Comment #8 on issue 3698 by asmeu...@gmail.com: Piecewise does solving with
subs
http://code.google.com/p/sympy/issues/detail?id=3698
Really there are three issues here that bother me:
- The constructor of Piecewise and .subs are performing different semantics.
- Doing this is too much magic for either (especially since solve can be
slow in general). This is better suited to a separate simplify method that
is not called automatically.
- Doing it is technically incorrect, since solve() is not as powerful as
the code assumes it is.
If you agree with any point in that list, then the points above it become
irrelevant, hence I've been trying to argue the last point. But even if
solve() were perfect, I would still argue the first two points.
For example, the second point surprised me, and for a bit I thought that
something strange was going on in your pull request when I removed the code
from DifferentialExtension, because it was still giving x == 1 even though
from what I could tell it should have been giving log(x) == 0. It was one
of those "clear the .pyc files and hope that helps" moments.
And the first point confused me further when I went to test it, because
when I entered Piecewise((..., Eq(log(x), 0)), it gave me just that. It
wasn't until I very carefully did exactly what the code said (i.e., use
subs on a dummy variable) that I noticed what was going on.
It's not part of the Zen of Python, but there's something to be said about
the so-called "principle of least astonishment" here.
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