2

Why should sys.argv with a negative index allow printing same value as sys.argv[0]? That too, it allows this up to number of arguments passed.

So, a call to hello.py at developers.google.com, such as the one below (with 3 arguments including script name) : python hello.py Sumit Test

would allow accessing sys.argv[-1], [-2] and [-3] with all of them printing the same value as argv[0] i.e. hello.py, but argv[-4] will throw the expected error:

Traceback (most recent call last):
  File "hello.py", line 35, in <module>
    main()
  File "hello.py", line 31, in main
    print (sys.argv[-4])
IndexError: list index out of range

The code is:

import sys

# Define a main() function that prints a little greeting.
def main():

  # Get the name from the command line, using 'World' as a fallback.
  if len(sys.argv) >= 2:
    name = sys.argv[1]
  else:
    name = 'World'
  print ('Hello', name)
  print (sys.argv[-3])

# This is the standard boilerplate that calls the main() function.
if __name__ == '__main__':
  main()
4

2 回答 2

5

因为您只传递了三个参数，所以下面的示例应该可以帮助您理解：

>>> [1,2,3][-1]   # working 
3
>>> [1,2,3][-2]   # working 
2
>>> [1,2,3][-3]   # working 
1
>>> [1,2,3][-4]   # exception as in your code 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

负索引从右侧打印一个值。

访问列表
例如我们的数组/列表的大小为n，那么对于正索引0是第一个索引，1第二个和最后一个索引将是n-1。对于负索引，-n是第一个索引，-(n-1)第二个，最后一个负索引将是–1。

根据您的评论，我添加了一个示例以进行澄清： 

import sys
# main()    
if __name__ == "__main__":
    print len(sys.argv)
    print sys.argv[-1], sys.argv[-2], sys.argv[-3]
    print sys.argv[0],  sys.argv[1], sys.argv[2]

请观察输出：

$ python main.py one two 
3
two one main.py
main.py one two

传递的参数数量为三个。argv[-1]是最后一个参数，即two

于 2013-06-11T06:36:28.403 回答
1

负索引从列表末尾开始计数：

>>> ['a', 'b', 'c'][-1]
'c'
>>> ['a', 'b', 'c'][-2]
'b'
>>> ['a', 'b', 'c'][-3]
'a'

要求 [-4] 将超出列表的末尾，给出一个例外。

于 2013-06-11T06:36:31.643 回答