BUG: ma.make_mask should always return nomask for nomask argument.

When the mask argument was nomask, commit 8da9c71 changed the behavior
to depend on shrink=True as well, resulting in array(False) false being
returned when shrink=True, which in turn led to bugs because nomask is a
singleton and is detected by `mask is nomask`. That dectection fails
when nomask is replaced by array(False).

Closes numpy#6667.

numpy/ma/core.py

@@ -1495,9 +1495,10 @@ def make_mask(m, copy=False, shrink=True, dtype=MaskType):
     shrink : bool, optional
         Whether to shrink `m` to ``nomask`` if all its values are False.
     dtype : dtype, optional
-        Data-type of the output mask. By default, the output mask has
-        a dtype of MaskType (bool). If the dtype is flexible, each field
-        has a boolean dtype.
+        Data-type of the output mask. By default, the output mask has a
+        dtype of MaskType (bool). If the dtype is flexible, each field has
+        a boolean dtype. This is ignored when `m` is ``nomask``, in which
+        case ``nomask`` is always returned.
 
     Returns
     -------
@@ -1547,7 +1548,7 @@ def make_mask(m, copy=False, shrink=True, dtype=MaskType):
           dtype=[('man', '|b1'), ('mouse', '|b1')])
 
     """
-    if m is nomask and shrink:
+    if m is nomask:
         return nomask
     elif isinstance(m, ndarray):
         # We won't return after this point to make sure we can shrink the mask

numpy/ma/tests/test_core.py

@@ -11,8 +11,10 @@
 import warnings
 import pickle
 import operator
+import itertools
 from functools import reduce
 
+
 import numpy as np
 import numpy.ma.core
 import numpy.core.fromnumeric as fromnumeric
@@ -3816,6 +3818,15 @@ def test_make_mask(self):
         assert_equal(test.dtype, bdtype)
         assert_equal(test, np.array([(0, 0), (0, 1)], dtype=bdtype))
 
+        # test that nomask is returned when m is nomask.
+        bools = [True, False]
+        dtypes = [MaskType, np.float]
+        msgformat = 'copy=%s, shrink=%s, dtype=%s'
+        for cpy, shr, dt in itertools.product(bools, bools, dtypes):
+            res = make_mask(nomask, copy=cpy, shrink=shr, dtype=dt)
+            assert_(res is nomask, msgformat % (cpy, shr, dt))
+
+
     def test_mask_or(self):
         # Initialize
         mtype = [('a', np.bool), ('b', np.bool)]