API: add droplevel() to flat indexes, for compatibility with MultiIndex

closes pandas-dev#21115

Author: toobaz

doc/source/whatsnew/v0.23.1.txt

@@ -65,6 +65,7 @@ Indexing
 ^^^^^^^^
 
 - Bug in :meth:`Series.reset_index` where appropriate error was not raised with an invalid level name (:issue:`20925`)
+- :meth:`Index.droplevel` is now implemented also for flat indexes, for compatibility with MultiIndex (:issue:`21115`)
 -
 
 I/O

pandas/core/indexes/base.py

@@ -3158,6 +3158,58 @@ def _get_level_values(self, level):
 
     get_level_values = _get_level_values
 
+    def droplevel(self, level=0):
+        """
+        Return index with requested level(s) removed. If resulting index has
+        only 1 level left, the result will be of Index type, not MultiIndex.
+
+        Parameters
+        ----------
+        level : int, str, or list-like, default 0
+            If a string is given, must be the name of a level
+            If list-like, elements must be names or indexes of levels.
+
+        Returns
+        -------
+        index : Index or MultiIndex
+        """
+        if not isinstance(level, (tuple, list)):
+            level = [level]
+
+        levnums = sorted(self._get_level_number(lev) for lev in level)[::-1]
+
+        if len(level) == 0:
+            return self
+        if len(level) >= self.nlevels:
+            raise ValueError("Cannot remove {} levels from an index with {} "
+                             "levels: at least one level must be "
+                             "left.".format(len(level), self.nlevels))
+        # The two checks above guarantee that here self is a MultiIndex
+
+        new_levels = list(self.levels)
+        new_labels = list(self.labels)
+        new_names = list(self.names)
+
+        for i in levnums:
+            new_levels.pop(i)
+            new_labels.pop(i)
+            new_names.pop(i)
+
+        if len(new_levels) == 1:
+
+            # set nan if needed
+            mask = new_labels[0] == -1
+            result = new_levels[0].take(new_labels[0])
+            if mask.any():
+                result = result.putmask(mask, np.nan)
+
+            result.name = new_names[0]
+            return result
+        else:
+            from .multi import MultiIndex
+            return MultiIndex(levels=new_levels, labels=new_labels,
+                              names=new_names, verify_integrity=False)
+
     _index_shared_docs['get_indexer'] = """
         Compute indexer and mask for new index given the current index. The
         indexer should be then used as an input to ndarray.take to align the

pandas/core/indexes/multi.py

@@ -1761,52 +1761,6 @@ def _drop_from_level(self, labels, level):
 
         return self[mask]
 
-    def droplevel(self, level=0):
-        """
-        Return Index with requested level removed. If MultiIndex has only 2
-        levels, the result will be of Index type not MultiIndex.
-
-        Parameters
-        ----------
-        level : int/level name or list thereof
-
-        Notes
-        -----
-        Does not check if result index is unique or not
-
-        Returns
-        -------
-        index : Index or MultiIndex
-        """
-        levels = level
-        if not isinstance(levels, (tuple, list)):
-            levels = [level]
-
-        new_levels = list(self.levels)
-        new_labels = list(self.labels)
-        new_names = list(self.names)
-
-        levnums = sorted(self._get_level_number(lev) for lev in levels)[::-1]
-
-        for i in levnums:
-            new_levels.pop(i)
-            new_labels.pop(i)
-            new_names.pop(i)
-
-        if len(new_levels) == 1:
-
-            # set nan if needed
-            mask = new_labels[0] == -1
-            result = new_levels[0].take(new_labels[0])
-            if mask.any():
-                result = result.putmask(mask, np.nan)
-
-            result.name = new_names[0]
-            return result
-        else:
-            return MultiIndex(levels=new_levels, labels=new_labels,
-                              names=new_names, verify_integrity=False)
-
     def swaplevel(self, i=-2, j=-1):
         """
         Swap level i with level j.