[Numpy-discussion] numpy vs algebra Was: Integers to negative integer powers...

[Yaroslav Halchenko]

On Tue, 03 Jan 2017, Stephan Hoyer wrote:

>    On Tue, Jan 3, 2017 at 9:00 AM, Yaroslav Halchenko <lists at onerussian.com>
>    wrote:

>      Sorry for coming too late to the discussion and after PR "addressing"
>      the issue by issuing an error was merged [1].A  I got burnt by new
>      behavior while trying to build fresh pandas release on Debian (we are
>      freezing for release way too soon ;) ) -- some pandas tests failed since
>      they rely on previous non-erroring behavior and we gotA  numpy 1.12.0~b1
>      which included [1] in unstable/testing (candidate release) now.

>      I quickly glanced over the discussion but I guess I have missed
>      actual description of the problem being fixed here...A  what was it??

>      previous behavior, int**int->int made sense to me as it seemed to be
>      consistent with casting Python's pow result to int, somewhat fulfilling
>      desired promise for in-place operations and being inline with built-in
>      pow results as far as I see it (up to casting).

>    I believe this is exactly the behavior we preserved. Rather, we turned
>    some cases that previously often gave wrong results (involving negative
>    integer powers) into errors.

hm... testing on current master (first result is from python's pow)

$> python -c "import numpy; print('numpy version: ', numpy.__version__); a=2; b=-2;  print(pow(a,b)); print(pow(numpy.array(a), b))" 
('numpy version: ', '1.13.0.dev0+02e2ea8')
0.25
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ValueError: Integers to negative integer powers are not allowed.


testing on Debian's packaged beta

$> python -c "import numpy; print('numpy version: ', numpy.__version__); a=2; b=-2;  print(pow(a,b)); print(pow(numpy.array(a), b))"
('numpy version: ', '1.12.0b1')
0.25
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ValueError: Integers to negative integer powers are not allowed.


testing on stable debian box with elderly numpy, where it does behave sensibly:

$> python -c "import numpy; print('numpy version: ', numpy.__version__); a=2; b=-2;  print(pow(a,b)); print(pow(numpy.array(a), b))"
('numpy version: ', '1.8.2')
0.25
0

what am I missing?

>    The pandas test suite triggered this behavior, but not intentionally, and
>    should be fixed in the next release:
>    https://github.com/pandas-dev/pandas/pull/14498

I don't think that was the full set of cases, e.g.

(git)hopa/sid-i386:~exppsy/pandas[bf-i386]
$> nosetests -s -v pandas/tests/test_expressions.py:TestExpressions.test_mixed_arithmetic_series
test_mixed_arithmetic_series (pandas.tests.test_expressions.TestExpressions) ... ERROR

======================================================================
ERROR: test_mixed_arithmetic_series (pandas.tests.test_expressions.TestExpressions)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/tests/test_expressions.py", line 223, in test_mixed_arithmetic_series
    self.run_series(self.mixed2[col], self.mixed2[col], binary_comp=4)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/tests/test_expressions.py", line 164, in run_series
    test_flex=False, **kwargs)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/tests/test_expressions.py", line 93, in run_arithmetic_test
    expected = op(df, other)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/core/ops.py", line 715, in wrapper
    result = wrap_results(safe_na_op(lvalues, rvalues))
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/core/ops.py", line 676, in safe_na_op
    return na_op(lvalues, rvalues)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/core/ops.py", line 652, in na_op
    raise_on_error=True, **eval_kwargs)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/computation/expressions.py", line 210, in evaluate
    **eval_kwargs)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/computation/expressions.py", line 63, in _evaluate_standard
    return op(a, b)
ValueError: Integers to negative integer powers are not allowed.


and being paranoid, I have rebuilt exact current master  of pandas with
master numpy in PYTHONPATH:

(git)hopa:~exppsy/pandas[master]git
$> PYTHONPATH=/home/yoh/proj/numpy nosetests -s -v pandas/tests/test_expressions.py:TestExpressions.test_mixed_arithmetic_series 
test_mixed_arithmetic_series (pandas.tests.test_expressions.TestExpressions) ... ERROR

======================================================================
ERROR: test_mixed_arithmetic_series (pandas.tests.test_expressions.TestExpressions)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/tests/test_expressions.py", line 223, in test_mixed_arithmetic_series
    self.run_series(self.mixed2[col], self.mixed2[col], binary_comp=4)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/tests/test_expressions.py", line 164, in run_series
    test_flex=False, **kwargs)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/tests/test_expressions.py", line 93, in run_arithmetic_test
    expected = op(df, other)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/core/ops.py", line 715, in wrapper
    result = wrap_results(safe_na_op(lvalues, rvalues))
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/core/ops.py", line 676, in safe_na_op
    return na_op(lvalues, rvalues)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/core/ops.py", line 652, in na_op
    raise_on_error=True, **eval_kwargs)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/computation/expressions.py", line 210, in evaluate
    **eval_kwargs)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/computation/expressions.py", line 63, in _evaluate_standard
    return op(a, b)
ValueError: Integers to negative integer powers are not allowed.

----------------------------------------------------------------------
Ran 1 test in 0.015s

FAILED (errors=1)

$> git describe --tags
v0.19.0-303-gb957f6f

$> PYTHONPATH=/home/yoh/proj/numpy python -c "import numpy; print('numpy version: ', numpy.__version__); a=2; b=-2;  print(pow(a,b)); print(pow(numpy.array(a), b))"                                                                                                                                            ('numpy version: ', '1.13.0.dev0+02e2ea8')
0.25
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ValueError: Integers to negative integer powers are not allowed.


-- 
Yaroslav O. Halchenko
Center for Open Neuroscience     http://centerforopenneuroscience.org
Dartmouth College, 419 Moore Hall, Hinman Box 6207, Hanover, NH 03755
Phone: +1 (603) 646-9834                       Fax: +1 (603) 646-1419
WWW:   http://www.linkedin.com/in/yarik