(Iron)Python formatting issue with modulo operator & "negative zero"

Question

Today i noticed following issue on IronPython2.7:

When formatting a format_string like so, i get a "negativ zero". I get the same result if i use Python2.7 or Python3.4 instead of IronPython.

>>> "%.2f" % -0.004
'-0.00'

Then i realized, that i can get rid of the negativ zero by passing two arguments to the format string like so:

>>> "%s%.2f" % ('x', -0.004)
'x0.00'

'x' is a random character. So, i format another string before the float value and the minus disappears. That does not work with Python2.7 and Python3.4 though, where i get:

>>> "%s%.2f" % ('x', -0.004)
'x-0.00'

Does anybody know what's going on here?

What is the purpose of the formatted "negativ zero" at all?

Sounds like a bug in IronPython. It's supposed to follow the CPython reference implementation wherever feasible. — Ignacio Vazquez-Abrams
– Ignacio Vazquez-Abrams, Commented Jan 10, 2017 at 8:30
I'm not sure if you're aware, but negative zero is a perfectly valid float value: -0.0. It's distinct from (but equal to) positive zero. No idea why IronPython makes the minus sign disappear with there's another value in the format string. — Blckknght
– Blckknght, Commented Jan 10, 2017 at 9:06

mguijarr · Accepted Answer · 2017-01-10 15:33:48Z

0

(edited to take @MarkDikinson comment into account)

You can format the number first, then doing the printf-like formatting:

"%.2f" % (round(-0.004, 2)+0)

The +0 is there to remove the negative zero sign.

round can also take a ndigits argument, to specify how many digits you need for the rounding of the number itself (not talking about display, which is handled by "%.2f").

edited Jan 10, 2017 at 15:33

answered Jan 10, 2017 at 8:57

mguijarr

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4 Comments

Mark Dickinson Over a year ago

Why use NumPy's around instead of Python's built-in round function? (And you probably want to round to 2 decimal places ...)

mguijarr Over a year ago

@MarkDickinson you're right ! I use numpy all the time at work but there is a standard round too ;)

Odomontois Over a year ago

@mguijarr installing numpy for ironpython could be problematic

mguijarr Over a year ago

@Odomontois indeed, that's why I simply removed numpy from my answer, I just originally forgot about standard round function ;)

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