gh-53032: support IEEE 754 contexts in the decimal module (#122003)

This was in C version from beginning, but available only
on conditional compilation (EXTRA_FUNCTIONALITY).  Current
patch adds function to create IEEE contexts to the
pure-python module as well.

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

2 files changed, 71 insertions, 44 deletions

diff --git a/Lib/_pydecimal.py b/Lib/_pydecimal.py
index d666c4133c3..4b09207eca6 100644
--- a/Lib/_pydecimal.py
+++ b/Lib/_pydecimal.py
@@ -38,10 +38,10 @@ __all__ = [
     'ROUND_FLOOR', 'ROUND_UP', 'ROUND_HALF_DOWN', 'ROUND_05UP',
 
     # Functions for manipulating contexts
-    'setcontext', 'getcontext', 'localcontext',
+    'setcontext', 'getcontext', 'localcontext', 'IEEEContext',
 
     # Limits for the C version for compatibility
-    'MAX_PREC',  'MAX_EMAX', 'MIN_EMIN', 'MIN_ETINY',
+    'MAX_PREC',  'MAX_EMAX', 'MIN_EMIN', 'MIN_ETINY', 'IEEE_CONTEXT_MAX_BITS',
 
     # C version: compile time choice that enables the thread local context (deprecated, now always true)
     'HAVE_THREADS',
@@ -83,10 +83,12 @@ if sys.maxsize == 2**63-1:
     MAX_PREC = 999999999999999999
     MAX_EMAX = 999999999999999999
     MIN_EMIN = -999999999999999999
+    IEEE_CONTEXT_MAX_BITS = 512
 else:
     MAX_PREC = 425000000
     MAX_EMAX = 425000000
     MIN_EMIN = -425000000
+    IEEE_CONTEXT_MAX_BITS = 256
 
 MIN_ETINY = MIN_EMIN - (MAX_PREC-1)
 
@@ -417,6 +419,27 @@ def localcontext(ctx=None, **kwargs):
     return ctx_manager
 
 
+def IEEEContext(bits, /):
+    """
+    Return a context object initialized to the proper values for one of the
+    IEEE interchange formats.  The argument must be a multiple of 32 and less
+    than IEEE_CONTEXT_MAX_BITS.
+    """
+    if bits <= 0 or bits > IEEE_CONTEXT_MAX_BITS or bits % 32:
+        raise ValueError("argument must be a multiple of 32, "
+                         f"with a maximum of {IEEE_CONTEXT_MAX_BITS}")
+
+    ctx = Context()
+    ctx.prec = 9 * (bits//32) - 2
+    ctx.Emax = 3 * (1 << (bits//16 + 3))
+    ctx.Emin = 1 - ctx.Emax
+    ctx.rounding = ROUND_HALF_EVEN
+    ctx.clamp = 1
+    ctx.traps = dict.fromkeys(_signals, False)
+
+    return ctx
+
+
 ##### Decimal class #######################################################
 
 # Do not subclass Decimal from numbers.Real and do not register it as such
diff --git a/Lib/test/test_decimal.py b/Lib/test/test_decimal.py
index 92dafc56dc2..9e298401dc3 100644
--- a/Lib/test/test_decimal.py
+++ b/Lib/test/test_decimal.py
@@ -4399,6 +4399,51 @@ class CContextSubclassing(ContextSubclassing, unittest.TestCase):
 class PyContextSubclassing(ContextSubclassing, unittest.TestCase):
     decimal = P
 
