如何统计字符串中小写字母的个数？

可以使用Python内置函数islower()来判断一个字符是否为小写字母，然后遍历字符串，统计小写字母的个数即可。以下是Python代码示例：

def count_lower(s):
    count = 0
    for c in s:
        if c.islower():
            count += 1
    return count

s = "Hello, World! This is a Test String."
n = count_lower(s)
print("The number of lowercase letters is:", n) # 输出：The number of lowercase letters is: 18