在Python中,我应该如何测试变量是None,True还是False

Question

我有一个函数可以返回三件事之一:

• 成功(True)
• 失败(False)
• 错误读取/解析流(None)

我的问题是,如果我不应该反对,True或者我应该False如何看待结果.以下是我目前的做法:

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"

它是否真的像删除== True部分一样简单,或者我应该添加一个tri-bool数据类型.我不希望simulate函数抛出异常,因为我希望外部程序执行错误是记录并继续.

Answer 1

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

保持简单明了.您当然可以预先定义字典.

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

如果您计划修改simulate函数以包含更多返回代码,则维护此代码可能会成为一个问题.

该simulate也可能提高对解析错误的异常,在这种情况下,你要么就在这里抓住它还是让它传播水平并打印位将减少到单行if-else语句.

Answer 2

不要害怕异常!让您的程序只需记录并继续即可轻松实现:

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,您可以从模拟方法获得更丰富的通知类型,以确定出现了什么问题,以防您发现错误/无错误不足以提供足够的信息.

Answer 3

永远,永远,永远不要说

if something == True:

决不.这很疯狂,因为你冗余地重复冗余指定的if语句的冗余条件规则.

更糟糕的是,永远,永远,永远,永远不要说

if something == False:

你有not.随意使用它.

最后,做法a == None效率低下.做a is None. None是一个特殊的单例对象,只能有一个.只需检查一下你是否有那个对象.

Answer 4

有很多很好的答案。我想再补充一点。如果您正在处理数值，那么错误可能会进入您的代码，而您的答案恰好是 0。

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.