Division in place acts differently according to Python version

[antnieszka]

This doctest works in py2.7, fails in 3.4.

        >>> c = Curve([[0, 0], [5, 5], [10, 10]], dtype=np.int)
        >>> c.rescale(-1, allow_cast=False)
        >>> print(c.y)
        [  0  -5 -10]

py3:

Failed example:
    c.rescale(-1, allow_cast=False)
Exception raised:
    Traceback (most recent call last):
      File "/home/ant6/bin/pycharm-2016.3/helpers/pycharm/docrunner.py", line 140, in __run
        compileflags, 1), test.globs)
      File "<doctest curve.Curve.rescale[7]>", line 1, in <module>
        c.rescale(-1, allow_cast=False)
      File "/home/ant6/PycharmProjects/beprof/beprof/curve.py", line 109, in rescale
        self.y /= factor
    TypeError: ufunc 'true_divide' output (typecode 'd') could not be coerced to provided output parameter (typecode 'l') according to the casting rule ''same_kind''

With allow_cast=True both act the same and pass.

[grzanka]

Beware that in py3 you have two division operators: // and /

[antnieszka]

https://www.python.org/dev/peps/pep-0238/

    In Python 3.0, the classic division semantics will be removed; the
    classic division APIs will become synonymous with true division.

Probably we could use from __future__ import division to have true division, not classic in py2.7?

[antnieszka]

@grzanka this too probably should be resolved before release to pypy?

[grzanka]

Yes. I was thinking a bit more about it and the conclusion was that we should prefer floats over ints.
Can you show an example how from __future__ import division works ?

[antnieszka]

According to this PEP: https://www.python.org/dev/peps/pep-0238/

• Classic division will remain the default in the Python 2.x
  series; true division will be standard in Python 3.0.

For Python 2:

• The future division statement, spelled "from __future__ import
  division", will change the / operator to mean true division
  throughout the module.

Classic division (default in Python 2)

>>> 1 / 2          # integer truncation (floor division)
0
>>> 1.0 / 2.0      # returns real quotient (true division)
0.5

True division

>>> from __future__ import division  # 2.2+-only
>>>
>>> 1 / 2               # returns real quotient
0.5
>>> 1.0 / 2.0           # returns real quotient
0.5

As said earlier - true division is default for Python 3.x, so is / used in our code.
More on topic: http://www.informit.com/articles/article.aspx?p=1439189

Oh, and __future__ import is ignored in Python 3, so it will only affect beprof if running with Python 2.