How to modify sympy's printing order?

Question

I am using sympy to write algebraic expressions and perform basic calculations with them. Sympy does not keep track of the order of the variables which can be a problem when it comes to printing expressions (the issue has already been raised here and here, so this is not a duplicate). e.g.

>>> from sympy import *
>>> var("p,a")
>>> l=p-1-a;
>>> print(l);
-a+p-1

However, sympy seems to print the variables in the alphabetical order. Is there a way to change the alphebetical order Python refers to and thus trick sympy into printing the variables in the desired order? Any other solution is welcome!

Will the Printing page in SymPy's documentation help? http://docs.sympy.org/latest/tutorial/printing.html — BoboDarph, Aug 10 '17 at 14:05
I have not seen anything relevant there. It seems to be a known issue, I don't think anything "conventional" will help. — Nre, Aug 10 '17 at 14:08
Possible duplicate of [Prevent Sympy from rearranging the equation](https://stackoverflow.com/questions/14624511/prevent-sympy-from-rearranging-the-equation) — Alexander McFarlane, Aug 10 '17 at 14:30
@AlexanderMcFarlane: OP is well aware of the 4 year old questions (linked them) and probably hopes the situaiton has changed since. Issues were reported to sympy before, unfortunately the links are broken. I do not consider this a duplicate. — laolux, Aug 10 '17 at 15:01

score 1 · Answer 1 · answered Aug 14 '17 at 17:01

1

Some new documentation on creating custom printers is in the pipe. Maybe that will help? I would create a custom printer which -- let's say we create a custom Add printer -- sorts the args based on some property like degree or sign of a term's coefficient, and then prints the resulting Add.

answered Aug 14 '17 at 17:01

smichr

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Thank you! I'll have a look if I manage to do something with that and will post it if it works. – Nre Aug 15 '17 at 11:01
Done! I would be happy to get any feedback on the way I implemented your idea. Thanks again – Nre Aug 21 '17 at 10:36
My initial feeling is that you are doing more than you need to. I would suggest that your print method take two args: the expression and a function that will sort args. That method would then get the args, sort them, and pass them as an unevaluated expression to the normal printer. Note: in your current code you use is_integer when I think you mean is_Integer and similar for is_symbol – smichr Aug 22 '17 at 13:01
Thank you. I will try to adjust the code as you suggest when I have time and update my answer. – Nre Aug 23 '17 at 13:09

Nre · Accepted Answer · 2017-08-21T10:42:33.400

Thanks to smichr's answer, I wrote a custom printer which does what I want. I am not a good programmer, so if you have any suggestions to make on my code, I would be happy to implement them.

from sympy import *
import math
import copy
import collections
import itertools
init_printing(use_unicode=True)
var("p,a,b,c,d,e,f");
ddict=collections.OrderedDict([(p, 1),(a, 2), (b, 3), (c, 4),(d, 5),(e, 6),(f, 7),])
from sympy import Basic, Function, Symbol
from sympy.printing.printer import Printer
from sympy.printing.latex import print_latex
from sympy.core.basic import Basic
class MyPrinter(Printer):
    printmethod = '_myprinter'
    def _print_Add(self,expr):
        expr_args=expr.args
        def new_place(el):
            if el in ddict:
                return ddict[el]
            else:
                return len(ddict)+1
        def get_place(el):
            if el.is_integer:
                return new_place(el)
            elif el.is_symbol:
                return new_place(el)
            elif len(el.args)>0:
                if el.args[len(el.args)-1].is_symbol:
                    return new_place(el.args[len(el.args)-1])
                else:
                    return 0
            else:
                return 0
        def write_coeff(el):
            if el.is_integer:
                if el>0:
                    return "+%s" %el
                else:
                    return "%s" %el
            elif el.is_symbol:
                return "+%s" %el
            elif len(el.args)>0:
                if el.args[len(el.args)-1].is_symbol:
                    if el.args[0].is_rational:
                        if el.args[0]>0:
                            return "+%s" %latex(el)
                        else:
                            return "%s" %latex(el)
                    else:
                        return "%s" %latex(el)
                else:
                    return "%s" %latex(el)
            else:
                return "%s" %el
        list_place=[get_place(a) for a in expr.args]
        expr_args=zip(*sorted(zip(list_place,expr_args)))[1]
        to_print=[write_coeff(a) for a in expr_args]
        to_print[0]=str(latex(expr_args[0]))
        return "".join(a for a in to_print)
def my_printer(expr):
    return (MyPrinter().doprint(expr))
de=-a+p+3+c-b
print(my_printer(de))

How to modify sympy's printing order?

2 Answers2

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