2 are no longer explicitly supported (:issue:`57387`)
 - pickled objects from pandas version less than ``1.0.0`` are no longer supported (:issue:`57155`)

Diff for: pandas/core/indexes/multi.py

@@ -212,8 +212,6 @@ class MultiIndex(Index):
         level).
     names : optional sequence of objects
         Names for each of the index levels. (name is accepted for compat).
-    dtype : Numpy dtype or pandas type, optional
-        Data type for the MultiIndex.
     copy : bool, default False
         Copy the meta-data.
     name : Label
@@ -305,7 +303,6 @@ def __new__(
         codes=None,
         sortorder=None,
         names=None,
-        dtype=None,
         copy: bool = False,
         name=None,
         verify_integrity: bool = True,

Diff for: pandas/tests/indexing/multiindex/test_loc.py

@@ -105,7 +105,7 @@ def test_loc_getitem_series(self):
         empty = Series(data=[], dtype=np.float64)
         expected = Series(
             [],
-            index=MultiIndex(levels=index.levels, codes=[[], []], dtype=np.float64),
+            index=MultiIndex(levels=index.levels, codes=[[], []]),
             dtype=np.float64,
         )
         result = x.loc[empty]
@@ -129,7 +129,7 @@ def test_loc_getitem_array(self):
         empty = np.array([])
         expected = Series(
             [],
-            index=MultiIndex(levels=index.levels, codes=[[], []], dtype=np.float64),
+            index=MultiIndex(levels=index.levels, codes=[[], []]),
             dtype="float64",
         )
         result = x.loc[empty]