BUG: count_nonzero treats empty axis tuples strangely

eric-wieser · eric-wieser · commit 3856a73ec59d · 2017-10-17T20:41:39.000-07:00
Fixes numpy#9728 This bug was introduced with the `axis` keyword in numpy#7177, as a misguided optimization.
diff --git a/doc/release/1.14.0-notes.rst b/doc/release/1.14.0-notes.rst
@@ -122,6 +122,13 @@ passed, despite not doing so under the simple cases::
 
 This change affects only ``float32`` and ``float16`` arrays.
 
+``count_nonzero(arr, axis=())`` now counts over no axes, not all axes
+---------------------------------------------------------------------
+Elsewhere, ``axis==()`` is always understood as "no axes", but
+`count_nonzero` had a special case to treat this as "all axes". This was
+inconsistent and surprising. The correct way to count over all axes has always
+been to pass ``axis == None``.
+
 ``__init__.py`` files added to test directories
 -----------------------------------------------
 This is for pytest compatibility in the case of duplicate test file names in
diff --git a/numpy/core/numeric.py b/numpy/core/numeric.py
@@ -406,7 +406,7 @@ def count_nonzero(a, axis=None):
     array([2, 3])
 
     """
-    if axis is None or (isinstance(axis, tuple) and axis == ()):
+    if axis is None:
         return multiarray.count_nonzero(a)
 
     a = asanyarray(a)
diff --git a/numpy/core/tests/test_numeric.py b/numpy/core/tests/test_numeric.py
@@ -1126,6 +1126,10 @@ def test_count_nonzero_axis_consistent(self):
                         np.count_nonzero(n, axis=perm),
                         err_msg=msg % (perm,))
 
+    def test_countnonzero_axis_empty(self):
+        a = np.array([[0, 0, 1], [1, 0, 1]])
+        assert_equal(np.count_nonzero(a, axis=()), a.astype(bool))
+
     def test_array_method(self):
         # Tests that the array method
         # call to nonzero works
