Question

可能重复：
Why does `letter==“A” or “a”` always evaluate to True?

当我在我的程序中运行它时，问题就会出现，但无论答案如何，“No”选项始终会运行。如果我切换选项顺序，“Y”选项将仅运行，它将始终直接启动。我确定我错过了一些简单的东西，我只是不知道是什么。

infoconf = raw_input("Is this information correct? Y/N: ")

    if infoconf == "N" or "No":
        print "Please try again."
    elif infoconf == "Y" or "Yes":
        start()
    else:
        print "That is not a yes or no answer, please try again."

Answer 1

原来是

infoconf = raw_input("Is this information correct? Y/N: ")

#you wrote:  infoconf == "N" or "No" but should be:
if infoconf == "N" or infoconf == "No": 
  print "Please try again."
#you wrote: infoconf == "Y" or "Yes" but should be
elif infoconf == "Y" or infoconf == "Yes": 
  start()
else:
  print "That is not a yes or no answer, please try again."

简短说明：

when value of x = 'N'
x == 'N' or 'No' will return True
when value of x = 'Y'
x == 'N' or 'No' will return 'No' i believe this is not what you want

在另一边

when value of x = 'N'
x == 'N' or x == 'No' will return True
when value of x = 'Y'
x == 'N' or x == 'No' will return False i believe this is what you want

Answer 2

Python会以不同于您的想法来解释infoconf == "N" or "No"。这有点是“运算符优先级”的情况，其中您的条件被解析为(infoconf == "N") or ("No")。

现在，infoconf == "N"可能是也可能不是，但"No"是“某事”，当被视为逻辑时，评估为真。实际上，您的条件infoconf == "N" or true将始终为真。

正如许多其他人所建议的那样，在第二个逻辑术语中比较infoconf到"No"就行了。

Answer 3

就个人而言，我会做这样的事情：

infoconf = raw_input("Is this information correct? Y/N: ")

if infoconf.lower().startswith('n'):
   # No
elif infoconf.lower().startswith('y'):
   # Yes
else:
   # Invalid

这意味着用户可以回答“Y / y / yes / yeah”为“是”，“N / n / no / nah”为“否”。

Answer 4

在Python中，这样做更容易：

infoconf = raw_input("Is this information correct? Y/N: ")

if infoconf in ["N", "No"]:
    print "Please try again."
elif infoconf in ["Y", "Yes"]:
    start()
else:
    print "That is not a yes or no answer, please try again."

正如其他人所说，if infoconf == "N" or "No"与if (infoconf == "N") or "No"是等价的，并且由于"No"（作为非空字符串）的计算结果为True，因此该语句将始终为真。

另外，为了对输入稍微挑剔，你可能希望在进行测试之前做infoconf = infoconf.strip().lower()（然后与小写版本进行比较）。

if-branch for x ==“N”或“No”始终运行

4 个答案: