diff options
| -rw-r--r-- | Lib/_pydecimal.py | 19 | ||||
| -rw-r--r-- | Lib/test/test_decimal.py | 25 | ||||
| -rw-r--r-- | Misc/NEWS.d/next/Library/2024-05-04-20-22-59.gh-issue-118164.9D02MQ.rst | 1 |
3 files changed, 40 insertions, 5 deletions
diff --git a/Lib/_pydecimal.py b/Lib/_pydecimal.py index de4561a..613123e 100644 --- a/Lib/_pydecimal.py +++ b/Lib/_pydecimal.py @@ -2131,10 +2131,16 @@ class Decimal(object): else: return None - if xc >= 10**p: + # An exact power of 10 is representable, but can convert to a + # string of any length. But an exact power of 10 shouldn't be + # possible at this point. + assert xc > 1, self + assert xc % 10 != 0, self + strxc = str(xc) + if len(strxc) > p: return None xe = -e-xe - return _dec_from_triple(0, str(xc), xe) + return _dec_from_triple(0, strxc, xe) # now y is positive; find m and n such that y = m/n if ye >= 0: @@ -2184,13 +2190,18 @@ class Decimal(object): return None xc = xc**m xe *= m - if xc > 10**p: + # An exact power of 10 is representable, but can convert to a string + # of any length. But an exact power of 10 shouldn't be possible at + # this point. + assert xc > 1, self + assert xc % 10 != 0, self + str_xc = str(xc) + if len(str_xc) > p: return None # by this point the result *is* exactly representable # adjust the exponent to get as close as possible to the ideal # exponent, if necessary - str_xc = str(xc) if other._isinteger() and other._sign == 0: ideal_exponent = self._exp*int(other) zeros = min(xe-ideal_exponent, p-len(str_xc)) diff --git a/Lib/test/test_decimal.py b/Lib/test/test_decimal.py index 7010c34..e927e24 100644 --- a/Lib/test/test_decimal.py +++ b/Lib/test/test_decimal.py @@ -4716,9 +4716,33 @@ class PyWhitebox(unittest.TestCase): c.prec = 1 x = Decimal("152587890625") ** Decimal('-0.5') + self.assertEqual(x, Decimal('3e-6')) + c.prec = 2 + x = Decimal("152587890625") ** Decimal('-0.5') + self.assertEqual(x, Decimal('2.6e-6')) + c.prec = 3 + x = Decimal("152587890625") ** Decimal('-0.5') + self.assertEqual(x, Decimal('2.56e-6')) + c.prec = 28 + x = Decimal("152587890625") ** Decimal('-0.5') + self.assertEqual(x, Decimal('2.56e-6')) + c.prec = 201 x = Decimal(2**578) ** Decimal("-0.5") + # See https://github.com/python/cpython/issues/118027 + # Testing for an exact power could appear to hang, in the Python + # version, as it attempted to compute 10**(MAX_EMAX + 1). + # Fixed via https://github.com/python/cpython/pull/118503. + c.prec = P.MAX_PREC + c.Emax = P.MAX_EMAX + c.Emin = P.MIN_EMIN + c.traps[P.Inexact] = 1 + D2 = Decimal(2) + # If the bug is still present, the next statement won't complete. + res = D2 ** 117 + self.assertEqual(res, 1 << 117) + def test_py_immutability_operations(self): # Do operations and check that it didn't change internal objects. Decimal = P.Decimal @@ -5705,7 +5729,6 @@ class CWhitebox(unittest.TestCase): with C.localcontext(rounding=C.ROUND_DOWN): self.assertEqual(format(y, '#.1f'), '6.0') - @requires_docstrings @requires_cdecimal class SignatureTest(unittest.TestCase): diff --git a/Misc/NEWS.d/next/Library/2024-05-04-20-22-59.gh-issue-118164.9D02MQ.rst b/Misc/NEWS.d/next/Library/2024-05-04-20-22-59.gh-issue-118164.9D02MQ.rst new file mode 100644 index 0000000..80dc868 --- /dev/null +++ b/Misc/NEWS.d/next/Library/2024-05-04-20-22-59.gh-issue-118164.9D02MQ.rst @@ -0,0 +1 @@ +The Python implementation of the ``decimal`` module could appear to hang in relatively small power cases (like ``2**117``) if context precision was set to a very high value. A different method to check for exactly representable results is used now that doesn't rely on computing ``10**precision`` (which could be effectively too large to compute). |