numpy · teoliphant · Jul 18, 2012 · Jul 12, 2012 · Jul 12, 2012 · Jul 12, 2012
diff --git a/doc/release/1.7.0-notes.rst b/doc/release/1.7.0-notes.rst
@@ -26,6 +26,13 @@ functions. To facilitate this transition, numpy 1.7 produces a
 FutureWarning if it detects that you may be attempting to write to
 such an array. See the documentation for np.diagonal for details.
 
+Similar to np.diagonal above, in a future version of numpy, indexing
+a record array by a list of field names will return a view onto the
+original array, instead of producing a copy as they do now. As with
+np.diagonal, numpy 1.7 produces a FutureWarning if it detects
+that you may be attemping to write to such an array. See the documentation
+for array indexing for details.
+
 The default casting rule for UFunc out= parameters has been changed from
 'unsafe' to 'same_kind'.  Most usages which violate the 'same_kind'
 rule are likely bugs, so this change may expose previously undetected

diff --git a/doc/source/reference/arrays.indexing.rst b/doc/source/reference/arrays.indexing.rst
@@ -335,6 +335,16 @@ sub-array) but of data type ``x.dtype['field-name']`` and contains
 only the part of the data in the specified field. Also record array
 scalars can be "indexed" this way.
 
+Indexing into a record array can also be done with a list of field names,
+*e.g.* ``x[['field-name1','field-name2']]``. Currently this returns a new
+array containing a copy of the values in the fields specified in the list.
+As of NumPy 1.7, returning a copy is being deprecated in favor of returning
+a view. A copy will continue to be returned for now, but a FutureWarning
+will be issued when writing to the copy. If you depend on the current
+behavior, then we suggest copying the returned array explicitly, i.e. use
+x[['field-name1','field-name2']].copy(). This will work with both past and
+future versions of NumPy.
+
 If the accessed field is a sub-array, the dimensions of the sub-array
 are appended to the shape of the result.
 

diff --git a/numpy/core/_internal.py b/numpy/core/_internal.py
@@ -287,18 +287,15 @@ def _newnames(datatype, order):
 def _index_fields(ary, fields):
     from multiarray import empty, dtype
     dt = ary.dtype
-    new_dtype = [(name, dt[name]) for name in fields if name in dt.names]
-    if ary.flags.f_contiguous:
-        order = 'F'
-    else:
-        order = 'C'
 
-    newarray = empty(ary.shape, dtype=new_dtype, order=order)
+    names = [name for name in fields if name in dt.names]
+    formats = [dt.fields[name][0] for name in fields if name in dt.names]
+    offsets = [dt.fields[name][1] for name in fields if name in dt.names]
 
-    for name in fields:
-        newarray[name] = ary[name]
+    view_dtype = {'names':names, 'formats':formats, 'offsets':offsets, 'itemsize':dt.itemsize}
+    view = ary.view(dtype=view_dtype)
 
-    return newarray
+    return view.copy()
 
 # Given a string containing a PEP 3118 format specifier,
 # construct a Numpy dtype

diff --git a/numpy/core/src/multiarray/arrayobject.c b/numpy/core/src/multiarray/arrayobject.c
@@ -694,9 +694,11 @@ array_might_be_written(PyArrayObject *obj)
 {
     const char *msg =
         "Numpy has detected that you (may be) writing to an array returned\n"
-        "by numpy.diagonal. This code will likely break in the next numpy\n"
-        "release -- see numpy.diagonal docs for details. The quick fix is\n"
-        "to make an explicit copy (e.g., do arr.diagonal().copy()).";
+        "by numpy.diagonal or by selecting multiple fields in a record\n"
+        "array. This code will likely break in the next numpy release --\n"
+        "see numpy.diagonal or arrays.indexing reference docs for details.\n"
+        "The quick fix is to make an explicit copy (e.g., do\n"
+        "arr.diagonal().copy() or arr[['f0','f1']].copy()).";
     if (PyArray_FLAGS(obj) & NPY_ARRAY_WARN_ON_WRITE) {
         if (DEPRECATE_FUTUREWARNING(msg) < 0) {
             return -1;

