Comment #5 on issue 1358 by wflynny: (e*f*e*f).subs(e*f,f*e+h) fails if e*f
is not commutative
http://code.google.com/p/sympy/issues/detail?id=1358
We were looking deeper into this problem today and we traced the problem to
the
following snippet of code in the '_eval_subs' definition in mul.py:
...
l1, l2 = len(terms_self), len(terms_old)
if l2 == 0:
# if old is just a number, go through the self.args one by one
return Mul(*[x._eval_subs(old, new) for x in self.args])
elif l2 < l1:
# old is some something more complex, like:
# (a*b*c*d).subs(b*c,x) -> a*x*d
# then we need to search where in self.args the "old" is, and
then
# correctly substitute both terms and coefficients.
vvvvvvvvvvvvvvvvvvvvv BEGIN: BROKEN CODE vvvvvvvvvvvvvvvvvvvvvvvvvvvv
"""
self_set = set(terms_self)
old_set = set(terms_old)
if old_set < self_set:
ret_set = self_set - old_set
return Mul(new, coeff_self/coeff_old, *[s._eval_subs(old,
new) for s
in ret_set])
^^^^^^^^^^^^^^^^^^^ END: BROKEN CODE ^^^^^^^^^^^^^^^^^^^^^^
out_set = [element._eval_subs(old,new) for element in
self_set]
return Mul(new, coeff_self/coeff_old,
*[element._eval_subs(old,new)
for element in out_set])
The method is using sets, which is the source of the problem. Sets in
python, as I
understand, are unordered and only contain one unique instance of an
element. Thus,
two problems arise. Firstly, there is my problem of it ignoring
commutativity. But
secondly, there is the larger problem of ignoring multiplicity. I have
tried my hand
at writing a patch using lists but it doesn't work perfectly yet. I would
like to
have it working properly tomorrow and will append my solution to this
thread. Any
experience help would be greatly appreciated.
I hope this helps pinpoint the problem and help solve a seemingly
widespread problem.
This could possible fix a bunch of problems in match, subs, collect and the
like.
Thanks!
-wflynny
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