Repository: numpy/numpy

Issue:
Strange behavior of np.ma.masked_where 
When the mask and array shapes do not match and the mask is always false, no exception is thrown, probably a warning could be good. Consider the example

```
x = np.arange(10)
y = np.arange(5) 
z = np.ma.masked_where(y > 6, x)
```



Discussion:
I would make it an outright error.  A simple patch like this:

``` diff
diff --git a/numpy/ma/core.py b/numpy/ma/core.py
index faf9896..9c982ed 100644
--- a/numpy/ma/core.py
+++ b/numpy/ma/core.py
@@ -1802,7 +1802,7 @@ def masked_where(condition, a, copy=True):

     """
     # Make sure that condition is a valid standard-type mask.
-    cond = make_mask(condition)
+    cond = make_mask(condition, shrink=False)
     a = np.array(a, copy=copy, subok=True)

     (cshape, ashape) = (cond.shape, a.shape)
```

would make your example behave like this:

``` python
>>> import numpy as np
>>> x = np.arange(10)
>>> y = np.arange(5)
>>> z = np.ma.masked_where(y > 6, x)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "numpy/ma/core.py", line 1811, in masked_where
    " (got %s and %s)" % (cshape, ashape))
IndexError: Inconsistant shape between the condition and the input (got (5,) and (10,))
```

(and fixing the spelling in the error message should be part of the patch as well:-)


Resolving patch:
diff --git a/numpy/ma/core.py b/numpy/ma/core.py
--- a/numpy/ma/core.py
+++ b/numpy/ma/core.py
@@ -1537,6 +1537,16 @@ def is_mask(m):
         return False
 
 
+def _shrink_mask(m):
+    """
+    Shrink a mask to nomask if possible
+    """
+    if not m.dtype.names and not m.any():
+        return nomask
+    else:
+        return m
+
+
 def make_mask(m, copy=False, shrink=True, dtype=MaskType):
     """
     Create a boolean mask from an array.
@@ -1621,10 +1631,9 @@ def make_mask(m, copy=False, shrink=True, dtype=MaskType):
     # Fill the mask in case there are missing data; turn it into an ndarray.
     result = np.array(filled(m, True), copy=copy, dtype=dtype, subok=True)
     # Bas les masques !
-    if shrink and (not result.dtype.names) and (not result.any()):
-        return nomask
-    else:
-        return result
+    if shrink:
+        result = _shrink_mask(result)
+    return result
 
 
 def make_mask_none(newshape, dtype=None):
@@ -1911,7 +1920,7 @@ def masked_where(condition, a, copy=True):
 
     """
     # Make sure that condition is a valid standard-type mask.
-    cond = make_mask(condition)
+    cond = make_mask(condition, shrink=False)
     a = np.array(a, copy=copy, subok=True)
 
     (cshape, ashape) = (cond.shape, a.shape)
@@ -1925,7 +1934,7 @@ def masked_where(condition, a, copy=True):
         cls = MaskedArray
     result = a.view(cls)
     # Assign to *.mask so that structured masks are handled correctly.
-    result.mask = cond
+    result.mask = _shrink_mask(cond)
     return result
 
 
@@ -3544,9 +3553,7 @@ def shrink_mask(self):
         False
 
         """
-        m = self._mask
-        if m.ndim and not m.any():
-            self._mask = nomask
+        self._mask = _shrink_mask(self._mask)
         return self
 
     baseclass = property(fget=lambda self: self._baseclass,
@@ -6659,12 +6666,11 @@ def concatenate(arrays, axis=0):
         return data
     # OK, so we have to concatenate the masks
     dm = np.concatenate([getmaskarray(a) for a in arrays], axis)
+    dm = dm.reshape(d.shape)
+
     # If we decide to keep a '_shrinkmask' option, we want to check that
     # all of them are True, and then check for dm.any()
-    if not dm.dtype.fields and not dm.any():
-        data._mask = nomask
-    else:
-        data._mask = dm.reshape(d.shape)
+    data._mask = _shrink_mask(dm)
     return data
 
 
@@ -7082,8 +7088,7 @@ def where(condition, x=_NoValue, y=_NoValue):
     mask = np.where(cm, np.ones((), dtype=mask.dtype), mask)
 
     # collapse the mask, for backwards compatibility
-    if mask.dtype == np.bool_ and not mask.any():
-        mask = nomask
+    mask = _shrink_mask(mask)
 
     return masked_array(data, mask=mask)
 


Test patch:
diff --git a/numpy/ma/tests/test_core.py b/numpy/ma/tests/test_core.py
--- a/numpy/ma/tests/test_core.py
+++ b/numpy/ma/tests/test_core.py
@@ -1642,6 +1642,12 @@ def test_shrink_mask(self):
         assert_equal(a, b)
         assert_equal(a.mask, nomask)
 
+        # Mask cannot be shrunk on structured types, so is a no-op
+        a = np.ma.array([(1, 2.0)], [('a', int), ('b', float)])
+        b = a.copy()
+        a.shrink_mask()
+        assert_equal(a.mask, b.mask)
+
     def test_flat(self):
         # Test that flat can return all types of items [#4585, #4615]
         # test simple access
@@ -3838,6 +3844,12 @@ def test_masked_where_structured(self):
         assert_equal(am["A"],
                     np.ma.masked_array(np.zeros(10), np.ones(10)))
 
+    def test_masked_where_mismatch(self):
+        # gh-4520
+        x = np.arange(10)
+        y = np.arange(5)
+        assert_raises(IndexError, np.ma.masked_where, y > 6, x)
+
     def test_masked_otherfunctions(self):
         assert_equal(masked_inside(list(range(5)), 1, 3),
                      [0, 199, 199, 199, 4])