Python列表索引（）

index()是Python中的一个内置函数，它从列表的开头搜索给定元素并返回该元素出现的最低索引。

Syntax:

list_name.index(element, start, end)

Parameters:  

• element – The element whose lowest index will be returned.
• start (Optional) – The position from where the search begins.
• end (Optional) – The position from where the search ends.

Returns:  

Returns the lowest index where the element appears.

Error: 

If any element which is not present is searched, it returns a ValueError

示例 1：查找元素的索引

Python3

# Python3 program for demonstration  
# of list index() method
  
list1 = [1, 2, 3, 4, 1, 1, 1, 4, 5]
  
# Will print the index of '4' in list1
print(list1.index(4))
  
list2 = ['cat', 'bat', 'mat', 'cat', 'pet']
  
# Will print the index of 'cat' in list2 
print(list2.index('cat'))

输出：

3
0

示例 1.1

Python3

# Python3 program for demonstration 
# of index() method
  
list1 = [1, 2, 3, 4, 1, 1, 1, 4, 5]
  
# Will print index of '4' in sublist
# having index from 4 to 8.
print(list1.index(4, 4, 8))
  
# Will print index of '1' in sublist
# having index from 1 to 7.
print(list1.index(1, 1, 7))
  
list2 = ['cat', 'bat', 'mat', 'cat', 
         'get', 'cat', 'sat', 'pet']
  
# Will print index of 'cat' in sublist
# having index from 2 to 6
print(list2.index('cat', 2, 6 ))

输出：

7
4
3

示例 1.2

Python3

# Python3 program for demonstration 
# of list index() method
  
# Random list having sublist and tuple also
list1 = [1, 2, 3, [9, 8, 7], ('cat', 'bat')]
  
# Will print the index of sublist [9, 8, 7]
print(list1.index([9, 8, 7]))
  
# Will print the index of tuple 
# ('cat', 'bat') inside list 
print(list1.index(('cat', 'bat')))

输出 ：

3
4

示例 2：列表中不存在的元素的索引 (ValueError)

Python3

# Python3 program for demonstration
# of index() method error
  
list1 = [1, 2, 3, 4, 1, 1, 1, 4, 5]
  
# Return ValueError
print(list1.index(10))

输出：

Traceback (most recent call last):
  File "/home/b910d8dcbc0f4f4b61499668654450d2.py", line 8, in 
    print(list1.index(10))
ValueError: 10 is not in list

示例 3：当传递 2 个参数时

当在 index函数中传递两个参数时，第一个参数被视为要搜索的元素，第二个参数是搜索开始的索引。

list_name.index(element, start)

Python3

# Python3 program for demonstration 
# of index() method
  
list1 = [6 , 8 , 5 , 6 , 1 , 2]
  
# Will print index of '3' in sublist
# having index from 1 to end of the list.
print(list1.index(6 , 1))

输出：

3

示例 4：不包括作为参数传递的结束索引

第三个参数是end，它本身不包含在start 到end 的范围内，即从start 到end-1 索引进行搜索。

Python3

# Python3 program for demonstration 
# of index() method
  
list1 = [6 , 2 , 14 , 8 , 9 , 10]
  
# return error as index '4' is not included in the range 
# having index from 1 to 4.
print(list1.index(9, 1, 4))

输出：

Traceback (most recent call last):
  File "/home/3cbe5b7d0595ab3f8564f16af7a15172.py", line 9, in 
    print(list1.index(9 , 1 , 4))
ValueError: 9 is not in list

示例 4.1

Python3

# Python3 program for demonstration 
# of index() method
  
list1 = [6 , 2 , 14 , 8 , 9 , 10]
  
  
# Will print index of '4' in sublist as now index '4' is included 
# having index from 1 to 5.
print(list1.index(9, 1, 5))

输出：

4