PERF: significantly improve performance of MultiIndex.shape

Author: qwhelan

doc/source/whatsnew/v0.25.0.rst

@@ -981,6 +981,7 @@ Performance improvements
 - For :meth:`to_datetime` changed default value of cache parameter to ``True`` (:issue:`26043`)
 - Improved performance of :class:`DatetimeIndex` and :class:`PeriodIndex` slicing given non-unique, monotonic data (:issue:`27136`).
 - Improved performance of :meth:`pd.read_json` for index-oriented data. (:issue:`26773`)
+- Improved performance of :meth:`MultiIndex.shape` (:issue:`27384`).
 
 .. _whatsnew_0250.bug_fixes:
 

pandas/core/indexes/base.py

@@ -5639,6 +5639,13 @@ def _add_logical_methods_disabled(cls):
         cls.all = make_invalid_op("all")
         cls.any = make_invalid_op("any")
 
+    @property
+    def shape(self):
+        """
+        Return a tuple of the shape of the underlying data.
+        """
+        return (len(self),)
+
 
 Index._add_numeric_methods_disabled()
 Index._add_logical_methods()

pandas/core/indexes/interval.py

@@ -405,11 +405,6 @@ def size(self):
         # Avoid materializing ndarray[Interval]
         return self._data.size
 
-    @property
-    def shape(self):
-        # Avoid materializing ndarray[Interval]
-        return self._data.shape
-
     @property
     def itemsize(self):
         msg = (