You might look into could_extract_minus_sign:
>>> sympify('-Gc*k/(-Gc*k - k*m1*s**2 - k*m2*s**2 -
m1*m2*s**4)').could_extract_minus_sign()
True
>>> sympify('Gc*k/(-Gc*k - k*m1*s**2 - k*m2*s**2 -
m1*m2*s**4)').could_extract_minus_sign()
False
Perhaps simplify() should do
if expr.could_extract_minus_sign():
n, d = expr.as_numer_denom()
expr = sympify(-1)*n/(sympify(-1)*d)
or some similar.
Aaron Meurer
On Jun 8, 2009, at 11:36 AM, Ryan Krauss wrote:
> So, I have yet another simplification question. What is the correct
> way to factor a minus one out of this:
>
> In [108]: type(x2_tf_ds)
> Out[108]: <class 'sympy.core.mul.Mul'>
>
> In [109]: x2_tf_ds
> Out[109]: -Gc*k/(-Gc*k - k*m1*s**2 - k*m2*s**2 - m1*m2*s**4)
>
> This didn't work:
> In [110]: sympy.simplify(x2_tf_ds)
> Out[110]: -Gc*k/(-Gc*k - k*m1*s**2 - k*m2*s**2 - m1*m2*s**4)
>
> This works, and is fine with me. But I wonder if there is a better
> way:
> In [111]: a,b = x2_tf_ds.as_numer_denom()
>
> In [112]: -a/-b
> Out[112]: Gc*k/(Gc*k + k*m1*s**2 + k*m2*s**2 + m1*m2*s**4)
>
>
> Thanks,
>
> Ryan
>
> >
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