Why does checking a variable against multiple valu

问题:

This question already has an answer here:

• How to test multiple variables against a value? 19 answers
• Elif and if not working or me not understanding [duplicate] 3 answers

I want to check if a variable has one of multiple values. I\'m confused about why or doesn\'t work in this situation. I was following a tutorial that gave the example if (a or b):, but when I try to do this it only checks the variable against the first value. What is wrong with my check?

name = raw_input(\'Please type in your name:\')

if len(name) < 5:
    print \"Your name has fewer than 5 characters\"
elif len(name) == 5:
    print \"Your name has exactly 5 characters\"
    if name == (\"Jesse\" or \"jesse\"):
        print \"Hey Jesse!\"
else:
    print \"Your name has greater than 5 characters\"

回答1:

(\"Jesse\" or \"jesse\")

The above expression tests whether or not \"Jesse\" evaluates to True. If it does, then the expression will return it; otherwise, it will return \"jesse\". The expression is equivalent to writing:

\"Jesse\" if \"Jesse\" else \"jesse\"

Because \"Jesse\" is a non-empty string though, it will always evaluate to True and thus be returned:

>>> bool(\"Jesse\")  # Non-empty strings evaluate to True in Python
True
>>> bool(\"\")  # Empty strings evaluate to False
False
>>>
>>> (\"Jesse\" or \"jesse\")
\'Jesse\'
>>> (\"\" or \"jesse\")
\'jesse\'
>>>

This means that the expression:

name == (\"Jesse\" or \"jesse\")

is basically equivalent to writing this:

name == \"Jesse\"

---

In order to fix your problem, you can use the in operator:

# Test whether the value of name can be found in the tuple (\"Jesse\", \"jesse\")
if name in (\"Jesse\", \"jesse\"):

Or, you can lowercase the value of name with str.lower and then compare it to \"jesse\" directly:

# This will also handle inputs such as \"JeSSe\", \"jESSE\", \"JESSE\", etc.
if name.lower() == \"jesse\":

回答2:

if name in (\"Jesse\", \"jesse\"):

would be the correct way to do it.

Although, if you want to use or, the statement would be

if name == \'Jesse\' or name == \'jesse\':

---

>>> (\"Jesse\" or \"jesse\")
\'Jesse\'

evaluates to \'Jesse\', so you\'re essentially not testing for \'jesse\' when you do if name == (\"Jesse\" or \"jesse\"), since it only tests for equality to \'Jesse\' and does not test for \'jesse\', as you observed.

回答3:

The or operator returns the first operand if it is true, otherwise the second operand. So in your case your test is equivalent to if name == \"Jesse\".

The correct application of or would be:

if (name == \"Jesse\") or (name == \"jesse\"):

回答4:

If you want case-insensitive comparison, use lower or upper:

if name.lower() == \"jesse\":