Why is Decimal('0') > 9999.0 True in Python?
This is somehow related to my question Why is ''>0 True in Python?
In Python 2.6.4:
>> Decimal('0') > 9999.0
True
From the answer to my original question I understand that when comparing objects of different types in Python 2.x the types are ordered by their name. But in this case:
>> type(Decimal('0')).__name__ > type(9999.0).__name__
False
Why is Decimal('0'开发者_开发技巧) > 9999.0 == True
then?
UPDATE: I usually work on Ubuntu (Linux 2.6.31-20-generic #57-Ubuntu SMP Mon Feb 8 09:05:19 UTC 2010 i686 GNU/Linux, Python 2.6.4 (r264:75706, Dec 7 2009, 18:45:15) [GCC 4.4.1] on linux2). On Windows (WinXP Professional SP3, Python 2.6.4 (r264:75706, Nov 3 2009, 13:23:17) [MSC v.1500 32 bit (Intel)] on win32) my original statement works differently:
>> Decimal('0') > 9999.0
False
I even more puzzled now. %-(
Because the decimal module does not compare against any type except long, int, and Decimal. In all other cases, decimal silently returns the "not something it knows about object" as greater. You can see this behavior in the _convert_other() function of decimal.py
Silly, silly Decimal class.
Oh, see http://bugs.python.org/issue2531 as well.
So, here is what happens:
- The interpreter calls the
Decimal.__gt__
comparison function. Decimal.__gt__
callsDecimal._convert_other
to convert the incoming float to a Decimal.Decimal._convert_other
doesn't understand floats. The implementation down inDecimal._convert_other
explicitly checks forlong
,int
, andDecimal
types of the operand. Yes, this is a bug, in that unexpected library implementations cause bugs further down the line. It would be cleaner to do the right thing or even just through aTypeException
. Instead it throughs the sameNotImplemented
that would happen comparing a Decimal to, say, a hash of Employee records.- A few other comparison operations are tried. Comparison gives up.
- The default comparison, down in CPython's Objects/object.c/default_3way_compare get called.
- In Python 3, this rightly barfs. In Python 2, it compares the id() functions.
- On Windows, a case insensitive comparison is used (sort of). On modern systems, a case sensitive comparison is used.
- So you get different results.
Are we there yet?
def __gt__(self, other, context=None):
other = _convert_other(other)
if other is NotImplemented:
return other
ans = self._compare_check_nans(other, context)
if ans:
return False
return self._cmp(other) > 0
def _convert_other(other, raiseit=False):
"""Convert other to Decimal.
Verifies that it's ok to use in an implicit construction.
"""
if isinstance(other, Decimal):
return other
if isinstance(other, (int, long)):
return Decimal(other)
if raiseit:
raise TypeError("Unable to convert %s to Decimal" % other)
return NotImplemented
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