math - Avoid sorting args in Python module Sympy

Question

I am currently developing a differential operator for sympy that can be placed in matricial form. In this case the order of the args list when creating a Mul object is very important to guarantee that the differentiation is performed where it is required only.

The issue is that, when the following is done:

input = (t,z,x)
Mul(*input).args

It returns (t, x, z) because some rearrangement in args took place. How to avoid args to be sorted?

Answer 1

Why is the arg ordering important for it to be correct?

The only way to prevent this is to set your symbols to be non-commutative (x = Symbol('x', commutative=False)). SymPy objects compare by comparing the args, so for x*y*z == y*x*z to work, the args have to be sorted canonically. There have been some attempts to get this working without explicit sorting (mainly for performance reasons), but note that even if we did that, there would be no guarantee at all about the arg order, especially if you perform any kind of operation on the expression. It is very common for functions to rebuild expressions in SymPy, which would in general use some other unrelated order from the original.