ENH: Changes (and tests) to allow exporting half-floats through the buffer interface. (#1789)

Code:
Added NPY_HALF to switch (descr->type_num) in _buffer_format_string.
Added 'e' keys to the _pep3118_native_map and _pep3118_standard_map.

Tests:
Added entries to the generic round-trip tests.
Added specialized half-float test that round-trips example values from the wikipedia page.

http://en.wikipedia.org/wiki/Half_precision_floating-point_format
http://mail.scipy.org/pipermail/numpy-discussion/2011-March/055795.html

3 files changed, 32 insertions, 6 deletions

diff --git a/numpy/core/_internal.py b/numpy/core/_internal.py
index 67ca1716f..5298f412b 100644
--- a/numpy/core/_internal.py
+++ b/numpy/core/_internal.py
@@ -363,6 +363,7 @@ _pep3118_native_map = {
     'L': 'L',
     'q': 'q',
     'Q': 'Q',
+    'e': 'e',
     'f': 'f',
     'd': 'd',
     'g': 'g',
@@ -388,6 +389,7 @@ _pep3118_standard_map = {
     'L': 'u4',
     'q': 'i8',
     'Q': 'u8',
+    'e': 'f2',
     'f': 'f',
     'd': 'd',
     'Zf': 'F',
diff --git a/numpy/core/src/multiarray/buffer.c b/numpy/core/src/multiarray/buffer.c
index 9f1ddc1f7..9bc45a76f 100644
--- a/numpy/core/src/multiarray/buffer.c
+++ b/numpy/core/src/multiarray/buffer.c
@@ -358,6 +358,7 @@ _buffer_format_string(PyArray_Descr *descr, _tmp_string_t *str,
             break;
         case NPY_LONGLONG:     if (_append_char(str, 'q')) return -1; break;
         case NPY_ULONGLONG:    if (_append_char(str, 'Q')) return -1; break;
+        case NPY_HALF:         if (_append_char(str, 'e')) return -1; break;
         case NPY_FLOAT:        if (_append_char(str, 'f')) return -1; break;
         case NPY_DOUBLE:       if (_append_char(str, 'd')) return -1; break;
         case NPY_LONGDOUBLE:   if (_append_char(str, 'g')) return -1; break;
diff --git a/numpy/core/tests/test_multiarray.py b/numpy/core/tests/test_multiarray.py
index 7cdd45eb0..e2010b6c2 100644
--- a/numpy/core/tests/test_multiarray.py
+++ b/numpy/core/tests/test_multiarray.py
@@ -2008,9 +2008,11 @@ if sys.version_info >= (2, 6):
                   ('l', 'S4'),
                   ('m', 'U4'),
                   ('n', 'V3'),
-                  ('o', '?')]
+                  ('o', '?'),
+                  ('p', np.half),
+                 ]
             x = np.array([(1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
-                           asbytes('aaaa'), 'bbbb', asbytes('xxx'), True)],
+                           asbytes('aaaa'), 'bbbb', asbytes('xxx'), True, 1.0)],
                          dtype=dt)
             self._check_roundtrip(x)
 
@@ -2042,6 +2044,25 @@ if sys.version_info >= (2, 6):
                 x = np.array([1,2,3], dtype='<q')
                 assert_raises(ValueError, self._check_roundtrip, x)
 
+        def test_roundtrip_half(self):
+            half_list = [
+                1.0,
+                -2.0,
+                6.5504 * 10**4, #  (max half precision)
+                2**-14, # ~= 6.10352 * 10**-5 (minimum positive normal)
+                2**-24, # ~= 5.96046 * 10**-8 (minimum strictly positive subnormal)
+                0.0,
+                -0.0,
+                float('+inf'),
+                float('-inf'),
+                0.333251953125, # ~= 1/3
+            ]
+
+            x = np.array(half_list, dtype='>e')
+            self._check_roundtrip(x)
+            x = np.array(half_list, dtype='<e')
+            self._check_roundtrip(x)
+
         def test_export_simple_1d(self):
             x = np.array([1,2,3,4,5], dtype='i')
             y = memoryview(x)
@@ -2092,9 +2113,11 @@ if sys.version_info >= (2, 6):
                   ('l', 'S4'),
                   ('m', 'U4'),
                   ('n', 'V3'),
-                  ('o', '?')]
+                  ('o', '?'),
+                  ('p', np.half),
+                 ]
             x = np.array([(1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
-                           asbytes('aaaa'), 'bbbb', asbytes('   '), True)],
+                           asbytes('aaaa'), 'bbbb', asbytes('   '), True, 1.0)],
                          dtype=dt)
             y = memoryview(x)
             assert_equal(y.shape, (1,))
@@ -2103,9 +2126,9 @@ if sys.version_info >= (2, 6):
 
             sz = sum([dtype(b).itemsize for a, b in dt])
             if dtype('l').itemsize == 4:
-                assert_equal(y.format, 'T{b:a:=h:b:i:c:l:d:^q:dx:B:e:@H:f:=I:g:L:h:^Q:hx:=f:i:d:j:^g:k:=Zf:ix:Zd:jx:^Zg:kx:4s:l:=4w:m:3x:n:?:o:}')
+                assert_equal(y.format, 'T{b:a:=h:b:i:c:l:d:^q:dx:B:e:@H:f:=I:g:L:h:^Q:hx:=f:i:d:j:^g:k:=Zf:ix:Zd:jx:^Zg:kx:4s:l:=4w:m:3x:n:?:o:@e:p:}')
             else:
-                assert_equal(y.format, 'T{b:a:=h:b:i:c:q:d:^q:dx:B:e:@H:f:=I:g:Q:h:^Q:hx:=f:i:d:j:^g:k:=Zf:ix:Zd:jx:^Zg:kx:4s:l:=4w:m:3x:n:?:o:}')
+                assert_equal(y.format, 'T{b:a:=h:b:i:c:q:d:^q:dx:B:e:@H:f:=I:g:Q:h:^Q:hx:=f:i:d:j:^g:k:=Zf:ix:Zd:jx:^Zg:kx:4s:l:=4w:m:3x:n:?:o:@e:p:}')
             assert_equal(y.strides, (sz,))
             assert_equal(y.itemsize, sz)