diff --git a/numpy/core/src/multiarray/mapping.c b/numpy/core/src/multiarray/mapping.c
@@ -976,6 +976,10 @@ array_subscript(PyArrayObject *self, PyObject *op)
             obj = PyObject_CallMethod(_numpy_internal,
                     "_index_fields", "OO", self, op);
             Py_DECREF(_numpy_internal);
+            if (obj == NULL) {
+                return NULL;
+            }
+            PyArray_ENABLEFLAGS((PyArrayObject*)obj, NPY_ARRAY_WARN_ON_WRITE);
             return obj;
         }
     }

diff --git a/numpy/core/tests/test_multiarray.py b/numpy/core/tests/test_multiarray.py
@@ -1909,7 +1909,8 @@ def test_unicode_field_names(self):
     def test_field_names(self):
         # Test unicode and 8-bit / byte strings can be used
         a = np.zeros((1,), dtype=[('f1', 'i4'),
-                                  ('f2', [('sf1', 'i4')])])
+                                  ('f2', 'i4'),
+                                  ('f3', [('sf1', 'i4')])])
         is_py3 = sys.version_info[0] >= 3
         if is_py3:
             funcs = (str,)
@@ -1934,12 +1935,18 @@ def test_field_names(self):
             assert_raises(IndexError, b[0].__setitem__, fnn, 1)
             assert_raises(IndexError, b[0].__getitem__, fnn)
             # Subfield
-            fn2 = func('f2')
+            fn3 = func('f3')
             sfn1 = func('sf1')
-            b[fn2][sfn1] = 1
-            assert_equal(b[fn2][sfn1], 1)
-            assert_raises(ValueError, b[fn2].__setitem__, fnn, 1)
-            assert_raises(ValueError, b[fn2].__getitem__, fnn)
+            b[fn3][sfn1] = 1
+            assert_equal(b[fn3][sfn1], 1)
+            assert_raises(ValueError, b[fn3].__setitem__, fnn, 1)
+            assert_raises(ValueError, b[fn3].__getitem__, fnn)
+            # multiple Subfields
+            fn2 = func('f2')
+            b[fn2] = 3
+            assert_equal(b[['f1','f2']][0].tolist(), (2, 3))
+            assert_equal(b[['f2','f1']][0].tolist(), (3, 2))
+            assert_equal(b[['f1','f3']][0].tolist(), (2, (1,)))
         # non-ascii unicode field indexing is well behaved
         if not is_py3:
             raise SkipTest('non ascii unicode field indexing skipped; '
@@ -1948,6 +1955,47 @@ def test_field_names(self):
             assert_raises(ValueError, a.__setitem__, u'\u03e0', 1)
             assert_raises(ValueError, a.__getitem__, u'\u03e0')
 
+    def test_field_names_deprecation(self):
+        import warnings
+        from numpy.testing.utils import WarningManager
+        def collect_warning_types(f, *args, **kwargs):
+            ctx = WarningManager(record=True)
+            warning_log = ctx.__enter__()
+            warnings.simplefilter("always")
+            try:
+                f(*args, **kwargs)
+            finally:
+                ctx.__exit__()
+            return [w.category for w in warning_log]
+        a = np.zeros((1,), dtype=[('f1', 'i4'),
+                                  ('f2', 'i4'),
+                                  ('f3', [('sf1', 'i4')])])
+        a['f1'][0] = 1
+        a['f2'][0] = 2
+        a['f3'][0] = (3,)
+        b = np.zeros((1,), dtype=[('f1', 'i4'),
+                                  ('f2', 'i4'),
+                                  ('f3', [('sf1', 'i4')])])
+        b['f1'][0] = 1
+        b['f2'][0] = 2
+        b['f3'][0] = (3,)
+
+        # All the different functions raise a warning, but not an error, and
+        # 'a' is not modified:
+        assert_equal(collect_warning_types(a[['f1','f2']].__setitem__, 0, (10,20)),
+                     [FutureWarning])
+        assert_equal(a, b)
+        # Views also warn
+        subset = a[['f1','f2']]
+        subset_view = subset.view()
+        assert_equal(collect_warning_types(subset_view['f1'].__setitem__, 0, 10),
+                     [FutureWarning])
+        # But the write goes through:
+        assert_equal(subset['f1'][0], 10)
+        # Only one warning per multiple field indexing, though (even if there are
+        # multiple views involved):
+        assert_equal(collect_warning_types(subset['f1'].__setitem__, 0, 10),
+                     [])
 
 class TestView(TestCase):
     def test_basic(self):