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Tue Apr 13 16:20:00 EDT 2004 · http://mail.python.org/pipermail/python-dev/2004-April/044228.html

[Shane Hathaway]

#- Should we expect this to work?
#- 
#-      d = {}
#-      d[Decimal('1.1')] = 1
#-      print d[float('1.1')]
#- 

No, because Decimal('1.1') is not the same number than float('1.1')!

>>> d = {}
>>> d[Decimal('1.1')] = 1
>>> d
{Decimal( (0, (1, 1), -1) ): 1}
>>> d[float('1.1')]
Traceback (most recent call last):
  File "<pyshell#10>", line 1, in ?
    d[float('1.1')]
KeyError: 1.1000000000000001

In part, this is why you can not compare Decimal with float, and why you can
not create a Decimal directly from float. You can use the alternative
method, but you should know what you're doing:

>>> Decimal.from_float(1.1)
Decimal( (0, (1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 8, 8, 8, 1,
7, 8, 4, 1, 9, 7, 0, 0, 1, 2, 5, 2, 3, 2, 3, 3, 8, 9, 0, 5, 3, 3, 4, 4, 7,
2, 6, 5, 6, 2, 5), -51L) )
>>> Decimal.Decimal.from_float(1.1, 1)
Decimal( (0, (1, 1), -1L) )

.	Facundo

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[Python-Dev] Decimal data type issues