Updates:
Cc: asmeurer
Comment #3 on issue 1705 by smichr: [PATCH] _separatevars_dict: Fix issue
where unintended symbols would be selected
http://code.google.com/p/sympy/issues/detail?id=1705
I looked at this and things look good. Since this function is being edited
(and since
there were a few typos in the docstring) could I suggest the following
edited version?
---
Separates variables in an expression, if possible. By default, it
separates
with respect to all symbols in an expression and collects constant
coefficients that are independent of symbols. If dict=True then only
those
symbols specified by `symbols` will be separated and keyed to their
respective symbol in the dictionary; any part that has no symbol (or
none
of those specified) will be returned with key 'coeff'.
Note that if the expression is not really separable, or is only
partially
separable it will do the best it can to separate it. It does not throw
any
errors if the expression is not separable. It just returns the
expression.
If dict=True and the expression is not separable, then None is returned.
Also, note that the order of the factors is determined by Mul, so that
the
separated expressions may not necessarily be grouped together.
---
Also, might I suggest that the one example with x, y, z and the dictionary
be changed
to the following to demonstrate that symbols not specified will be grouped
into coef:
>>> separatevars(2*x**2*z*sin(y)+2*z*x**2, dict=True, symbols=(x, y))
{'coeff': 2*z, x: x**2, y: 1 + sin(y)}
And finally, that a filter of given symbols be done so only symbols are
returned in
the dictionary. If this isn't done (and someone uses atoms() instead of
atoms(Symbol)) then the dictionary will have some numerical keys...it's not
a show
stopper, but perhaps it could be more discriminating.
e.g.
>>> var('x y z a')
(x, y, z, a)
>>> eq=3*x+a*(x+1)-a
>>> separatevars(eq, dict=True, symbols=eq.atoms())
{'coeff': 1, x: x, 1: 1, 3: 1, -1: 1, a: 3 + a}
Perhaps something in separatevars like
if dict:
symbols = [x for x in symbols if x.is_Symbol]
return _separatevars_dict(_separatevars(expr), *symbols)
else:
return _separatevars(expr)
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