Why is this giving me an index error in python?

Question

In the code below, s refers to a string (although I have tried converting it to a list and I still have the same problem).

s = "".join(s)
if s[-1] == "a":
    s += "gram"

我在字符串中的最后一项是字母"a",然后程序需要将字符串"gram"添加到字符串's'表示的末尾.

例如输入:

s = "insta"

输出:

instagram

但我不断得到一个IndexError,任何想法为什么？

Answer 1

如果s是空字符串s[-1]导致IndexError:

>>> s = ""
>>> s[-1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: string index out of range

而不是s[-1] == "a",您可以使用s.endswith("a"):

>>> s = ""
>>> s.endswith('a')
False
>>> s = "insta"
>>> s.endswith('a')
True

Answer 2

如果s是空的,则没有最后一个字母要测试:

>>> ''[-1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: string index out of range

str.endswith()改为使用:

if s.endswith('a'):
    s += 'gram'

str.endswith()不不,当字符串为空抛出一个异常:

>>> 'insta'.endswith('a')
True
>>> ''.endswith('a')
False

或者,使用切片也可以:

if s[-1:] == 'a':

因为切片总是返回一个结果(至少是一个空字符串),但str.endswith()对于它对代码的随意读者所做的操作更为明显.