From string to sympy expression

Question

Recently I was working on a web application, using Flask and sympy libraries. The user enters his equation in a textarea and Flask rechieve it as a string. I would like to have the possibility to calculate the result of this equation,by using sympy function solve(). But for this I must convert this string to an sympy expression. How could I do that?

from flask import Flask,request,render_template,flash
from sympy import *
from sympy.parsing.sympy_parser import *

x = symbols('x')
app = Flask(__name__)
app.secret_key = 'mysecretkey'


def calculate_():
    first_eq = request.form['first_eq']

    first_eq2= parse_expr(first_eq)
    result = solve(first_eq2)
    return result


@app.route("/",methods=['GET', 'POST'])
def myGet():

    return render_template("my-form.html")

@app.route("/myPost/",methods=['GET', 'POST'])
def myPost():
    answer = request.form['answer']
    result_of_answer=solve(Eq(answer),x)
    result = calculate_()
    try:
        if result == result_of_answer:
            flash("you got it!")
        else:
            flash("no, it's wrong")
            return render_template("my-form.html")
    except:
        flash("sorry, wrong type. Try again!")
        return render_template("my-form.html")
if __name__ == '__main__':
    app.run()

You can use `parse_expr` which converts the string `s` to a SymPy expression as follows: http://docs.sympy.org/dev/modules/parsing.html — Avión, Nov 09 '15 at 10:13
I tryed to use this in my calculate_ func. It gives me a sympifyError: `def calculate_(): first_eq = request.form['first_eq'] first_eq2= parse_expr(first_eq) result = solve(first_eq2) return result` — Vasile, Nov 09 '15 at 11:47
What's the content of `first_eq`? Please, edit your main post with your code. — Avión, Nov 09 '15 at 12:06
I added my code. The content of the first_eq is an expression equaled to zero. — Vasile, Nov 09 '15 at 13:14

score 12 · Answer 1 · answered Apr 05 '20 at 06:28

12

sympify() uses eval() which is dangerous. Use sympy.parsing.sympy_parser.parse_expr() instead.

answered Apr 05 '20 at 06:28

Alec

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score 10 · Accepted Answer · answered Nov 09 '15 at 17:33

10

The function you're looking for is sympify. http://docs.sympy.org/latest/modules/core.html#sympy.core.sympify.sympify

answered Nov 09 '15 at 17:33

asmeurer

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This answer uses `eval()` and is dangerous – Alec Apr 05 '20 at 06:14
1

See also: [How safe is simpify?](https://stackoverflow.com/questions/16718644/how-safe-is-sympys-sympifystring-evalf) (tl;dr: it's not) – Stef Sep 09 '22 at 08:38

From string to sympy expression

2 Answers2

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