Closed
Description
Bug report
Bug description:
decimal.Decimal(2) ** 117 does not terminate when using the pure Python implementation _pydecimal.py of the decimal module, and setting the precision to MAX_PREC:
import sys
if len(sys.argv) > 1 and sys.argv[1] == "pydecimal":
# block _decimal
sys.modules['_decimal'] = None
import decimal
with decimal.localcontext() as ctx:
ctx.prec = decimal.MAX_PREC
ctx.Emax = decimal.MAX_EMAX
ctx.Emin = decimal.MIN_EMIN
ctx.traps[decimal.Inexact] = 1
D2 = decimal.Decimal(2)
res = D2 ** 117
print(res)this behaves as follows:
./python decbug.py # instant
166153499473114484112975882535043072
$ ./python decbug.py pydecimal # hangs
^CTraceback (most recent call last):
File "/home/cfbolz/projects/cpython/decbug.py", line 14, in <module>
res = D2 ** 117
~~~^^~~~~
File "/home/cfbolz/projects/cpython/Lib/_pydecimal.py", line 2436, in __pow__
ans = self._power_exact(other, context.prec + 1)
~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/cfbolz/projects/cpython/Lib/_pydecimal.py", line 2284, in _power_exact
if xc > 10**p:
~~^^~
KeyboardInterrupt
The bug happens because p is the precision + 1 in the last traceback entry. MAX_PREC = 999999999999999999 on 64-bit systems. Therefore the the code in _power_exact tries to compute 10**1000000000000000000, which obviously won't work.
CPython versions tested on:
3.9, 3.10, 3.11, 3.12, 3.13, CPython main branch
Operating systems tested on:
Linux