How to write nested list back to text file so that there is one list per line?

Question

Question:

How to write a nested list from a variable into a text file so that each list inside the nested list is written into it's own line?

The list is needed to be compiled back into the text file in exactly the same format as it was before pulling it out with the f.readfiles() function.

Other than variable based solutions and multi-variable solutions are also accepted. csv module is also an option but I'd rather learn the logic of this first without it.

Introduction to the problem

Assume there is a text file that contains list in each and every line like this:

['some_text','foo', some_int]
['some_text','foo', some_int]
['some_text','foo', some_int]
...

Each list contains several items. All the lines are wanted to be read from the text file into a one big nested list and stored into a variable like this:

variable = [['some_text','foo', some_int],
            ['some_text','foo', some_int],
            ['some_text','foo', some_int]]

The format of the information stored into the variable above is irrelevant. The list stored into the variable is now edited with some list tools. When editing is complete, the variable containing the edited list is written back to the same text file again, overwriting all the data with new changed data like this:

['some_text','bar', some_int]
['edited_tx','foo', some_int]
['some_text','foo', edtd_int]
...

Reading the lines

I managed to read the lines with this code:

with open("my_lists.txt") as f:
    listed_text = f.readlines()

All the text inside the text file is now stored into a variable named 'listed_text' which by then is being edited with some list tools prefered.

More about the code above is here:

How do I read a file line-by-line into a list?

Incomplete code example for reading and writing the file

This is incomplete and inoperative coding example of what should have been done with the lists.

# Let us open 'my_lists.txt' and read lines
# into a variable as a list.


def open_file():
    with open("my_lists.txt") as f:
        listed_text = f.readlines()
    list_item_editor(listed_text)

# Now let us replace 'foo' every index(1) in
# nested lists with word 'bar' with
# this imaginary editor 'list_item_editor()'.


def list_item_editor(argument):
    edited_list = []
    for item in nested_list:
        nested_list[item][i] = "bar"
    ...
    ...
    write_file(edited_list)

# Now we write the edited list back to 'my_lists.txt'
# by replacing the whole contents with new edited list.


def write_file(arg):
    with open("my_lists.txt", "w") as write_file:
        write_file.write(arg)

Preferred output

This is example of the preferred output. Each item (list) inside the nested list is written into it's own line. The 'new line mark' \n is not visible inside text file.

# This is text_file.txt containing edited lists
# New line inserted after every list

['some_text','bar', some_int]
['edited_tx','foo', some_int]
['some_text','foo', edtd_int]
['some_text','bar', some_int]
['edited_tx','foo', some_int]
['some_text','foo', edtd_int]
['some_text','bar', some_int]
['edited_tx','foo', some_int]
['some_text','foo', edtd_int]

I'm not really sure if you want the file to contain literally: `['some_text','bar',some_int]`, or is it just an example but really you want `['some_text','bar',1]`. Or specifically if you run `x=1 ; variable=[['foo','bar',x]]`, what exactly do you want your file to contain? — viraptor, Sep 23 '17 at 01:27
Do you really want the file to look like Python list format? It's easier to just use the `csv` module and have file lines like `some_text,bar,5` in the file. — Mark Tolonen, Sep 23 '17 at 01:29
@MarkTolonen I look into that `csv` module, but I would really like to learn the logig on this one also. — Kasperi Koski, Sep 23 '17 at 01:41
@viraptor Yes, sorry, the arguments `'some_text'`, `'bar'` and `some_int` were just examples. The value of each index can be what ever one wants it to be. — Kasperi Koski, Sep 23 '17 at 01:43

score 1 · Accepted Answer · answered Sep 23 '17 at 01:42

With an input file of:

['some_text','foo', 1]
['some_text','foo', 2]
['some_text','foo', 3]

This will read/edit/write the file:

import ast

with open('input.txt') as f:
    data = [ast.literal_eval(line) for line in f]

print(data)

data[0][1] = 'bar'
data[-1][-1] = 5

print(data)

with open('output.txt','w') as f:
    for line in data:
        f.write(str(line) + '\n')

But writing Python lists as text to a file isn't really the best way to go about it. A better way is to use the csv module of you are willing to change the input file to this:

some_text,foo,1
some_text,foo,2
some_text,foo,3

And use this code:

import csv

with open('input.txt',newline='') as f:
    r = csv.reader(f)
    data = list(r)

print(data)

# Note, with csv, all columns are read as strings,
# so to do calculations convert:
data = [[a,b,int(c)] for a,b,c in data]

print(data)

data[0][1] = 'bar'
data[-1][-1] += 2

print(data)

with open('output.txt','w',newline='') as f:
    w = csv.writer(f)
    w.writerows(data)

Also, look into the pandas third-party module.

You're welcome! Feel free to accept the answer if it is useful and/or upvote. That's the way to thank on SO :^) — Mark Tolonen, Sep 23 '17 at 01:53