在Python中如何测试变量是否为None、True或False

Question

我有一个函数，它可以返回三种结果：

• 成功（True）
• 失败（False）
• 读取/解析流时出错（None）

我想问的是，如果我不应该直接检查 True 或 False，那我该怎么判断结果呢？下面是我现在的做法：

result = simulate(open("myfile"))
if result == None:
    print "error parsing stream"
elif result == True: # shouldn't do this
    print "result pass"
else:
    print "result fail"

是不是只要去掉 == True 的部分就可以了，还是说我应该添加一个三值数据类型？我不想让 simulate 函数抛出异常，因为我只希望外部程序在遇到错误时记录一下，然后继续运行。

Answer 1

这里有很多不错的回答。我想再补充一点。如果你在处理数字的时候，结果恰好是0，那么你的代码里可能会出现错误。

a = 0 
b = 10 
c = None

### Common approach that can cause a problem

if not a:
    print(f"Answer is not found. Answer is {str(a)}.") 
else:
    print(f"Answer is: {str(a)}.")

if not b:
    print(f"Answer is not found. Answer is {str(b)}.") 
else:
    print(f"Answer is: {str(b)}")

if not c:
    print(f"Answer is not found. Answer is {str(c)}.") 
else:
    print(f"Answer is: {str(c)}.")

Answer is not found. Answer is 0.   
Answer is: 10.   
Answer is not found. Answer is None.

### Safer approach 
if a is None:
    print(f"Answer is not found. Answer is {str(a)}.") 
else:
    print(f"Answer is: {str(a)}.")

if b is None:
    print(f"Answer is not found. Answer is {str(b)}.") 
else:
    print(f"Answer is: {str(b)}.")

if c is None:
    print(f"Answer is not found. Answer is {str(c)}.") 
else:
    print(f"Answer is: {str(c)}.")

Answer is: 0.
Answer is: 10.
Answer is not found. Answer is None.

Answer 2

if result is None:
    print "error parsing stream"
elif result:
    print "result pass"
else:
    print "result fail"

保持简单明了。当然，你可以提前定义一个字典。

messages = {None: 'error', True: 'pass', False: 'fail'}
print messages[result]

如果你打算修改你的 simulate 函数，加入更多的返回代码，维护这段代码可能会变得有点麻烦。

而且，simulate 可能会因为解析错误而抛出异常，这种情况下你要么在这里捕获这个异常，要么让它往上层传播，这样打印的部分就可以简化成一行的if-else语句。

Answer 3

别害怕异常！让你的程序记录错误并继续运行其实很简单：

try:
    result = simulate(open("myfile"))
except SimulationException as sim_exc:
    print "error parsing stream", sim_exc
else:
    if result:
        print "result pass"
    else:
        print "result fail"

# execution continues from here, regardless of exception or not

现在，当你在模拟方法中遇到问题时，可以得到更详细的通知，这样如果你觉得仅仅知道有没有错误不够有用，就能更清楚地知道到底出了什么问题。