Python - Read from text file and put into a list

Question

As title, I have been trying to read text from file, convert to int and store it in a list. My text file look like this:

1,2
3,4
5,6

I want to read this file, put each pair of numbers into a list m , and store those lists in a bigger list lis . Here is my attempt:

def read_file():        
    lis = []
    m = [0,0]

    with open("data.txt") as f:
        for line in f:
            m[0], m[1] = line.split(",")    # assign to list m
            m[0] = int(m[0])   # cut off '\n' and for later use
            m[1] = int(m[1])   
            lis.append(m)      # store in lis

    print lis

I expect the lis to be like this:

[[1, 2], [3, 4], [5, 6]]

But instead, it is:

[[5, 6], [5, 6], [5, 6]]

I have tried insert instead of append but it seems like that's not where it has problems. I need some help - thank you in advance!

Answer 1

You are reusing the same list m in each iteration of the loop, each time overwriting the values set in the previous iteration. In the end, lis holds many references to the same list.

Instead, assign a new value to m as a whole in the loop:

for line in f:
    m = [0,0]
    m[0], m[1] = line.split(",")
    m[0] = int(m[0])
    m[1] = int(m[1])   
    lis.append(m)

Or shorter:

for line in f:
    m = line.split(",")
    m[0] = int(m[0])
    m[1] = int(m[1])   
    lis.append(m)

Or even shorter, using map:

for line in f:
    m = list(map(int, line.split(",")))
    lis.append(m)

Or even more shorter, using a list comprehension:

lis = [list(map(int, line.split(","))) for line in f]

Answer 2

def read_file():        
    lis = []
    with open("data.txt") as f:
        for line in f:
            m, n = line.split(",")    
            lis.append([int(m), int(n)])      

    print lis

Answer 3

In lis all index reference to one list m . If any update in m is takes place, m is update every where

Try this

lis.append( list(map(int,line.split(","))))    

Output

[['1', '2'], ['3', '4'], ['5', '6']]

Answer 4

Use csv.reader object instead which uses , (comma) as default field separator:

import csv

with open('data.txt') as f:
    reader = csv.reader(f)
    result = [list(map(int, lst)) for lst in reader]
    print(result)

The output:

[[1, 2], [3, 4], [5, 6]]

https://docs.python.org/3/library/csv.html?highlight=csv#csv.reader