将字符串解析为布尔值？

Question

是否有可能将if语句中的字符串解析为字符串？像

if "1 > 2": 
    print "1 is greater than 2" 

但一些被解析为

if 1 > 2: 
    print "1 is greater than 2" 

这可能吗？这是一个程序吗？

Answer 1

这就是eval的用途。

if eval("1 > 2"): 
    print "1 is greater than 2" 

不过要注意eval。它会调用提供给它的任何函数。像os.system('rm -rf /')：/

Answer 2

如果您只是比较数值，这种方法通常会更安全。

这也可以用于非数字值。

from operator import gt, ge, lt, le, eq, ne 

def compare(expression): 
    parts = expression.split() 
    if len(parts) != 3: 
     raise Exception("Can only call this with 'A comparator B', like 1 > 2") 
    a, comp, b = parts 
    try: 
     a, b = float(a), float(b) 
    except: 
     raise Exception("Comparison only works for numerical values") 
    ops = {">": gt, '<': lt, '>=': ge, '<=': le, '==': eq, '!=': ne} 
    if comp not in ops: 
     raise Exception("Can only compare with %s" % (", ".join(ops))) 
    return ops.get(comp)(a, b) 


def run_comp(expression): 
    try: 
     print("{} -> {}".format(expression, compare(expression))) 
    except Exception as e: 
     print str(e) 

if __name__ == "__main__": 
    run_comp("1.0 > 2") 
    run_comp("2.0 > 2") 
    run_comp("2 >= 2") 
    run_comp("2 <= 1") 
    run_comp("5 == 5.0") 
    run_comp("5 <= 5.0") 
    run_comp("5 != 5.0") 
    run_comp("7 != 5.0") 
    run_comp("pig > orange") 
    run_comp("1 ! 2") 
    run_comp("1 >") 

输出

1.0 > 2 -> False 
2.0 > 2 -> False 
2 >= 2 -> True 
2 <= 1 -> False 
5 == 5.0 -> True 
5 <= 5.0 -> True 
5 != 5.0 -> False 
7 != 5.0 -> True 
Comparison only works for numerical values 
Can only compare with >=, ==, <=, !=, <, > 
Can only call this with 'A comparator B', like 1 > 2