+class IEEEContexts:
+
+    def test_ieee_context(self):
+        # issue 8786: Add support for IEEE 754 contexts to decimal module.
+        IEEEContext = self.decimal.IEEEContext
+
+        def assert_rest(self, context):
+            self.assertEqual(context.clamp, 1)
+            assert_signals(self, context, 'traps', [])
+            assert_signals(self, context, 'flags', [])
+
+        c = IEEEContext(32)
+        self.assertEqual(c.prec, 7)
+        self.assertEqual(c.Emax, 96)
+        self.assertEqual(c.Emin, -95)
+        assert_rest(self, c)
+
+        c = IEEEContext(64)
+        self.assertEqual(c.prec, 16)
+        self.assertEqual(c.Emax, 384)
+        self.assertEqual(c.Emin, -383)
+        assert_rest(self, c)
+
+        c = IEEEContext(128)
+        self.assertEqual(c.prec, 34)
+        self.assertEqual(c.Emax, 6144)
+        self.assertEqual(c.Emin, -6143)
+        assert_rest(self, c)
+
+        # Invalid values
+        self.assertRaises(ValueError, IEEEContext, -1)
+        self.assertRaises(ValueError, IEEEContext, 123)
+        self.assertRaises(ValueError, IEEEContext, 1024)
+
+    def test_constants(self):
+        # IEEEContext
+        IEEE_CONTEXT_MAX_BITS = self.decimal.IEEE_CONTEXT_MAX_BITS
+        self.assertIn(IEEE_CONTEXT_MAX_BITS, {256, 512})
+
+@requires_cdecimal
+class CIEEEContexts(IEEEContexts, unittest.TestCase):
+    decimal = C
+class PyIEEEContexts(IEEEContexts, unittest.TestCase):
+    decimal = P
+
 @skip_if_extra_functionality
 @requires_cdecimal
 class CheckAttributes(unittest.TestCase):
@@ -4410,6 +4455,7 @@ class CheckAttributes(unittest.TestCase):
         self.assertEqual(C.MAX_EMAX, P.MAX_EMAX)
         self.assertEqual(C.MIN_EMIN, P.MIN_EMIN)
         self.assertEqual(C.MIN_ETINY, P.MIN_ETINY)
+        self.assertEqual(C.IEEE_CONTEXT_MAX_BITS, P.IEEE_CONTEXT_MAX_BITS)
 
         self.assertTrue(C.HAVE_THREADS is True or C.HAVE_THREADS is False)
         self.assertTrue(P.HAVE_THREADS is True or P.HAVE_THREADS is False)
@@ -4894,42 +4940,6 @@ class CFunctionality(unittest.TestCase):
     """Extra functionality in _decimal"""
 
     @requires_extra_functionality
-    def test_c_ieee_context(self):
-        # issue 8786: Add support for IEEE 754 contexts to decimal module.
-        IEEEContext = C.IEEEContext
-        DECIMAL32 = C.DECIMAL32
-        DECIMAL64 = C.DECIMAL64
-        DECIMAL128 = C.DECIMAL128
-
-        def assert_rest(self, context):
-            self.assertEqual(context.clamp, 1)
-            assert_signals(self, context, 'traps', [])
-            assert_signals(self, context, 'flags', [])
-
-        c = IEEEContext(DECIMAL32)
-        self.assertEqual(c.prec, 7)
-        self.assertEqual(c.Emax, 96)
-        self.assertEqual(c.Emin, -95)
-        assert_rest(self, c)
-
-        c = IEEEContext(DECIMAL64)
-        self.assertEqual(c.prec, 16)
-        self.assertEqual(c.Emax, 384)
-        self.assertEqual(c.Emin, -383)
-        assert_rest(self, c)
-
-        c = IEEEContext(DECIMAL128)
-        self.assertEqual(c.prec, 34)
-        self.assertEqual(c.Emax, 6144)
-        self.assertEqual(c.Emin, -6143)
-        assert_rest(self, c)
-
-        # Invalid values
-        self.assertRaises(OverflowError, IEEEContext, 2**63)
-        self.assertRaises(ValueError, IEEEContext, -1)
-        self.assertRaises(ValueError, IEEEContext, 1024)
-
-    @requires_extra_functionality
     def test_c_context(self):
         Context = C.Context
 
@@ -4949,12 +4959,6 @@ class CFunctionality(unittest.TestCase):
             C.DecSubnormal, C.DecUnderflow
         )
 
-        # IEEEContext
-        self.assertEqual(C.DECIMAL32, 32)
-        self.assertEqual(C.DECIMAL64, 64)
-        self.assertEqual(C.DECIMAL128, 128)
-        self.assertEqual(C.IEEE_CONTEXT_MAX_BITS, 512)
-
         # Conditions
         for i, v in enumerate(cond):
             self.assertEqual(v, 1<<i)