allow custom pytype return values

[vtjnash]

this extends pytype_query() to allow the user to add custom type mappings.

example usage (based upon SymPy):

type Sym
     x::PyCall.PyObject
end
Sym(s::Sym) = s
convert(::Type{Sym}, o::PyCall.PyObject) = Sym(o)
PyCall.pytype_query_add(sympy.basic[Basic], Sym)

note: I see this would address item 3 in #9

[vtjnash]

I wasn't sure if this needed to be handled differently given the above extension to pytype_query, so I left is alone

[stevengj]

Thanks! Perhaps pytype_mapping(pyT, T) (or pytype_mapping[pyT] = T?) would be a better name than pytype_query_add? And exported?

Also, maybe pytype_queries should be a Dict instead of an Array of pairs, so that it avoids adding duplicate entries if pytype_mapping is called more than once.

(What I had in mind for #9 was more like an automatic mapping of a Julia composite type to a Python composite type with the same fields, which is different from this.)

[vtjnash]

Thanks, I'll update when I have time. My thought was that an Array would be much faster for iteration (which will happen much more often than insertion), so I think I'll keep it the same, but I'll also add a check for duplicates to mimic a Dict.

[stevengj]

Sounds good.

[vtjnash]

OK. This should be set.

As an example, note that this would have allowed BigFloat support to be added without editing PyCall (except perhaps for adding finalizer support). This would by done by adding to mpmath_init the following lines:

pytype_mapping(PyCall.mpf, BigFloat)
pytype_mapping(PyCall.mpc, Complex{BigFloat})

instead of adding it directly to pytype_query and creating pymp_query

[vtjnash]

bump @stevengj

[stevengj]

Sorry about the delay. I'm on vacation this week with family and may not get to PyCall until I get back in the last week of June.