BUG: fix length_of_indexer with boolean mask

pandas/core/indexers.py

@@ -219,7 +219,7 @@ def maybe_convert_indices(indices, n: int):
 
 def length_of_indexer(indexer, target=None) -> int:
     """
-    Return the length of a single non-tuple indexer which could be a slice.
+    Return the expected length of target[indexer]
 
     Returns
     -------
@@ -245,6 +245,12 @@ def length_of_indexer(indexer, target=None) -> int:
             step = -step
         return (stop - start + step - 1) // step
     elif isinstance(indexer, (ABCSeries, ABCIndexClass, np.ndarray, list)):
+        if isinstance(indexer, list):
+            indexer = np.array(indexer)
+
+        if indexer.dtype == bool:
+            # GH#25774
+            return indexer.sum()
         return len(indexer)
     elif not is_list_like_indexer(indexer):
         return 1

pandas/core/indexing.py

@@ -1702,31 +1702,17 @@ def _setitem_with_indexer(self, indexer, value):
 
                 plane_indexer = tuple([idx]) + indexer[info_axis + 1 :]
                 lplane_indexer = length_of_indexer(plane_indexer[0], index)
+                # lplane_indexer gives the expected length of obj[idx]
 
                 # require that we are setting the right number of values that
                 # we are indexing
-                if (
-                    is_list_like_indexer(value)
-                    and np.iterable(value)
-                    and lplane_indexer != len(value)
-                ):
-
-                    if len(obj[idx]) != len(value):
-                        raise ValueError(
-                            "cannot set using a multi-index "
-                            "selection indexer with a different "
-                            "length than the value"
-                        )
-
-                    # make sure we have an ndarray
-                    value = getattr(value, "values", value).ravel()
+                if is_list_like_indexer(value) and lplane_indexer != len(value):
 
-                    # we can directly set the series here
-                    obj._consolidate_inplace()
-                    obj = obj.copy()
-                    obj._data = obj._data.setitem(indexer=tuple([idx]), value=value)
-                    self.obj[item] = obj
-                    return
+                    raise ValueError(
+                        "cannot set using a multi-index "
+                        "selection indexer with a different "
+                        "length than the value"
+                    )
 
             # non-mi
             else:

pandas/tests/test_indexers.py

@@ -0,0 +1,11 @@
+# Tests aimed at pandas.core.indexers
+import numpy as np
+
+from pandas.core.indexers import length_of_indexer
+
+
+def test_length_of_indexer():
+    arr = np.zeros(4, dtype=bool)
+    arr[0] = 1
+    result = length_of_indexer(arr)
+    assert result == 1