Python：我的代码出错，我不知道如何修复它

Question

The example below is failing upper_lower('abcXYZ') , it returns true

def upper_lower(s: str) -> bool:
    """Return True if and only if there is at least one alphabetic character
    in s and the alphabetic characters in s are either all uppercase or all
    lowercase.
    >>> upper_lower('abc')
    True
    >>> upper_lower('abcXYZ')
    False
    >>> upper_lower('XYZ')
    True
    """
    for char in s:
        if char.isalpha():
            if char.isupper() or char.islower():
                return True
            if char.swapcase():
                return False
            else:
                return False

Answer 1

Try that:

def upper_lower(s):
    return s.isupper() or s.islower()

print(upper_lower("abc"))  # True
print(upper_lower("12abc45"))  # True
print(upper_lower("ABC"))  # True
print(upper_lower("ABC45"))  # True
print(upper_lower("aBC"))  # False
print(upper_lower("123"))  # False

Your code currently returns True if the first alpha character is eiher lowcase or upcase:

if char.isupper() or char.islower():
    return True   # return cause the function to end, other cars are not tested

Answer 2

I would suggest using list comprehension for this:

def upper_lower(word):
    if not word:
        return False
    else: 
        return all([s.isupper() for s in word]) or all([s.islower() for s in word])

Answer 3

def upper_lower(s):
    s = ''.join(c for c in s if c.isalpha())
    return bool(s) and (s.isupper() or s.islower())