NotImplemented used in Decimal

[Chris Rebert]

On Fri, Apr 23, 2010 at 11:25 PM, Steven D'Aprano
<steve at remove-this-cybersource.com.au> wrote:
> On Sat, 24 Apr 2010 09:05:14 +0530, Kushal Kumaran wrote:
>> On Sat, Apr 24, 2010 at 8:54 AM, Steven D'Aprano
>> <steve at remove-this-cybersource.com.au> wrote:
>>> I'm reading the source code for decimal.Decimal, and I see that the
>>> arithmetic operations (__add__, __sub__, etc.) start with code like
>>> this:
>>>
>>>        if other is NotImplemented:
>>>            return other
>>>
>>>
>>> I don't understand the purpose of this. I presume that it is *not* for
>>> the use-case of:
>>>
>>> d = Decimal('123.456')
>>> result = d + NotImplemented
>>>
>>> which not only doesn't make sense to me, but when I try it, it raises
>>> TypeError. So I find myself confused why the arithmetic methods do
>>> this.
>>>
>> There's a _convert_other method which is called before the test that can
>> return NotImplemented.
>
> Yes, I can see that, but why?

If the conversion to Decimal in  _convert_other() fails, the operator
method returns NotImplemented to signal to the interpreter that
Decimal doesn't know how to do the requested operation with an operand
of the given type; the interpreter will fall back by calling the
reflected method of the other operand. And if that also fails, the
interpreter raises TypeError:
>>> Decimal(4)+'g'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'Decimal' and 'str'

Note how the traceback doesn't mention decimal.py

See also NotImplemented's entry in the docs:
http://docs.python.org/reference/datamodel.html#the-standard-type-hierarchy

Now, they could have had _convert_other() return some other sentinel
value (or raise an exception) to indicate that the conversion failed,
but they didn't; it probably made the code marginally simpler.

Cheers,
Chris
--
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