#13028: mpmath import problem warning?
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Reporter: dsm | Owner: jason, jkantor
Type: enhancement | Status: new
Priority: minor | Milestone: sage-5.1
Component: numerical | Keywords:
Work issues: | Report Upstream: N/A
Reviewers: | Authors:
Merged in: | Dependencies:
Stopgaps: |
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A rather subtle bug just ate several hours of my (and Benjamin's) time, so
for the sake of future sufferers, it's probably worth documenting and
wondering if we can find a way to make sure this doesn't happen. The bug
relates to trac #12455, which introduces symbolic Airy functions and uses
mpmath for numerical evaluation.
The manifestation is unusual: without the patch, we have
{{{
sage: import mpmath
sage: from sage.libs.mpmath import utils as mpmath_utils
sage: mpmath_utils.call(mpmath.airyai, 3,4, prec=500)
0.0354943008052438353507905704856075851248207412675096681263890254113384424359941551501461978539207259766557679623911357156186928332753550261698746599899
}}}
but with the patch (after moving an import to avoid a circularity), we
have
{{{
sage: import mpmath
sage: from sage.libs.mpmath import utils as mpmath_utils
sage: mpmath_utils.call(mpmath.airyai, 3,4, prec=500)
[...]
/Users/mcneil/sagedev/sage-5.1.beta0/local/lib/python2.7/site-
packages/mpmath/libmp/libintmath.py in python_bitcount(n)
88 if bc != 300:
89 return bc
---> 90 bc = int(math.log(n, 2)) - 4
91 return bc + bctable[n>>bc]
92
OverflowError: cannot convert float infinity to integer
}}}
even though we're not using any of the new functions, and it works fine
for (say) prec=300 and lower!
This turns out to be because the patch created a new module `airy.py` in
functions, and it was being imported in functions/all.py. mpmath
determines its backend in libmp/backend.py by attempting to import
sage.all, which fails (because it's not ready yet), and so mpmath decides
it's not in Sage and uses the Python backend. This results in using
math.log on a really big number, which leads to the OverflowError. It's
quite unlikely that a beginner would ever guess this happens -- because it
*works* whenever the precision is low enough, so many of the standard
tests one might try would be passed.
Would there be a good place to insert a verification that our mpmath knows
it's in Sage?
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/13028>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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