Updates:
Summary: Remove automatic distribution 2*(x + y) => 2*x + 2*y
Comment #9 on issue 1497 by asmeurer: Remove automatic distribution 2*(x +
y) => 2*x + 2*y
http://code.google.com/p/sympy/issues/detail?id=1497
I have changed the issue to removing automatic distribution of numbers.
See also the list thread I linked to
above. Ideally, we would only have distribution of numbers whenever there
are exactly two arguments in a Mul,
a number and an Add (as in 2*(x + y) to 2*x + 2*y), but because of the
iterative nature of Mul. For example, with
2*(x + y)*z, Mul first sees 2*(x + y), which is combined, then it
multiplies that with the z term, giving (2*x +
2*y)*z. Currently there is no way to tell if there is more remaining to be
iteratively combined, as far as I know.
This also creates contradictory automatic simplification. For example, if
you force Mul to evaluate (x + y)*z first,
either with parentheses or order, the 2 will not distribute.
In [5]: 2*(x + y)*z
Out[5]: z⋅(2⋅x + 2⋅y)
In [6]: 2*((x + y)*z)
Out[6]: 2⋅z⋅(x + y)
In [7]: (x + y)*z*2
Out[7]: 2⋅z⋅(x + y)
Another benefit noted from not doing this automatically is that you can
have factor(2*x + 2*y) return 2*(x + y),
something which is currently impossible.
Basically to do this, you need to remove (from line 304 in core/mul.py):
# we are done
if len(c_part)==2 and c_part[0].is_Number and c_part[1].is_Add:
# 2*(1+a) -> 2 + 2 * a
coeff = c_part[0]
c_part = [Add(*[coeff*f for f in c_part[1].args])]
and fix any tests that fail (which unfortunately is quite a few).
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