Comparing float and decimal

Terry Reedy tjreedy at udel.edu
Tue Sep 23 20:31:49 CEST 2008


Gerhard Häring wrote:
> D'Arcy J.M. Cain wrote:
>> I'm not sure I follow this logic.  Can someone explain why float and
>> integer can be compared with each other and decimal can be compared to
>> integer but decimal can't be compared to float?
>>
>>>>> from decimal import Decimal
>>>>> i = 10
>>>>> f = 10.0
>>>>> d = Decimal("10.00")
>>>>> i == f
>> True
>>>>> i == d
>> True
>>>>> f == d
>> False
> 
> I can give you the technical answer after reading the sources of the 
> decimal module: you can only compare to Decimal what can be converted to 
> Decimal. And that is int, long and another Decimal.

The new fractions module acts differently, which is to say, as most 
would want.

 >>> from fractions import Fraction as F
 >>> F(1) == 1.0
True
 >>> F(1.0)
Traceback (most recent call last):
   File "<pyshell#20>", line 1, in <module>
     F(1.0)
   File "C:\Program Files\Python30\lib\fractions.py", line 97, in __new__
     numerator = operator.index(numerator)
TypeError: 'float' object cannot be interpreted as an integer
 >>> F(1,2) == .5
True
 >>> .5 == F(1,2)
True

so Fraction obviously does comparisons differently.

Decimal is something of an anomaly in Python because it was written to 
exactly follow an external standard, with no concessions to what would 
be sensible for Python.  It is possible that that standard mandates that 
Decimals not compare to floats.

tjr




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