[New-bugs-announce] [issue20561] Decimal handling error in statistics module

Wolfgang Maier report at bugs.python.org
Sat Feb 8 12:10:27 CET 2014

New submission from Wolfgang Maier:

Can this still be fixed in 3.4 ??

I came across this bug in the statistics module today:

>>> import statistics
>>> data = [Decimal('1e4')]
>>> statistics.mean(data)
Traceback (most recent call last):
  File "<pyshell#465>", line 1, in <module>
  File "C:/Python33\statistics.py", line 316, in mean
    return _sum(data)/n
  File "C:/Python33\statistics.py", line 167, in _sum
    total += Fraction(n, d)
  File "C:\Python33\lib\fractions.py", line 163, in __new__
    raise TypeError("both arguments should be "
TypeError: both arguments should be Rational instances

The reason for this is that the statistics module converts Decimals to exact ratios internally like this:

def _decimal_to_ratio(d):
    """Convert Decimal d to exact integer ratio (numerator, denominator).

    >>> from decimal import Decimal
    >>> _decimal_to_ratio(Decimal("2.6"))
    (26, 10)

    sign, digits, exp = d.as_tuple()
    if exp in ('F', 'n', 'N'):  # INF, NAN, sNAN
        assert not d.is_finite()
        raise ValueError
    num = 0
    for digit in digits:
        num = num*10 + digit
    if sign:
        num = -num
    den = 10**-exp
    #if type(den) != int:
    #    print (d, sign, digits, exp, num, den)
    return (num, den)
The important part here is the line:

den = 10**-exp

which makes den an int if - and **only** if - exp is negative.

However, the exponent in the namedtuple returned by Decimal.as_tuple() is not guaranteed to be negative and in fact:

DecimalTuple(sign=0, digits=(1,), exponent=4)

With this den in the above function becomes a float, which raises an error subsequently in _sum, when it essentially tries to do:
Fraction(num, den)

IMPORTANT: I have been running this example on my laptop with Python3.3.0 and the statistics module imported from sys.path.
Thus, I cannot exclude that the decimal module has changed since 3.3.0 and is now always returning a negative exponent with
.as_tuple(). I'm abroad and cannot verify this against the Python3.4dev build.
Could somebody confirm this for 3.4 and reject this issue immediately if the error doesn't occur ?

If this is not version dependent though, then there is a very simple bug-fix for it, which I suggest should still be incorporated into 3.4:

if exp >= 0:
    return int(x), 1

inserted right after the raise ValueError line is doing the job.
I cannot generate the diff for this myself, so could somebody do this.

Thanks a lot for your efforts!

messages: 210613
nosy: oscarbenjamin, python-dev, skrah, stevenjd, wolma
priority: normal
severity: normal
status: open
title: Decimal handling error in statistics module
type: behavior
versions: Python 3.4

Python tracker <report at bugs.python.org>

More information about the New-bugs-announce mailing list