how to check a math expression in a string Python

Question

I have a math expression in a string.

So,how can i check is math expression right ?

Maybe there are some ways to do that using Python libraries without creating own functional?

Examples:

exp = "(5+5)-(5)+(-2)" #True
exp = "(5+5)/2-1" #True
exp = "(5+5)/(3-1)" #True
exp = "(5+5)/(3-1" #False
exp = "5+" #False
exp = "(5+5)/()" #False
exp = "(a+b)/5" #False (only numbers)

Answer 1

You can use the eval function in pandas

import pandas as pd
exp = "(5+5)-(5)+(-2)"
pd.eval(exp)

If you intend on using expressions like "(a+b)/5" make sure you define the variables like this

a = 1
b = 2
exp = "(a+b)/5"
pd.eval(exp)

else the snippet will yield the following error:

UndefinedVariableError: name 'a' is not defined

You can try using this function

def ismath(exp):
 try:
 result = pd.eval(exp)
 return True
 except Exception:
 return False
ismath("(5+5)-(5)+(-2)") # Returns True
ismath("(5+5)-(5)+(a)") # Returns False

Answer 2

As @Luke has suggested, eval() can be used, but I think it's important to expand due to the risks associated with the use of eval - especially with user-controlled input.

2 big reasons to pre-filter input with a regex pattern

1. In order to filter out expressions that contain anything other than valid mathematical expressions (without variables), you will want to first perform a regex filter, only passing to eval() strings which contain only characters that are valid in a math-context (which was part of your question)

2. The second reason to use a regex, is that eval() can be very dangerous when users are able to inject arbitrary values as direct input. By limiting to only the characters valid in a math equation, your risk is reduced significantly

The Regex itself

You can validate that inputs match a pattern along the lines of: r'^[\d+-/*()]+$' (example here)

While the version above wouldn't account for syntactically incorrect operations (e.g. "(5+5)/(3-1", "5+" or "(5+5)/()"), you should be able to play with it to improve filtering.

... or just catch the errors from eval() that result from invalid math

More on security

This page gives some more context/examples of the risks of eval() as well as some ways to mitigate.

tl;dr

1. Use a regex to limit input to eval() to only strings made up exclusively or math characters

2. make the function call like so: eval(expr, {}, {}) such that the eval function will not have access to the program's global and local variables (which it doesn't need in the first place).