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How to write multiple lists into a text file where each list is written into a new line?

May 26, 2022

I have two python lists:

a_list = [1, 2, 3]
b_list = [4, 5, 6]

How can I do the following:

create new text file —> convert lists to json string —> write lists into the file (each list should have its own line in the file) —> open the file —> read each line into a new variable and convert back from json string to python list?

I am stuck here:

import json

a_list = [1, 2, 3]
b_list = [4, 5, 6]

with open('test.txt', 'w') as f:
    f.write(json.dumps(a_list))
    f.write(json.dumps(b_list))

(The json string is written on the same line.)

Thanks

>Solution :

Using JSON

first you should put those lists into a dict

d = {
    "a" : a_list,
    "b" : b_list
}

Then you can dump it into .json file

json.dump(d,open("file.json","w"))

To read/load the files, you can use

d = json.load(open("file.json","r"))

which will return the original dictionary. ie,

{'a': [1, 2, 3], 'b': [4, 5, 6]}

Using Pickle

dumping

pickle.dump(a_list,open("b.ls","wb"))
pickle.dump(b_list,open("a.ls","wb"))

pickle.load(open("a.ls","rb"))
pickle.load(open("b.ls","rb"))

fyi: I’ve never used pickle before 🙂

Comparison with json

There are fundamental differences between the pickle protocols and
JSON (JavaScript Object Notation):
JSON is a text serialization format (it outputs unicode text,
although most of the time it is then encoded to utf-8), while pickle
is a binary serialization format;
JSON is human-readable, while pickle is not;
JSON is interoperable and widely used outside of the Python
ecosystem, while pickle is Python-specific;
JSON, by default, can only represent a subset of the Python built-in
types, and no custom classes; pickle can represent an extremely large
number of Python types (many of them automatically, by clever usage
of Python’s introspection facilities; complex cases can be tackled by
implementing specific object APIs);
Unlike pickle, deserializing untrusted JSON does not in itself create
an arbitrary code execution vulnerability.