查找列表中两个标签之间的元素

Question

Language: Python 3.4 OS: Windows 8.1

I have some lists like the following:

a = ['text1', 'text2', 'text3','text4','text5']
b = ['text1', 'text2', 'text3','text4','New_element', 'text5']

What is the simplest way to find the elements between two tags in a list?

I want to be able to get it even if the lists and tags have variable number of elements or variable length.

Ex: get elements between text1 and text4 or text1 or text5, etc. Or get the elements between text1 and text5 that has longer length.

I tried using regular expressions like:

re.findall(r'text1(.*?)text5', a)

This will give me an error I guess because you can only use this in a string but not lists.

Answer 1

To get the location of an element in a list use index() . Then use the discovered index to create a slice of the list like:

Code:

print(b[b.index('text3')+1:b.index('text5')])

Results:

['text4', 'New_element']

Answer 2

You can use the list.index method to find the first occurrence of each of your tags, then slice the list to get the value between the indexes.

def find_between_tags(lst, start_tag, end_tag):
    start_index = lst.index(start_tag)
    end_index = lst.index(end_tag, start_index)
    return lst[start_index + 1: end_index]

If either of the tags is not in the list (or if the end tag only occurs before the start tag), one of the index calls will raise a ValueError . You could suppress the exception if you want to do something else, but just letting the caller deal with it seems like a reasonable option to me, so I've left the exception uncaught.

If the tags might occur in this list multiple times, you could extend the logic of the function above to find all of them. For this you'll want to use the start argument to list.index , which will tell it not to look at values before the previous end tag.

def find_all_between_tags(lst, start_tag, end_tag):
    search_from = 0
    try:
        while True:
            start_index = lst.index(start_tag, search_from)
            end_index = lst.index(end_tag, start_index + 1)
            yield lst[start_index + 1:end_index]
            search_from = end_index + 1
    except ValueError:
        pass

This generator does suppress the ValueError , since it keeps on searching until it can't find another pair of tags. If the tags don't exist anywhere in the list, the generator will be empty, but it won't raise any exception (other than StopIteration ).

Answer 3

You can get the items between the values by utilizing the index function to search for the index of both objects in the list. Be sure to add one to the index of the first object so it won't be included in the result. See my code below:

def get_sublist_between(e1, e2, li): 
    return li[li.index(e1) + 1:li.index(e2)]