This is my patch:

[ 1181301 ] make float packing copy bytes when they can

which hasn't been reviewed, despite numerous threats to check it in
anyway if noone reviews it.  Please read the diff on the checkin list,
at least!

The basic idea is to examine the bytes of some 'probe values' to see if
the current platform is a IEEE 754-ish platform, and if so
_PyFloat_{Pack,Unpack}{4,8} just copy bytes around.

The rest is hair for testing, and tests.

Lib/test/test_float.py

@@ -0,0 +1,110 @@
+
+import unittest, struct
+from test import test_support
+
+class FormatFunctionsTestCase(unittest.TestCase):
+
+    def setUp(self):
+        self.save_formats = {'double':float.__getformat__('double'),
+                             'float':float.__getformat__('float')}
+
+    def tearDown(self):
+        float.__setformat__('double', self.save_formats['double'])
+        float.__setformat__('float', self.save_formats['float'])
+
+    def test_getformat(self):
+        self.assert_(float.__getformat__('double') in
+                     ['unknown', 'IEEE, big-endian', 'IEEE, little-endian'])
+        self.assert_(float.__getformat__('float') in
+                     ['unknown', 'IEEE, big-endian', 'IEEE, little-endian'])
+        self.assertRaises(ValueError, float.__getformat__, 'chicken')
+        self.assertRaises(TypeError, float.__getformat__, 1)
+
+    def test_setformat(self):
+        for t in 'double', 'float':
+            float.__setformat__(t, 'unknown')
+            if self.save_formats[t] == 'IEEE, big-endian':
+                self.assertRaises(ValueError, float.__setformat__,
+                                  t, 'IEEE, little-endian')
+            elif self.save_formats[t] == 'IEEE, little-endian':
+                self.assertRaises(ValueError, float.__setformat__,
+                                  t, 'IEEE, big-endian')
+            else:
+                self.assertRaises(ValueError, float.__setformat__,
+                                  t, 'IEEE, big-endian')
+                self.assertRaises(ValueError, float.__setformat__,
+                                  t, 'IEEE, little-endian')
+            self.assertRaises(ValueError, float.__setformat__,
+                              t, 'chicken')
+        self.assertRaises(ValueError, float.__setformat__,
+                          'chicken', 'unknown')
+
+BE_DOUBLE_INF = '\x7f\xf0\x00\x00\x00\x00\x00\x00'
+LE_DOUBLE_INF = ''.join(reversed(BE_DOUBLE_INF))
+BE_DOUBLE_NAN = '\x7f\xf8\x00\x00\x00\x00\x00\x00'
+LE_DOUBLE_NAN = ''.join(reversed(BE_DOUBLE_NAN))
+
+BE_FLOAT_INF = '\x7f\x80\x00\x00'
+LE_FLOAT_INF = ''.join(reversed(BE_FLOAT_INF))
+BE_FLOAT_NAN = '\x7f\xc0\x00\x00'
+LE_FLOAT_NAN = ''.join(reversed(BE_FLOAT_NAN))
+
+# on non-IEEE platforms, attempting to unpack a bit pattern
+# representing an infinity or a NaN should raise an exception.
+
+class UnknownFormatTestCase(unittest.TestCase):
+    def setUp(self):
+        self.save_formats = {'double':float.__getformat__('double'),
+                             'float':float.__getformat__('float')}
+        float.__setformat__('double', 'unknown')
+        float.__setformat__('float', 'unknown')
+        
+    def tearDown(self):
+        float.__setformat__('double', self.save_formats['double'])
+        float.__setformat__('float', self.save_formats['float'])
+
+    def test_double_specials_dont_unpack(self):
+        for fmt, data in [('>d', BE_DOUBLE_INF),
+                          ('>d', BE_DOUBLE_NAN),
+                          ('<d', LE_DOUBLE_INF),
+                          ('<d', LE_DOUBLE_NAN)]:
+            self.assertRaises(ValueError, struct.unpack, fmt, data)
+
+    def test_float_specials_dont_unpack(self):
+        for fmt, data in [('>f', BE_FLOAT_INF),
+                          ('>f', BE_FLOAT_NAN),
+                          ('<f', LE_FLOAT_INF),
+                          ('<f', LE_FLOAT_NAN)]:
+            self.assertRaises(ValueError, struct.unpack, fmt, data)
+
+
+# on an IEEE platform, all we guarantee is that bit patterns
+# representing infinities or NaNs do not raise an exception; all else
+# is accident (today).
+
+class IEEEFormatTestCase(unittest.TestCase):
+    if float.__getformat__("double").startswith("IEEE"):
+        def test_double_specials_do_unpack(self):
+            for fmt, data in [('>d', BE_DOUBLE_INF),
+                              ('>d', BE_DOUBLE_NAN),
+                              ('<d', LE_DOUBLE_INF),
+                              ('<d', LE_DOUBLE_NAN)]:
+                struct.unpack(fmt, data)
+
+    if float.__getformat__("float").startswith("IEEE"):
+        def test_float_specials_do_unpack(self):
+            for fmt, data in [('>f', BE_FLOAT_INF),
+                              ('>f', BE_FLOAT_NAN),
+                              ('<f', LE_FLOAT_INF),
+                              ('<f', LE_FLOAT_NAN)]:
+                struct.unpack(fmt, data)
+
+
+def test_main():
+    test_support.run_unittest(
+        FormatFunctionsTestCase,
+        UnknownFormatTestCase,
+        IEEEFormatTestCase)
+
+if __name__ == '__main__':
+    test_main()