read lists from .txt file python

Question

I want to create .txt which contains several lists

a = ['210','210','210','210']
b = ['0.3','0.3','0.3','0.3']
c = ['7.85e-06','7.85e-06','7.85e-06','7.85e-06']

with open("abcd.txt", "w") as output:
    output.write(str(a))
    
with open("abcd.txt", "a") as output:
    output.write(str(b))
    
with open("abcd.txt", "a") as output:
    output.write(str(c))

So test file is generated like

['210', '210', '210', '210']['0.3', '0.3', '0.3', '0.3']['7.85e-06', '7.85e-06', '7.85e-06', '7.85e-06']

Now I want to extract the first element of all lists. How can I do that? Here each list contain 4 elements but it may be higher that also.

try this: `print(open("abcd.txt").readline().split("]")[0].replace('[', ""))` — Grijesh Chauhan, Sep 09 '20 at 05:47
Do you need to support nested lists? Like `['1', ['2a', '2b'], 3]` — Adirio, Sep 09 '20 at 05:49
@GrijeshChauhan It is giving the output like '210', '210', '210', '210' . But I want like first element of list a then first element of list b and then first element of list c — Aakash Patel, Sep 09 '20 at 05:58
why no `'\n'` between the lists? then you had 1 list per line and could concentrate on parsing a list per line? Which btw is handled here: [eval-vs-ast-literal-eval](https://stackoverflow.com/questions/15197673/using-pythons-eval-vs-ast-literal-eval) — Patrick Artner, Sep 09 '20 at 06:01

Adirio · Answer 1 · 2020-09-09T06:42:50.727

You can use .index() to find the position of ']' and .count() to find how many of them you have:

with open('abcd.txt', 'r') as input:
    content = input.read()


strings = []
start = 0
for _ in range(content.count(']')):
    end = content.index(']', start) + 1
    strings.append(content[start:end]
    start = end

for string in strings:
    print(string[1:string.index(',')])

The second argument to .index() tells it from which position to start searching from, in order to avoid returning always the position of the first ']'.

Output:

'210'
'0.3'
'7.85e-06'

However, if you are going to store data in a file, I suggest you do it in something a bit more structured like JSON which every language has a parser for.

score 1 · Answer 2 · answered Sep 09 '20 at 05:58

firstly read the file and store the data in a variable.

with open('abcd.txt','r') as myFile:
    data = myFile.read()

then remove the first and last square brackets because they are not needed.

data = data[1:-1]

then split the data from '][' because this is where two lists meet.

lists=data.split("][")

then get the first values of all elements and store it in another list like:

firstelements = []
for aList in lists:
    firstelements.append(aList.split(",")[0])

balderman · Answer 3 · 2020-09-09T06:14:08.380

1

If you can work with json (why not??) - here is a json based solution

import json

a = ['210', '210', '210', '210']
b = ['0.3', '0.3', '0.3', '0.3']
c = ['7.85e-06', '7.85e-06', '7.85e-06', '7.85e-06']

data = {'a': a, 'b': b, 'c': c}

# save to a json file
with open('data.json', 'w') as f:
    json.dump(data, f)

# read from json file and get the data you need
with open('data.json') as f:
    data = json.load(f)
first_values = [x[0] for x in data.values()]
print(first_values)

output

['210', '0.3', '7.85e-06']

edited Sep 09 '20 at 06:14

answered Sep 09 '20 at 06:08

balderman

22,927
7
34
52

1

May I suggest moving the last two statements out of the `with` block as there is no reason to keep that file opened? – Adirio Sep 09 '20 at 06:10
@Adirio makes sense. – balderman Sep 09 '20 at 06:13
@balderman this is working good in windows but in linux it is giving error like...Traceback (most recent call last): File "Untitled3 (Run from Script Editor)", line 10, in OSError: [Errno 28] No space left on device: 'data.json' – Aakash Patel Sep 13 '20 at 14:45
Disk is full. Check it. – balderman Sep 13 '20 at 14:50

score 0 · Accepted Answer · answered Sep 09 '20 at 06:07

0

Here is another solution using regex,

Regex Demo

import re
from ast import literal_eval

with open('abcd.txt', "r") as f:
    for x in re.findall(r".+?\]", f.read()):
        print(literal_eval(x)[0])

210
0.3
7.85e-06

answered Sep 09 '20 at 06:07

sushanth

8,275
3
17
28

`eval`, or in this case `ast.literal_eval` is the reason why I think storing as plain text is not a very good idea. Now anyone can edit the file, place some python code ending in `#]` to trigger your pattern and that code will be run. I know that `eval` is the easiest way to parse a list which would require a lof of string parsing but that's the main reason why formats like CSV and JSON already have their parsers in the standard library so that you can store them in a file and retrieve them later without the need to `eval` potentially malicious code. – Adirio Sep 09 '20 at 06:38

score -1 · Answer 5 · answered Sep 09 '20 at 06:07

-1

You can concatenate strings using the + operator. Try this:

a = ['210','210','210','210']
b = ['0.3','0.3','0.3','0.3']
c = ['7.85e-06','7.85e-06','7.85e-06','7.85e-06']

to_save = str(a) + str(b) + str(c)
with open("abcd.txt", "wb") as output:
    output.write(to_save)

If you want to duplicate the code it might be better to write it as a function.

answered Sep 09 '20 at 06:07

Alon

1

This is not an answer, you suggested an improvement on part of the code he provided but the question was about reading not about writing. Not trying to sound too harsh. – Adirio Sep 09 '20 at 06:11

read lists from .txt file python

5 Answers5