PERF: significantly improve performance of MultiIndex.shape

Author: qwhelan

doc/source/whatsnew/v0.25.0.rst

@@ -939,6 +939,7 @@ Performance improvements
 - Improved performance by removing the need for a garbage collect when checking for ``SettingWithCopyWarning`` (:issue:`27031`)
 - 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:`MultiIndex.shape`
 
 .. _whatsnew_0250.bug_fixes:
 

pandas/core/indexes/multi.py

@@ -3426,6 +3426,13 @@ def isin(self, values, level=None):
             else:
                 return np.lib.arraysetops.in1d(level_codes, sought_labels)
 
+    @property
+    def shape(self):
+        """
+        Return a tuple of the shape of the underlying data.
+        """
+        return (len(self),)
+
 
 MultiIndex._add_numeric_methods_disabled()
 MultiIndex._add_numeric_methods_add_sub_disabled()