list - Copy the last three lines of a text file in python?

Question

I'm new to python and the way it handles variables and arrays of variables in lists is quite alien to me. I would normally read a text file into a vector and then copy the last three into a new array/vector by determining the size of the vector and then looping with a for loop a copy function for the last size-three into a new array.

I don't understand how for loops work in python so I can't do that.

so far I have:

    #read text file into line list
            numberOfLinesInChat = 3
    text_file = open("Output.txt", "r")
    lines = text_file.readlines()
    text_file.close()
    writeLines = []
    if len(lines) > numberOfLinesInChat:
                    i = 0
        while ((numberOfLinesInChat-i) >= 0):
            writeLine[i] = lines[(len(lines)-(numberOfLinesInChat-i))]
                            i+= 1

    #write what people say to text file
    text_file = open("Output.txt", "w")
    text_file.write(writeLines)
    text_file.close()

Answer 1

To get the last three lines of a file efficiently, use deque:

from collections import deque

with open('somefile') as fin:
    last3 = deque(fin, 3)

This saves reading the whole file into memory to slice off what you didn't actually want.

To reflect your comment - your complete code would be:

from collections import deque

with open('somefile') as fin, open('outputfile', 'w') as fout:
    fout.writelines(deque(fin, 3))