Python - How to read lines of lists in a text file directly into lists in python

Question

I have received a text file generated by a json dump in python that looks like this:

[0.1,0.1,0.2,0.3]
[0.1,0.3,0.4,0.3]
[0.1,0.1,0.3,0.3]
[0.3,0.1,0.5,0.3]
.
.
.
[0.1,0.1,0.3,0.3]
[0.3,0.4,0.6,0.3]

and so on for a considerable amount of lines ~>10,000,000

I would like to figure out the quickest/most efficient way to read from the file and actually convert them into lists.

I have a program that has a for loop that runs a particular operation with lists:

for x in range(filelength):
    for y in list(each line from the file):
        use the numbers from each list to perform certain operations

I was thinking of parsing out all the brackets from the text file and feeding each value comma separated into a blank list for each line (which would probably be slow and time consuming), but I thought there might be a feature of python to convert a list represented as a string easily into an actual list in python quickly.

Any thoughts or suggestions would be appreciated.

Answer 1

Use ast.literal_eval() to parse each line back into a Python list:

import ast

with open(filename, 'r') as fh:
    for line in fh:
        listobj = ast.literal_eval(line)

ast.literal_eval() takes a string and interprets it as Python literal values; lists and floating point values are directly supported:

>>> ast.literal_eval('[0.1,0.1,0.2,0.3]\n')
[0.1, 0.1, 0.2, 0.3]

Answer 2

You say this was "generated by a json dump", and each line looks like valid JSON, so the right thing to do is to parse each line as JSON:

import json
with open(filename) as f:
    the_lists = map(json.loads, f)

Since you just want to iterate directly over the lists, it might be simpler to do the loads right in your loop:

import json
with open(filename) as f:
    for line in f:
        for column in json.loads(line):
            # your code here