在Python3上评估没有eval()的数学表达式

Question

我正在研究一种"复制粘贴计算器",它可以检测复制到系统剪贴板的任何数学表达式,对它们进行评估并将答案复制到准备粘贴的剪贴板上.但是,虽然代码使用了eval()函数,但考虑到用户通常知道他们正在复制什么,我并不十分担心.话虽如此,我想找到一种更好的方法,而不会给计算带来障碍(例如,删除计算乘法或指数的能力).

这是我的代码的重要部分:

#! python3
import pyperclip, time

parsedict = {"×": "*",
             "÷": "/",
             "^": "**"} # Get rid of anything that cannot be evaluated

def stringparse(string): # Remove whitespace and replace unevaluateable objects
    a = string
    a = a.replace(" ", "")
    for i in a:
        if i in parsedict.keys():
            a = a.replace(i, parsedict[i])
    print(a)
    return a

def calculate(string):
    parsed = stringparse(string)
    ans = eval(parsed) # EVIL!!!
    print(ans)
    pyperclip.copy(str(ans))

def validcheck(string): # Check if the copied item is a math expression
    proof = 0
    for i in mathproof:
        if i in string:
            proof += 1
        elif "http" in string: #TODO: Create a better way of passing non-math copies
            proof = 0
            break
    if proof != 0:
        calculate(string)

def init(): # Ensure previous copies have no effect
    current = pyperclip.paste()
    new = current
    main(current, new)

def main(current, new):
    while True:
        new = pyperclip.paste()
        if new != current:
            validcheck(new)
            current = new
            pass
        else:
            time.sleep(1.0)
            pass

if __name__ == "__main__":
    init()

问:我应该使用什么而不是eval()来计算答案？

Answer 1

你应该使用ast.parse:

import ast

try:
    tree = ast.parse(expression, mode='eval')
except SyntaxError:
    return    # not a Python expression
if not all(isinstance(node, (ast.Expression,
        ast.UnaryOp, ast.unaryop,
        ast.BinOp, ast.operator,
        ast.Num)) for node in ast.walk(tree)):
    return    # not a mathematical expression (numbers and operators)
result = eval(compile(tree, filename='', mode='eval'))

请注意,为了简单起见这允许所有的一元运算符(+,-,~,not),以及该算术和逐位二进制运算符(+,-,*,/,%,// **,<<,>>,&,|,^),但不逻辑或比较运算符.如果应该直接改进或扩展允许的运算符.