统计字符串中字母、数字、空格和其他字符的个数

可以通过遍历字符串，对每个字符判断其类型，然后统计相应的计数器。
例如：

def count_chars(s):
    letter_count = 0
    digit_count = 0
    space_count = 0
    other_count = 0
    for c in s:
        if c.isalpha():
            letter_count += 1
        elif c.isdigit():
            digit_count += 1
        elif c.isspace():
            space_count += 1
        else:
            other_count += 1
    return letter_count, digit_count, space_count, other_count