python - 在没有任何内置 e​​val 函数或外部库的情况下在 python 中求解数学

Question

我需要编写一个可以从中获取值的程序

Answer 1

非常有趣的问题。实际上，就您而言，这很简单。您可以使用 ast 模块解析整个源字符串，如下所示

import ast
import operator

functions = {
    "add": operator.add,
    "abs": operator.abs,
    "multiply": operator.mul
}

def recursive_evaluation(current_element):
    if isinstance(current_element, ast.Module):
        return recursive_evaluation(current_element.body[0].value)
    elif isinstance(current_element, ast.Call):
        function = functions[current_element.func.id]
        args = [recursive_evaluation(item) for item in current_element.args]
        return function(*args)
    elif isinstance(current_element, ast.Num):
        return current_element.n
    else:
        raise ValueError("Unknown Element " + str(current_element))

source = "abs(add(add(9465,38),multiply(add(63303,146),46)))"
print recursive_evaluation(ast.parse(source))

source = "add(1, -2)"
print recursive_evaluation(ast.parse(source))

source = "abs(add(1, -2))"
print recursive_evaluation(ast.parse(source))

输出

2928157
-1
1