[Python-Dev] Adventures with Decimal

[Nick Coghlan]

Michael Chermside wrote:
> Frankly, I have no idea WHAT purpose is served by passing a context
> to the decimal constructor... I didn't even realize it was allowed!

As Tim pointed out, it's solely to control whether or not ConversionSyntax 
errors are exceptions or not:

Py> decimal.Decimal("a")
Traceback (most recent call last):
   File "<stdin>", line 1, in ?
   File "c:\python24\lib\decimal.py", line 571, in __new__
     self._sign, self._int, self._exp = context._raise_error(ConversionSyntax)
   File "c:\python24\lib\decimal.py", line 2266, in _raise_error
     raise error, explanation
decimal.InvalidOperation
Py> context = decimal.getcontext().copy()
Py> context.traps[decimal.InvalidOperation] = False
Py> decimal.Decimal("a", context)
Decimal("NaN")

I'm tempted to suggest deprecating the feature, and say if you want invalid 
strings to produce NaN, use the create_decimal() method of Context objects. That 
would mean the standard construction operation becomes genuinely context-free. 
Being able to supply a context, but then have it be mostly ignored is rather 
confusing.

Doing this may also fractionally speed up Decimal creation from strings in the 
normal case, as the call to getcontext() could probably be omitted from the 
constructor.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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