当执行Sympify时，将切换表达式的顺序（Python的Sympy）

Question

I ran the following program

from sympy import *

str = "Abs(a)*(β-α)**3/6"
print(str)
print(sympify(str))

The execution result was as follows.

Abs(a)*(β-α)**3/6
(-α + β)**3*Abs(a)/6

As a result of executing sympify , the order of the expressions has changed.

I want to match the execution results as follows.

Abs(a)*(β-α)**3/6
Abs(a)*(β-α)**3/6

What should I do?

---

The reason why I want to do this is that I don't want to make it look weird when converting an expression to mathml format.

str = "Abs(a)*(β-α)**3/6"
print(mathml(sympify(str),printer='presentation'))

When the above program is executed, the following is output.

<mrow><mfrac><mrow><msup><mfenced><mrow><mrow><mo>-</mo><mi>&#945;</mi></mrow><mo>+</mo><mi>&#946;</mi></mrow></mfenced><mn>3</mn></msup><mo>&InvisibleTimes;</mo><mrow><mfenced clos
e="|" open="|"><mi>a</mi></mfenced></mrow></mrow><mn>6</mn></mfrac></mrow>

It looks like the image below.

I want the formula to look like the image below.

Answer 1

If you apply the following diff to SymPy, I think your case will work:

diff --git a/sympy/printing/str.py b/sympy/printing/str.py
index ee560ca..cb0db5e 100644
--- a/sympy/printing/str.py
+++ b/sympy/printing/str.py
@@ -51,6 +51,8 @@ def _print_Add(self, expr, order=None):

         PREC = precedence(expr)
         l = []
+        if len(terms) == 2 and str(terms[0])[0] == '-' and str(terms[1])[0] != '-':
+            terms.reverse()
         for term in terms:
             t = self._print(term)
             if t.startswith('-'):

(It prints b - a as the same instead of as -a + b with that change.)