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The += and *= operators have strange behavior when the
LHS is a Decimal and the RHS is a float (as of
2005年11月13日 CVS decimal.py).
Example:
>>> d = Decimal('1.02')
>>> d += 2.1
>>> d
NotImplemented
A blatant violation of "Errors should never pass silently."
Also, a bad error description is produced for the *=
operator:
>>> d = Decimal('1.02')
>>> d *= 2.9
Traceback (most recent call last):
 File "<stdin>", line 1, in ?
TypeError: can't multiply sequence by non-int

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Hmmm. __add__ returns NotImplemented which works with
classic classes, but not new-style classes. I wonder if
NotImplementedError is supposed to be raised for new-style
classes. 

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The += and *= operations also give the same strange behavior
when the LHS is a Decimal and the RHS is str or unicode:
>>> d = Decimal("1.0")
>>> d += "5"
>>> d
NotImplemented
>>> d = Decimal("1.0")
>>> d *= "1.0"
Traceback (most recent call last):
 File "<stdin>", line 1, in ?
TypeError: can't multiply sequence by non-int

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Facundo, can you look into this? Are you still working on
Decimal?

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Regarding problem 1:
Nick also detected this behaviour, back in March
(http://mail.python.org/pipermail/python-dev/2005-March/051834.html),
in python-dev discussions about how integrate better the
Decimal behaviour into Python framework.
Even knowing this, Raymond Hettinger and I made a patch
(almost exactly the same), and corrected another behaviour.
Will this issue be resolved somewhen? Raymond said that this
problem is also present in sets.py and datetime objects
(http://mail.python.org/pipermail/python-dev/2005-March/051825.html),
and that should be addressed in a larger context than decimal.
As Neil Schemenauer proposed
(http://mail.python.org/pipermail/python-dev/2005-March/051829.html),
Decimal now returns NotImplemented instead of raise
TypeError, which should be the correct way to deal with
operation capabilities in the numbers.
And look at this:
>>> d
Decimal("1") # using decimal.py rev. 39328 from svn
>>> d + 1.2
Traceback (most recent call last):
 File "<stdin>", line 1, in ?
TypeError: unsupported operand type(s) for +: 'Decimal' and
'float'
>>> d += 1.2
>>> d
NotImplemented
>>>
Why this happens? Really don't know, it's beyond my actual
knowledge, I'll keep searching. But I'm not so sure that
this is a Decimal issue.
Regarding problem 2:
I'll fix that.

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Hi Facundo,
the problem you are seeing seems to be in the way new-style
classes implement number coercion. 
Apparently some part in the (not so new-style anymore)
coercion logic is masking an exception which then triggers
later during processing. 
The NotImplemented singleton should never make it to the
interactive shell since it is normally only used internally
by the number coercion logic to signal "object method
doesn't handle mixed type operation".

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See proposed patch: #1390657 

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The patch was committed and fixed this, but only in SVN
HEAD, not for 2.4.

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Backported as r42511.