Python如何读取.txt文件的最后三行，并将这些项目放入列表中？

Question

I am trying to have python read the last three lines of a .txt file. I am also trying to add each line as an element in a list.

So for instance:

**list.txt**
line1
line2
line3

**python_program.py**
(read list.txt, insert items into line_list)
line_list[line1,line2,line3]

However I am a bit confused on this process.

Any help would be greatly appreciated!

Answer 1

What if you are dealing with a very big file? Reading all the lines in memory is going to be quite wasteful. An alternative approach may be:

from collections import deque 
d=deque([], maxlen=3)
with open("file.txt") as f:
    for l in f:
       d.append(l) 

This keeps in memory at a given time only the last three rows read (the deque discards the oldest elements in excess at each append).

---

As @user2357112 points out, this will work as well, and is more synthetic:

from collections import deque 
d=None
with open("file.txt") as f:
    d=deque(f, maxlen=3)

Answer 2

with open('list.txt') as f:
    lines = f.readlines()
line_list = lines[-3:]

Answer 3

Try these:

#!/usr/local/cpython-3.3/bin/python

import pprint

def get_last_3_variant_1(file_):
    # This is simple, but it also reads the entire file into memory
    lines = file_.readlines()
    return lines[-3:]

def get_last_3_variant_2(file_):
    # This is more complex, but it only keeps three lines in memory at any given time
    three_lines = []
    for index, line in zip(range(3), file_):
        three_lines.append(line)

    for line in file_:
        three_lines.append(line)
        del three_lines[0]

    return three_lines

get_last_3 = get_last_3_variant_2

def main():
    # /etc/services is a long file
    # /etc/adjtime is exactly 3 lines long on my system
    # /etc/libao.conf is exactly 2 lines long on my system
    for filename in ['/etc/services', '/etc/adjtime', '/etc/libao.conf']:
        with open (filename, 'r') as file_:
            result = get_last_3(file_)
        pprint.pprint(result)

main()