Parse math formulas in python and put parentheses around Mult/Div

main.py

from __future__ import print_function
import ast


def recurse(node):
    if isinstance(node, ast.BinOp):
        if isinstance(node.op, ast.Mult) or isinstance(node.op, ast.Div):
            print('(', end='')
        recurse(node.left)
        recurse(node.op)
        recurse(node.right)
        if isinstance(node.op, ast.Mult) or isinstance(node.op, ast.Div):
            print(')', end='')
    elif isinstance(node, ast.Add):
        print('+', end='')
    elif isinstance(node, ast.Sub):
        print('-', end='')
    elif isinstance(node, ast.Mult):
        print('*', end='')
    elif isinstance(node, ast.Div):
        print('/', end='')
    elif isinstance(node, ast.Num):
        print(node.n, end='')
    else:
        for child in ast.iter_child_nodes(node):
            recurse(child)


def search_expr(node):
    returns = []
    for child in ast.iter_child_nodes(node):
        if isinstance(child, ast.Expr):
            return child
        returns.append(search_expr(child))
    for ret in returns:
        if isinstance(ret, ast.Expr):
            return ret
    return None


formula = '3+7*8/2'

a = ast.parse(formula)

expr = search_expr(a)
if expr is not None:
    recurse(expr)

This is exactly what I am after but I cant get it to run in my Flask app, it always returns None. Have you tried to use it with the formula as an input variable and the parsing returning the formatted formula or None?

def fix(formula):
    def recurse(node):
        if isinstance(node, ast.BinOp):
            if isinstance(node.op, ast.Mult) or isinstance(node.op, ast.Div):
                print('(', end='')
            recurse(node.left)
            recurse(node.op)
            recurse(node.right)
            if isinstance(node.op, ast.Mult) or isinstance(node.op, ast.Div):
                print(')', end='')
        elif isinstance(node, ast.Add):
            print('+', end='')
        elif isinstance(node, ast.Sub):
            print('-', end='')
        elif isinstance(node, ast.Mult):
            print('*', end='')
        elif isinstance(node, ast.Div):
            print('/', end='')
        elif isinstance(node, ast.Num):
            print(node.n, end='')
        else:
            for child in ast.iter_child_nodes(node):
                recurse(child)
    
    
    def search_expr(node):
        returns = []
        for child in ast.iter_child_nodes(node):
            if isinstance(child, ast.Expr):
                return child
            returns.append(search_expr(child))
        for ret in returns:
            if isinstance(ret, ast.Expr):
                return ret
        return None
    a = ast.parse(formula)
    expr = search_expr(a)
    output = recurse(expr)
    return output

The issue is that this gist is printing the results, not putting them in a variable. That's why it returns None

Would it be easy/possible to switch it over to variable? I could try but I am greener than grass at this.

yep like this:

from __future__ import print_function
import ast

def fix(formula):
    def recurse(node):
        result = ''
        if isinstance(node, ast.BinOp):
            if isinstance(node.op, ast.Mult) or isinstance(node.op, ast.Div):
                result += '('
            result += recurse(node.left)
            result += recurse(node.op)
            result += recurse(node.right)
            if isinstance(node.op, ast.Mult) or isinstance(node.op, ast.Div):
                result += ')'
        elif isinstance(node, ast.Add):
            result += '+'
        elif isinstance(node, ast.Sub):
            result += '-'
        elif isinstance(node, ast.Mult):
            result += '*'
        elif isinstance(node, ast.Div):
            result += '/'
        elif isinstance(node, ast.Num):
            result += str(node.n)
        else:
            for child in ast.iter_child_nodes(node):
                result += recurse(child)
        return result
    
    
    def search_expr(node):
        returns = []
        for child in ast.iter_child_nodes(node):
            if isinstance(child, ast.Expr):
                return child
            returns.append(search_expr(child))
        for ret in returns:
            if isinstance(ret, ast.Expr):
                return ret
        return None
    a = ast.parse(formula)
    expr = search_expr(a)
    output = recurse(expr)
    return output

Legend Thankyou, just tested perfectly. I will trawl over it and see if I can learn more about it and not have to ask next time.
All part of my new COVID skills in python

Looking good! So nice to see that there are good things coming out of COVID :)