Status: Valid
Owner: ----
Labels: Type-Enhancement Priority-Medium Printing
New issue 3613 by [email protected]: Save the order that an expression was
entered
http://code.google.com/p/sympy/issues/detail?id=3613
If you enter b*c*a, the original order, (b, c, a), is completely lost. The
order in Mul's args is canonicalized, and the printing order is computed
just by sorting alphabetically. But it would be nice to be able to print
expressions exactly as they were entered. This shouldn't be too hard, but
there are some subtleties I can foresee:
- We want to make sure this is general (i.e., put it on Basic, not Mul).
But not all classes are commutative.
- We want this to be part of the class, but not part of the equality
comparison, so that x*y == y*x still compares true.
- We want it to inherit. So Mul(Mul(x, y), z) has the same order as Mul(x,
y, z).
- We need it to work correctly with __op__ and __rop__.
- Probably the most important, it should not slow down the core. If it
makes things slow, we could disable it by default, but allow people to
enable it (e.g., when enabling the printing option that prints expressions
as they are entered).
Of course, if we got this to work, there would be the caveat that just
about any SymPy function could rearrange the order that an expression is
printed in, because there is no guarantee that it will manipulate an
expression in the same order that it was originally built.
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