You are testing whether the result from simplify is structurally the
same as a predefined answer, but simplify does not guarantee that it
will give the same result each time.

It is giving mathematically equivalent result, it just decided that it
should factor out a minus sign.

The simplest solution would be to rewrite tests of the form:

>>> a.simplify()
b

into

>> (a-b).simplify()
0

but this is somewhat ugly, especially for doctests which should be
more of a documentation tool. However, if a doctest is so complicated
it might be a good idea to move it to a test file and write a new,
simpler doctest.

On 7 September 2013 16:52, Sachin Joglekar <[email protected]> wrote:
> I am currently working on a PR to modify the mechanics core - #2412. The
> Travis build seems to be failing for a couple of doctests in the
> doc/src/modules/physics/mechanics/bicycle_example.rst file. Have a look here
> - https://travis-ci.org/sympy/sympy/jobs/11069792
> The problem is, neither Gilbert nor I are able to figure out why. The
> weirder stuff is, doing
> ./bin/doctest -n doc/src/modules/physics/mechanics/bicycle_example.rst
> on my machine gives no error once, and 5 errors the next time I run it
> (Without altering any code in the meantime). Same goes for the command
> without the '-n' parameter.
> Can anybody help us out with this? The PR is stuck at this issue, and we
> need to resolve it soon. (I have attached a file with the errors shown)
>
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