I seem to have fixed my problem by making changing in
sympy/functions/elementary/trigonometric.py this definition in class sin
(and making a corresponding change in class cos). The change was to tell
sin/cos not evaluate in their constructors.
I haven't run any tests on this except for my code where I found the
problem.
def _eval_expand_trig(self, **hints):
from sympy import expand_mul
arg = self.args[0]
x = None
if arg.is_Add: # TODO, implement more if deep stuff here
# TODO: Do this more efficiently for more than two terms
x, y = arg.as_two_terms()
#GCB 120920 added ",evaluate=False" to the next 4 lines
sx = sin(x,evaluate=False)._eval_expand_trig()
sy = sin(y,evaluate=False)._eval_expand_trig()
cx = cos(x,evaluate=False)._eval_expand_trig()
cy = cos(y,evaluate=False)._eval_expand_trig()
return sx*cy + sy*cx
else:
n, x = arg.as_coeff_Mul(rational=True)
if n.is_Integer: # n will be positive because of .eval
# canonicalization
# See
http://mathworld.wolfram.com/Multiple-AngleFormulas.html
if n.is_odd:
return (-1)**((n - 1)/2)*C.chebyshevt(n, sin(x))
else:
return expand_mul((-1)**(n/2 - 1)*cos(x)*C.chebyshevu(n
-
1, sin(x)), deep=False)
return sin(arg)
On Wednesday, September 19, 2012 7:02:11 PM UTC-7, Aaron Meurer wrote:
>
> Ah, that is a bug. You should report that at
> http://code.google.com/p/sympy/issues/. The reason it happens is that
> it tries to do the expansion recursively, but sin(-x) becomes -sin(x).
> The fix is not too hard either, if you want to give it a shot.
>
> The work around is to do the expansion with +y and then do
> expr.subs(y, -y) at the end.
>
> By the way, x - y and x + -y are exactly the same thing in SymPy.
> Both are represented internally as Add(x, Mul(-1, y)).
>
> Aaron Meurer
>
> On Wed, Sep 19, 2012 at 7:55 PM, G B <[email protected] <javascript:>>
> wrote:
> > I'm still new to sympy, so I might be doing something stupid, but I
> think
> > there's a problem here:
> >
> > In [1]: from sympy import *
> > In [2]: x,y=symbols('x,y')
> > In [3]: cos(x+y).expand(trig=True)
> > Out[3]: -sin(x)*sin(y) + cos(x)*cos(y)
> >
> > So far so good. but then:
> > In [4]: cos(x-y).expand(trig=True)
> >
> > Leads to a long traceback that culminates with: AttributeError: 'Mul'
> object
> > has no attribute '_eval_expand_trig'
> >
> > For giggles, I also tried:
> > In [5]: cos(x + (-y)).expand(trig=True)
> >
> > Which fails in the same way. I can post the full traceback if it helps.
> >
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