Python，在匹配前后提取3行

Question

I am trying to figure out how to extract 3 lines before and after a matched word.

At the moment, my word is found. I wrote up some text to test my code. And, I figured out how to print three lines after my match.

But, I am having difficulty trying to figure out how to print three lines before the word, " secure ".

Here is what I have so far:

from itertools import islice
with open("testdoc.txt", "r") as f:
for line in f:
    if "secure" in line:
        print("".join(line))
        print ("".join(islice(f,3)))

Here is the text I created for testing:

----------------------------
 This is a test to see
if i can extract information
using this code
I hope, I try, 
maybe secure shell will save thee
Im adding extra lines to see my output
hoping that it comes out correctly
boy im tired, sleep is nice
until then, time will suffice

Answer 1

i came up with this solution, just adding the previous lines in a list, and deleting the first one after 4 elements

from itertools import islice

with open("testdoc.txt", "r") as f:
    linesBefore = list()
    for line in f:
        linesBefore.append(line.rstrip())
        if len(linesBefore) > 4: #Adding up to 4 lines
            linesBefore.pop(0)
        if "secure" in line:
            if len(linesBefore) == 4: # if there are at least 3 lines before the match
                for i in range(3):
                    print(linesBefore[i])
            else: #if there are less than 3 lines before the match
                print(''.join(linesBefore))
            print("".join(line.rstrip()))
            print ("".join(islice(f,3)))

Answer 2

You need to buffer your lines so you can recall them. The simplest way is to just load all the lines into a list:

with open("testdoc.txt", "r") as f:
    lines = f.readlines()  # read all lines into a list
    for index, line in enumerate(lines):  # enumerate the list and loop through it
        if "secure" in line:  # check if the current line has your substring
            print(line.rstrip())  # print the current line (stripped off whitespace)
            print("".join(lines[max(0,index-3):index]))  # print three lines preceeding it

But if you need maximum storage efficiency you can use a buffer to store the last 3 lines as you loop over the file line by line. A collections.deque is ideal for that.

Answer 3

如果我需要对多次出现的“安全”一词执行相同的操作，并且只取最后一次出现并提取上下两行