[New-bugs-announce] [issue21227] Decimal class error messages for integer division aren't good

[leewz]

New submission from leewz:

Python's `decimal.Decimal` doesn't seem to like taking modulo or intdiv of large Decimals by integers (where "large" depends on how many digits are internally stored).

    >>> from decimal import *
    >>> getcontext().prec
    28
    >>> Decimal(10**29)%1
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    decimal.InvalidOperation: [<class 'decimal.DivisionImpossible'>]
    >>> getcontext().prec=50
    >>> Decimal(10**29)%1
    Decimal('0')

Same for `Decimal(10**29)//1`

This is a logical problem: "Alice has a 100-digit number which begins with 1543256. What is the last digit?"

But I found the error hard to understand. Searching for "DivisionImpossible" didn't turn up anything immediate (it wasn't added to the official docs?). I was most of the way through writing out a question to StackOverflow when it clicked. (Also, why is it an InvalidOperation that holds an exception as a message? Never saw that before.)

Suggestions:
- Improve docs with further examples. The examples of InvalidOperation are logical MATH errors (e.g. division by 0), not logical PROGRAMMING errors.
- Uh, maybe the error message could be changed to something like, "The answer you seek lies beyond the mantissa." Or more sanely, "Not enough precision to compute the answer." Or something else that hints to me to look into precision.

----------
assignee: docs at python
components: Documentation, Library (Lib)
messages: 216253
nosy: docs at python, leewz
priority: normal
severity: normal
status: open
title: Decimal class error messages for integer division aren't good
type: enhancement
versions: Python 3.3, Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21227>
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