How to save a list to a file and read it as a list type?

Question

Say I have the list score=[1,2,3,4,5] and it gets changed whilst my program is running. How could I save it to a file so that next time the program is run I can access the c

Answer 1

If you want you can use numpy's save function to save the list as file. Say you have two lists

sampleList1=['z','x','a','b']
sampleList2=[[1,2],[4,5]]

here's the function to save the list as file, remember you need to keep the extension .npy

def saveList(myList,filename):
    # the filename should mention the extension 'npy'
    np.save(filename,myList)
    print("Saved successfully!")

and here's the function to load the file into a list

def loadList(filename):
    # the filename should mention the extension 'npy'
    tempNumpyArray=np.load(filename)
    return tempNumpyArray.tolist()

a working example

>>> saveList(sampleList1,'sampleList1.npy')
>>> Saved successfully!
>>> saveList(sampleList2,'sampleList2.npy')
>>> Saved successfully!

# loading the list now 
>>> loadedList1=loadList('sampleList1.npy')
>>> loadedList2=loadList('sampleList2.npy')

>>> loadedList1==sampleList1
>>> True

>>> print(loadedList1,sampleList1)

>>> ['z', 'x', 'a', 'b'] ['z', 'x', 'a', 'b']

Answer 2

I decided I didn't want to use a pickle because I wanted to be able to open the text file and change its contents easily during testing. Therefore, I did this:

score = [1,2,3,4,5]

with open("file.txt", "w") as f:
    for s in score:
        f.write(str(s) +"\n")

with open("file.txt", "r") as f:
  for line in f:
    score.append(int(line.strip()))

So the items in the file are read as integers, despite being stored to the file as strings.

Answer 3

If you don't want to use pickle, you can store the list as text and then evaluate it:

data = [0,1,2,3,4,5]
with open("test.txt", "w") as file:
    file.write(str(data))

with open("test.txt", "r") as file:
    data2 = eval(file.readline())

# Let's see if data and types are same.
print(data, type(data), type(data[0]))
print(data2, type(data2), type(data2[0]))

[0, 1, 2, 3, 4, 5] class 'list' class 'int'

[0, 1, 2, 3, 4, 5] class 'list' class 'int'

Answer 4

You can use pickle module for that. This module have two methods,

1. Pickling(dump): Convert Python objects into string representation.
2. Unpickling(load): Retrieving original objects from stored string representstion.

https://docs.python.org/3.3/library/pickle.html

Code:

>>> import pickle
>>> l = [1,2,3,4]
>>> with open("test.txt", "wb") as fp:   #Pickling
...   pickle.dump(l, fp)
... 
>>> with open("test.txt", "rb") as fp:   # Unpickling
...   b = pickle.load(fp)
... 
>>> b
[1, 2, 3, 4]

---

Also Json

1. dump/dumps: Serialize
2. load/loads: Deserialize

https://docs.python.org/3/library/json.html

Code:

>>> import json
>>> with open("test.txt", "w") as fp:
...     json.dump(l, fp)
...
>>> with open("test.txt", "r") as fp:
...     b = json.load(fp)
...
>>> b
[1, 2, 3, 4]

Answer 5

Although the accepted answer works, you should really be using python's json module:

import json

score=[1,2,3,4,5]

with open("file.json", 'w') as f:
    # indent=2 is not needed but makes the file 
    # human-readable for more complicated data
    json.dump(score, f, indent=2) 

with open("file.json", 'r') as f:
    score = json.load(f)

print(score)

The main advantages of using a json are that:

1. json is a widely adopted and standardized data format, so non-python programs can easily read and understand the data inside the json file.
2. json files are human-readable.
3. You can literally save any list/dictionary in python to a json (as long as all the contents are serializable, so most objects/functions can't be saved into a json without explicit conversion to a serializable object)

The main disadvantages of using a json are that:

1. The data is stored in plain-text, meaning the data is completely uncompressed, making it a slow and bloated option for large amounts of data. I wouldn't recommend storing gigabytes of vectors in a json format (hdf5 is a much better candidate for that).
2. The contents of a list/dictionary need to be serializable (python objects need to be explicitly serialized when dumping, and then deserialized when loading).

The json format is used for all kinds of purposes:

• node.js uses it to track dependencies with package.json
• many REST APIs use it to transmit and receive data
• jsons are also a great way to store complicated nested lists/dictionaries (like an array of records).

In general, if I want to store something I know I'm only ever going to use in the context of a python program, I typically go to pickle. If I need a platform agnostic solution, or I need to be able to inspect my data once it's stored, I go with json.

Answer 6

errorlist = ['aaaa', 'bbbb', 'cccc', 'ffffdd']

f = open("filee.txt", "w")
f.writelines(nthstring + '\n' for nthstring in errorlist)

f = open("filee.txt", "r")
cont = f.read()
contentlist = cont.split()
print(contentlist)