Converting Input List that is read in String to List in Python

Question

I am reading a file that has list. Below is the input file.

[1,2,3,4]
[42]
[1,1,2,3,5,8]
[]

Now as you can see there are lists which is read as string character and I am trying to convert it into an actual list. Below is the code I'm using.

_list = list(sys.stdin.readline().strip())
_final_list = []
    for n in _list:
        try:
            n = int(n)
        except ValueError:
            continue
        if isinstance(n, int):
            _final_list.append(n)

Now my code works fine until a number is more than one digit. For example 42 would become 4, 2. Which is not what I want.

My code generates below result.

[1, 2, 3, 4]
[4, 2] <--- FALSE
[1, 1, 2, 3, 5, 8]
[]

What should I do here to accomplish this task without using libraries such as ast

Why don't you want to use ast? Why are you doing this in the first place? Why not use a built in serialization method like json or pickle? — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Feb 3, 2020 at 17:32

kederrac · Accepted Answer · 2020-02-03 17:24:23Z

2

you can use ast.literal_eval:

import ast

_final_list = []
for line in _list:
    python_object = ast.literal_eval(line)        
    if isinstance(python_object, list):
        _final_list.extend(python_object)

or you can use regular expression:

import re

_final_list = []
for line in _list:
    my_list = [e.group() for e in re.finditer(r'\d+', line)]
    _final_list.extend(my_list)

edited Feb 3, 2020 at 17:24

answered Feb 3, 2020 at 17:13

kederrac

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6 Comments

Khan Over a year ago

Thanks for your suggestion! But As I mentioned before, I don't want to use ast.

kederrac Over a year ago

any reason to not use?

Khan Over a year ago

I am sure there is a way to do it without using libraries. I am preparing for a competition and I will not be able to memorize all libraries there are in the competition. I am trying to solve this problem just in python.

Khan Over a year ago

For practice wise, I do not want to use it. But to solve the problem, I would've used ast right away

kederrac Over a year ago

@Khan check now my answer

|

snnguyen · Accepted Answer · 2020-02-03 17:21:45Z

0

I would strip the endings and split by commas.

def str_to_list(s):
    return [int(i) for i in s.strip('][').split(',') if i]

answered Feb 3, 2020 at 17:21

snnguyen

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1 Comment

Khan Over a year ago

Awesome! This is perfect!

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