Document `pyconvert_add_rule`

[jverzani]

Well, I wanted this to be an "issue", but really I have a request for wee bit of help. I asked this on discourse (https://discourse.julialang.org/t/pythoncall-equivalent-to-pycalls-pytype-mapping/77086) but found no answers.

I want to see if I can use PythonCall to replace PyCall with SymPy.jl and to do so am looking for someway to automatically convert the underlying sympy objects into julia objects (such as Py, though with a type from SymPy.) I think the right approach is pyconvert_add_rule but can't seem to get it working. As a test case, my current use of PyCall::pytype_mapping to do this is basically: pytype_mapping(sympy.core.basic.Basic, Sym).

[cjdoris]

Ah sorry I do hang out on discourse but don't necessarily see everything.

I have now documented pyconvert_add_rule here: https://cjdoris.github.io/PythonCall.jl/dev/conversion-to-julia/#py2jl-conversion-custom

You'll need to use PythonCall from the main branch (pkg> add PythonCall#main) for now until I make a release, because I changed the function a little before documenting it.

[jverzani]

Awesome. Thanks so much!

[jverzani]

Sorry to trouble you again, but you could guide me as to why the following fails to convert to Sym type of the last line:

using PythonCall # [6099a3de] PythonCall v0.6.1 `https://github.com/cjdoris/PythonCall.jl.git#main`
struct Sym{P}
    py::P
end

sympy = pyimport("sympy")
x = sympy.symbols("x")
x.__module__ # Python str: 'sympy.core.symbol'
x.__class__  # ython ManagedProperties: <class 'sympy.core.symbol.Symbol'>

pyconvert_sym(::Type{S}, x::Py) where {S} = PythonCall.pyconvert_return(Sym(x))::S
PythonCall.pyconvert_add_rule("sympy.core.symbol:Symbol", Sym, pyconvert_sym)

x = sympy.symbols("x")

[cjdoris]

Ah, PythonCall does not automatically convert Python objects to Julia ones when you do things like call Python functions. This differs from PyCall. So you must do one of:

• pyconvert(Any, sympy.symbols("x"))
• pyconvert(Sym, sympy.symbols("x"))
• Sym(sympy.symbols("x")) - this of course doesn't require you to define a pyconvert rule first

Also, as the Important paragraph in the documentation for pyconvert_add_rule says, you are not allowed to return anything which references x from your conversion rule. So you must return Sym(Py(x)) or add an inner constructor like Sym(x) = new(Py(x)) to Sym.

[jverzani]

Thanks for the clarification. Most appreciated!