Auto-converting one-dimensional AbstractArray to Matrix breaks type-checking logic in Python library

[utensil]

In https://github.com/JuliaPy/SymPy.jl/blob/master/src/matrix.jl#L37:

## This allows abstract arrays of Sym Objects to slip through sympy.meth() calls
PyCall.PyObject(A::AbstractArray{Sym,2}) =
    PyCall.pycall(sympy.Matrix, PyCall.PyObject, [PyCall.PyObject.(A[i,:]) for i in 1:size(A)[1]])

PyCall.PyObject(V::AbstractArray{Sym,1}) =
    PyCall.pycall(sympy.Matrix, PyCall.PyObject,[[PyCall.PyObject(v)] for v in V])

This is fine for 2 dimensional AbstractArray and above, but it may break type-checking logic in Python library. And in GAlgebra and its Julia wrapper GAlgebra.jl, this is exactly the case.

The parameter g of galgebra.ga.Ga expects a metric which can be specified in many ways, as a string, as a Python list, or as a SymPy Matrix. The last two has a subtle difference:

• a Python list is treated as diagonal elements of the metric matrix
• a SymPy Matrix is treated as a complete matrix

For example, in the Python version of galgebra.ga.Ga, we can use:

#Define spherical coordinate system in 3-d

coords = (r, th, phi) = symbols('r,theta,phi', real=True)

s3d = Ga('e_r,e_th,e_ph', g=[1, r**2, r**2*sin(th)**2], coords=coords)
(er, eth, ephi) = s3d.mv()

Convert these to Julia syntax, it naturally becomes something like:

import SymPy: sympy
using GAlgebra

Ga = galgebra.ga.Ga

(r, th, phi) = coords = sympy.symbols("r theta phi", real=true)
s3d = Ga("e_r e_theta e_phi", g=[1, r^2, r^2 * sympy.sin(th)^2], coords=coords)
(er, eth, ephi) = s3d.mv()

But this won't work because SymPy automatically converted one-dimensional AbstractArray to Matrix, and GAlgebra will require a Matrix to be a complete metric Matrix instead of a list of diagonal elements of the metric matrix.

so I'll have to expand the Ga() call to

s3d = Ga("e_r e_theta e_phi", g=[1 0 0; 0 r^2 0; 0 0 r^2 * sympy.sin(th)^2], coords=coords, norm=true)

Of course, I can change GAlgebra to handle this scenario by treating one-dimensional sympy.Matrix as a list. But this might not work for other Python wrappers, and the core of the problem is one now can't pass a list of SymPy.jl objects to Python as a list of SymPy objects.

So I raised this issue to see if it could also cause problems for others and if it can be handled better.

[jverzani]

Yes, reasonable point caused by https://github.com/JuliaPy/SymPy.jl/blob/master/src/SymPy.jl#L128

Do you know if vectors are a distinct python class?

[utensil]

Do you know if vectors are a distinct python class?

Do you mean list?

[jverzani]

I think this happens in PyCall (the conversion from PyList to an array) in the conversions.jl file. I'm not sure how to get a List type to flow through. What happens if you remove

PyCall.PyObject(V::AbstractArray{Sym,1}) =
    PyCall.pycall(sympy.Matrix, PyCall.PyObject,[[PyCall.PyObject(v)] for v in V])

and try and pass a Python list through?

[utensil]

Sorry for the late reply. I'll experiment a little bit and see what happens.