In Python, how can I read just the first x lines from a file using the “strip() for line in file” format?

Question

See code below where I try to use 2 different functions to read different parts from a text file (essentially a save file for a game board). The first one tries to read the first 5 lines and assign them to a matrix (list of lists). The second one tries to read the 6th line and assign it to a string. However I can't seem to get the code to work. Any ideas?

def load_board():
    with open("savefile.txt","r") as savefile:
        loadBoard = [line.strip().split(",") for line in savefile]
        return loadBoard

def load_side():
    with open("savefile.txt","r") as savefile:
        loadBoard = [line.strip() for line in savefile]
        loadSide = loadBoard.pop()
        return loadSide

savefile.txt looks like this:

R,R,R,M,R
R,R,R,R,-
R,R,M,R,R
R,R,R,R,R
M,R,R,R,R
M

Answer 1

in both functions you are reading complete file. You need read all lines and then use required lines , see below example:

def load_board():
    with open("savefile.txt","r") as savefile:
        lines = savefile.readlines()
        loadBoard = [line.strip().split(",") for line in lines[:5]]
        loadside = lines[6]
        return loadBoard, loadside

Answer 2

Instead of opening the file in each function, take an already open file as an argument:

import itertools

# Pre-condition: the input is at the beginning of the file
def load_board(savefile):
    return [line.strip().split(",") for line in itertools.islice(savefile, 5)]


# Pre-condition: the first 5 lines have already been read
def load_side(savefile):
    return next(savefile).strip()

Then open the file once before calling each function. Note that load_from_file should be used to ensure that load_board and load_side are called in the correct order and with no other reads from savefile to break the pre-conditions.

def load_from_file(fname):
    with open(fname) as savefile:
        board = load_board(savefile)
        side = load_side(savefile)
    return board, side

loadBoard, loadSide = load_from_file("savefile.txt")

Answer 3

savefile as in code, is a file instance, not a list of lines. Instead, use savefile.readlines() for all the lines. Also,

loadBoard = [line.strip().split(",") for line in savefile.readlines()]

would give you all 6 lines, not just 5. So loadBoard[-1] would be [M] .

Answer 4

Why not load the file once and then use the output to parse out what you want?

def load_save(save_filename):
    with open(save_filename, 'r') as save_file:
        return save_file.read().split('\n')  # Return a list of all lines from the save file

def load_board(save_lines):
    return [line.strip().split(',') for line in save_lines[:5]]  # Won't include last line (side line)

def load_side(save_lines):
    return [save_lines[5:].strip()] # Just the last line

This can be used like so:

save_lines = load_save('savefile.txt')
board = load_board(save_lines)
side = load_side(save_lines)

Answer 5

You can also enumerate over the file object.

def load_board():
    with open("savefile.txt","r") as savefile:
        loadBoard = [line.strip().split(",") for i, line in enumerate(savefile) if i<=4]
        return loadBoard

def load_side():
    with open("savefile.txt","r") as savefile:
        loadBoard = [line.strip() for i, line in enumerate(savefile) if i==5]  #only 6th ( index starts at 0 )
        loadSide = loadBoard.pop()
        return loadSide

print(load_board())
print(load_side())

Output

[['R', 'R', 'R', 'M', 'R'], ['R', 'R', 'R', 'R', '-'], ['R', 'R', 'M', 'R', 'R'], ['R', 'R', 'R', 'R', 'R'], ['M', 'R', 'R', 'R', 'R']]
M