[New-bugs-announce] [issue15815] Add numerator to ZeroDivisionError messages

[Terry J. Reedy]

New submission from Terry J. Reedy:

I propose that we add 'of ####' to all ZeroDivisionError messagesso it is less necessary to use a debugger to add print statements to recover the information currently tossed away. (I also propose to change denominator from 'zero' to '0', '0.0', '0+0j', as appropriate.) 

>>> 3//0
...
ZeroDivisionError: integer division or modulo by zero

# augment to
ZeroDivisionError: integer division or modulo of 3 by 0

Similarly for

>>> 3 / 0
ZeroDivisionError: division by zero

# perhaps this should be 'true division of 3 by 0'

>>> 3.0 / 0.0
ZeroDivisionError: float division by zero

>>> (3+3j) / (0+0j)
ZeroDivisionError: complex division by zero

In #7482 it was proposed to make float and complex messages same as int message by adding 'or modulo'. I an not sure why not to do that, or if it was just deferred.

Fractions currently print the numerator as part of an invalid numerator / 0 result from *either* construction or division.

>>> from fractions import Fraction as F
>>> F(3, 0)
Traceback (most recent call last):
  File "<pyshell#20>", line 1, in <module>
    F(3, 0)
  File "C:\Programs\Python33\lib\fractions.py", line 167, in __new__
    raise ZeroDivisionError('Fraction(%s, 0)' % numerator)
ZeroDivisionError: Fraction(3, 0)

# Here, 'Fraction(3, 0)' is invalid input.

>>> F(3, 1) / F(0, 1)
Traceback (most recent call last):
  File "<pyshell#19>", line 1, in <module>
    F(3, 1) / F(0, 1)
  File "C:\Programs\Python33\lib\fractions.py", line 367, in forward
    return monomorphic_operator(a, b)
  File "C:\Programs\Python33\lib\fractions.py", line 417, in _div
    a.denominator * b.numerator)
  File "C:\Programs\Python33\lib\fractions.py", line 167, in __new__
    raise ZeroDivisionError('Fraction(%s, 0)' % numerator)
ZeroDivisionError: Fraction(3, 0)

# Here, 'Fraction(3, 0)' is invalid output.
I found this confusing until I worked out the dual meaning. I think

ZeroDevisionError: invalid Fraction(3, 0) from construction or division

might be a bit clearer.

I have not looked at decimals.

----------
components: Interpreter Core
messages: 169410
nosy: ezio.melotti, mark.dickinson, terry.reedy
priority: normal
severity: normal
stage: needs patch
status: open
title: Add numerator to ZeroDivisionError messages
type: enhancement
versions: Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue15815>
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