Merge pull request numpy#6556 from shoyer/fix-broadcast-arrays

BUG: error in broadcast_arrays with as_strided array

numpy/lib/stride_tricks.py

@@ -62,11 +62,14 @@ def _broadcast_to(array, shape, subok, readonly):
     if any(size < 0 for size in shape):
         raise ValueError('all elements of broadcast shape must be non-'
                          'negative')
+    needs_writeable = not readonly and array.flags.writeable
+    extras = ['reduce_ok'] if needs_writeable else []
+    op_flag = 'readwrite' if needs_writeable else 'readonly'
     broadcast = np.nditer(
-        (array,), flags=['multi_index', 'refs_ok', 'zerosize_ok'],
-        op_flags=['readonly'], itershape=shape, order='C').itviews[0]
+        (array,), flags=['multi_index', 'refs_ok', 'zerosize_ok'] + extras,
+        op_flags=[op_flag], itershape=shape, order='C').itviews[0]
     result = _maybe_view_as_subclass(array, broadcast)
-    if not readonly and array.flags.writeable:
+    if needs_writeable and not result.flags.writeable:
         result.flags.writeable = True
     return result
 

numpy/lib/tests/test_stride_tricks.py

@@ -390,6 +390,14 @@ def test_writeable():
     _, result = broadcast_arrays(0, original)
     assert_equal(result.flags.writeable, False)
 
+    # regresssion test for GH6491
+    shape = (2,)
+    strides = [0]
+    tricky_array = as_strided(np.array(0), shape, strides)
+    other = np.zeros((1,))
+    first, second = broadcast_arrays(tricky_array, other)
+    assert_(first.shape == second.shape)
+
 
 def test_reference_types():
     input_array = np.array('a', dtype=object)