help needed on decimal formatting issue

Robin Becker robin at
Mon Sep 19 08:42:30 EDT 2011

I'm not really very used to the decimal module so I'm asking here if any one can 
help me with a problem in a well known third party web framework

The code in question is

def format_number(value, max_digits, decimal_places):
     Formats a number into a string with the requisite number of digits and
     decimal places.
     if isinstance(value, decimal.Decimal):
         context = decimal.getcontext().copy()
         context.prec = max_digits
         return u'%s' % str(
          value.quantize(decimal.Decimal(".1") ** decimal_places,
         return u"%.*f" % (decimal_places, value)

we have set up decimal fields with max_digits=7 decimal_places=2 and 
max_digits=10, decimal_places=4. We are getting issues with quantize failing 
with the message 'quantize result has too many digits for current context'.

Anyhow, I believe that we need to adjust the above code so that the precision is 
actually set to

     context.prec = max_digits+decimal_places

but I'm not certain that is right. Presumably the author thought that the fields 
could have large values of both max_digits and decimal_places. Initially I 
thought precision must be to do with decimal_places only, but changing
     context.prec = decimal_places

did not fix the issue. Can someone enlighten me? The failing case for the 
original code was


with context.prec = decimal_places we failed differently with


so I adopted context.prec = max_digits+decimal_places and that seems to work. If 
this is indeed a proper fix I will send a bug report to the well known jazz 
based framework.
Robin Becker

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