isalpha()

中文的isalpha()会被判定为True，需要使用unicode来区分中文和英文。

def calculate_character_start_end(strs, idx):
    start = idx - 1
    end = idx + 1
    while (strs[start].encode('utf-8').isalpha()):
        start = start - 1
        if start == 0:
            break
    if not strs[start].encode('utf-8').isalpha():
        start = start + 1

    while (strs[end].encode('utf-8').isalpha()):
        end = end + 1
        if end == len(strs) - 1:
            break
    if not strs[end].encode('utf-8').isalpha():
        end = end - 1

    return start, end

参考资料
Python学习笔记（六）：isalpha()函数的用法