如何在Python的计算中使用像x，y这样的字符串符号作为未定义变量？

symbol_table = {}

def assign_symbol(symbol, value):
    symbol_table[symbol] = value

def evaluate_expression(expression):
    for symbol in symbol_table:
        expression = expression.replace(symbol, str(symbol_table[symbol]))
    return eval(expression)

# 示例用法
assign_symbol('x', 5)
assign_symbol('y', 10)

result = evaluate_expression('2 * x + y')
print(result)  # 输出：20