isinstance and MatrixSymbol

[Lorenzo Monacelli]

Dear all,

I would like to implement some basic manipulation with matrices that I need. 

The problem I'm facing is that I cannot figure out how to recognize correctly a MatrixSymbol:

from sympy.abc import i,j

A = sy.MatrixSymbol("A",3,3)

B = sy.MatrixSymbol("B",3,3)

C = sy.MatrixSymbol("C",3,3)

pippo = A[i,j].subs(A, B+C)

isinstance(pippo.args[0].args[0].func, sy.MatrixSymbol)  # False
isinstance(B.func, sy.MatrixSymbol) #True

Note, that  pippo.args[0].args[0] is B and if you print pippo.args[0].args[0].func it returns 

sympy.matrices.expressions.matexpr.MatrixSymbol

So why is it not working? How can I check if it is a matrix and stop the recursion?

Bests,

Lorenzo

[Oscar Benjamin]

On Thu, 5 Mar 2020 at 11:30, Lorenzo Monacelli
<lorenzo92...@gmail.com> wrote:
>
> Dear all,
> I would like to implement some basic manipulation with matrices that I need.
> The problem I'm facing is that I cannot figure out how to recognize correctly a MatrixSymbol:
> from sympy.abc import i,j
> A = sy.MatrixSymbol("A",3,3)
> B = sy.MatrixSymbol("B",3,3)
> C = sy.MatrixSymbol("C",3,3)
>
> pippo = A[i,j].subs(A, B+C)
> isinstance(pippo.args[0].args[0].func, sy.MatrixSymbol) # False
> isinstance(B.func, sy.MatrixSymbol) #True

The .func attribute gives MatrixSymbol so you could use:

pippo.args[0].args[0].func is sy.MatrixSymbol

or (note no .func):

isinstance(pippo.args[0].args[0], sy.MatrixSymbol)

> Note, that pippo.args[0].args[0] is B and if you print pippo.args[0].args[0].func it returns
> sympy.matrices.expressions.matexpr.MatrixSymbol
>
> So why is it not working?

Because you are asking if MatrixSymbol is an instance of itself:

In [11]: isinstance(MatrixSymbol, MatrixSymbol)
Out[11]: False

--
Oscar

[S.Y. Lee]

I think that this is a gotcha with Python.

Python already returns False when checking out isinstance(int, int).

And in general OOP theory, class and instance should be different.

If you want to compare the class by class, then comparing with == or issubclass should suffice.

[Lorenzo Monacelli]

Thank you very much!

Indeed, now it works!