Python cdecimal InvalidOperation

Question

I am trying to read financial data and store it. The place I get the financial data from stores the data with incredible precision, however I am only interested in 5 figures after the decimal point. Therefore, I have decided to use t = .quantize(cdecimal.Decimal('.00001'), rounding=cdecimal.ROUND_UP) on the Decimal I create, but I keep getting an InvalidOperation exception. Why is this?

>>> import cdecimal
>>> c = cdecimal.getcontext()
>>> c.prec = 5
>>> s = '45.2091000080109'
>>> # s = '0.257585003972054' works!
>>> t = cdecimal.Decimal(s).quantize(cdecimal.Decimal('.00001'), rounding=cdecimal.ROUND_UP)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  cdecimal.InvalidOperation: [<class 'cdecimal.InvalidOperation'>]

Why is there an invalid operation here? If I change the precision to 7 (or greater), it works. If I set s to be '0.257585003972054' instead of the original value, that also works! What is going on?

Thanks!

Answer 1

decimal version gives a better description of the error:

Python 2.7.2+ (default, Feb 16 2012, 18:47:58) 
>>> import decimal
>>> s = '45.2091000080109'
>>> decimal.getcontext().prec = 5
>>> decimal.Decimal(s).quantize(decimal.Decimal('.00001'), rounding=decimal.ROUND_UP)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/decimal.py", line 2464, in quantize
    'quantize result has too many digits for current context')
  File "/usr/lib/python2.7/decimal.py", line 3866, in _raise_error
    raise error(explanation)
decimal.InvalidOperation: quantize result has too many digits for current context
>>> 

Docs:

Unlike other operations, if the length of the coefficient after the quantize operation would be greater than precision, then an InvalidOperation is signaled. This guarantees that, unless there is an error condition, the quantized exponent is always equal to that of the right-hand operand.

But i must confess i don't know what this means.