Safely evaluate simple string equation

Question

I'm writing a program in which an equation is inputted as a string, then evaluated. So far, I've come up with this:

test_24_string = str(input("Enter your answer: "))
test_24 = eval(test_24_string)

I need both a string version of this equation and an evaluated version. However, eval is a very dangerous function. Using int() doesn't work, though, because it's an equation. Is there a Python function that will evaluate a mathematical expression from a string, as if inputting a number?

Answer 1

One way would be to use numexpr. It's mostly a module for optimizing (and multithreading) numpy operations but it can also handle mathematical python expressions:

>>> import numexpr
>>> numexpr.evaluate('2 + 4.1 * 3')
array(14.299999999999999)

You can call .item on the result to get a python-like type:

>>> numexpr.evaluate('17 / 3').item()
5.666666666666667

It's a 3rd party extension module so it may be total overkill here but it's definetly safer than eval and supports quite a number of functions (including numpy and math operations). If also supports "variable substitution":

>>> b = 10
>>> numexpr.evaluate('exp(17) / b').item()
2415495.27535753

---

One way with the python standard library, although very limited is ast.literal_eval. It works for the most basic data types and literals in Python:

>>> import ast
>>> ast.literal_eval('1+2')
3

But fails with more complicated expressions like:

>>> ast.literal_eval('import os')
SyntaxError: invalid syntax

>>> ast.literal_eval('exec(1+2)')
ValueError: malformed node or string: <_ast.Call object at 0x0000023BDEADB400>

Unfortunatly any operator besides + and - isn't possible:

>>> ast.literal_eval('1.2 * 2.3')
ValueError: malformed node or string: <_ast.BinOp object at 0x0000023BDEF24B70>

I copied part of the documentation here that contains the supported types:

Safely evaluate an expression node or a string containing a Python literal or container display. The string or node provided may only consist of the following Python literal structures: strings, bytes, numbers, tuples, lists, dicts, sets, booleans, and None.

Answer 2

It is not that difficult to write a postfix expression evaluator. Below is a working example. (Also available on github.)

import operator
import math

_add, _sub, _mul = operator.add, operator.sub, operator.mul
_truediv, _pow, _sqrt = operator.truediv, operator.pow, math.sqrt
_sin, _cos, _tan, _radians = math.sin, math.cos, math.tan, math.radians
_asin, _acos, _atan = math.asin, math.acos, math.atan
_degrees, _log, _log10 = math.degrees, math.log, math.log10
_e, _pi = math.e, math.pi
_ops = {'+': (2, _add), '-': (2, _sub), '*': (2, _mul), '/': (2, _truediv),
        '**': (2, _pow), 'sin': (1, _sin), 'cos': (1, _cos), 'tan': (1, _tan),
        'asin': (1, _asin), 'acos': (1, _acos), 'atan': (1, _atan),
        'sqrt': (1, _sqrt), 'rad': (1, _radians), 'deg': (1, _degrees),
        'ln': (1, _log), 'log': (1, _log10)}
_okeys = tuple(_ops.keys())
_consts = {'e': _e, 'pi': _pi}
_ckeys = tuple(_consts.keys())


def postfix(expression):
    """
    Evaluate a postfix expression.

    Arguments:
        expression: The expression to evaluate. Should be a string or a
                    sequence of strings. In a string numbers and operators
                    should be separated by whitespace

    Returns:
        The result of the expression.
    """
    if isinstance(expression, str):
        expression = expression.split()
    stack = []
    for val in expression:
        if val in _okeys:
            n, op = _ops[val]
            if n > len(stack):
                raise ValueError('not enough data on the stack')
            args = stack[-n:]
            stack[-n:] = [op(*args)]
        elif val in _ckeys:
            stack.append(_consts[val])
        else:
            stack.append(float(val))
    return stack[-1]

Usage:

In [2]: from postfix import postfix

In [3]: postfix('1 2 + 7 /')
Out[3]: 0.42857142857142855

In [4]: 3/7
Out[4]: 0.42857142857142855