Status: New
Owner: ----
CC: [email protected]
Labels: Type-Defect Priority-Medium Quantum
New issue 2974 by [email protected]: Bug in sympy.physics.quantum "qapply"
http://code.google.com/p/sympy/issues/detail?id=2974
(Note: this was originally reported at
https://github.com/sympy/sympy/issues/945)
There seems to be a bug in `qapply`, both in 0.7.1 and in the current git
HEAD.
I was triggering it when trying `qapply(vacuum.dual*A)` , where
`vacuum.dual` is a custom Bra-class that I'm hacking on, and A is a quantum
operator. I am not sure if my Bra-class does indeed conform to what is
required for such, but I believe the bug is independent from that. The main
requirements are that the two objects are non-commutative things that do
not know how to be applied to each other (i.e. `lhs._apply_operator(rhs)`
and vice versa fails). In this case `qapply_Mul` will continue to try the
previous pair in the multiplication via line 166 of `qapply.py`:
```
return qapply_Mul(e._new_rawargs(*(args+[lhs])), **options)*rhs
```
Now if this was actually the first pair of the multiplication,
`e._new_rawargs(*(args+[lhs]))` will return `lhs` itself (might this be the
bug indeed?). The new call of `qapply_Mul` will then work on the `lhs`
object, assuming it is `Mul`, which it isn't. It probably most often goes
unnoticed since it has a high chance of bailing out early either due to
`lhs` not having two arguments or one of them being commutative.
The solution should probably be simple, just check for for an empty `args`
at that line.
A testcase will need a lefthandside object that is not commutative and at
least two non-commutative arguments. I was unable to find a good example,
and my quick try at a simple example Bra-class was not successful.
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