Status: Accepted
Owner: asmeurer
Labels: Type-Defect Priority-High Milestone-Release0.7.0
New issue 2070 by asmeurer: All elements of .args should be Basic
http://code.google.com/p/sympy/issues/detail?id=2070
There are two invariants that any .args attribute should uphold. The first
is that Object.__class__(*Object.args) == Object, i.e., you should be able
to rebuild the object exactly by calling it with its .args. Actually,
strictly speaking, it should really be Object.__class__(*Object.args,
**Object.kwargs), where kwargs is assumptions or options or something. We
actually still need to fix that up once we completely dump the old
assumptions.
But other than that, the first invariant holds pretty well among Basic
objects in SymPy (except for maybe issue 1206). The second invariant is
the one that is often violated. It is that all(isinstance(Basic.args,
Basic)) == True should hold, i.e., every element of .args should itself be
a instance of Basic. This is so it will be easy to recurse through .args,
because every element of .args will itself have a .args.
Currently, many classes use tuples in their .args, violating this rule, and
creating headaches like issue 2068. I know that Integral and Sum are two.
There might also be others. The fix is to use the new Tuple container,
which acts like a tuple but is also an element of Basic, so it works
with .args.
*Note* .args itself should remain a tuple. Only the elements need to be
fixed.
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