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 changed list as a list type    I have tried   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      score    line rstrip   n   for line in f    print score    But this results in the elements in the list being strings not integers

User · Answer

pickle and other serialization packages work   So does writing it to a  py file that you can then import    gt  gt  gt  score    1 2 3 4 5   gt  gt  gt    gt  gt  gt  with open  file py    w   as f        f write  score    s    score        gt  gt  gt  from file import score as my list  gt  gt  gt  print my list   1  2  3  4  5

User · Answer

You can use pickle  module for that  This module have two methods   Pickling dump   Convert Python objects into string representation  Unpickling load   Retrieving original objects from stored string representstion   https   docs python org 3 3 library pickle html Code   gt  gt  gt  import pickle  gt  gt  gt  l    1 2 3 4   gt  gt  gt  with open  quot test txt quot    quot wb quot   as fp     Pickling       pickle dump l  fp        gt  gt  gt  with open  quot test txt quot    quot rb quot   as fp      Unpickling       b   pickle load fp        gt  gt  gt  b  1  2  3  4    Also Json  dump dumps  Serialize load loads  Deserialize  https   docs python org 3 library json html Code   gt  gt  gt  import json  gt  gt  gt  with open  quot test txt quot    quot w quot   as fp          json dump l  fp       gt  gt  gt  with open  quot test txt quot    quot r quot   as fp          b   json load fp       gt  gt  gt  b  1  2  3  4

User · Answer

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   gt  gt  gt  saveList sampleList1  sampleList1 npy    gt  gt  gt  Saved successfully   gt  gt  gt  saveList sampleList2  sampleList2 npy    gt  gt  gt  Saved successfully     loading the list now   gt  gt  gt  loadedList1 loadList  sampleList1 npy    gt  gt  gt  loadedList2 loadList  sampleList2 npy     gt  gt  gt  loadedList1  sampleList1  gt  gt  gt  True   gt  gt  gt  print loadedList1 sampleList1    gt  gt  gt    z    x    a    b     z    x    a    b

User · Answer

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  quot file txt quot    quot w quot   as f      for s in score          f write str s    quot  n quot     score      with open  quot file txt quot    quot r quot   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

User · Answer

Although the accepted answer works  you should really be using python s json module  import json  score  1 2 3 4 5   with open  quot file json quot    w   as f        indent 2 is not needed but makes the file human-readable     json dump score  f  indent 2    with open  quot file json quot    r   as f      score   json load f   print score   Advantages   json is a widely adopted and standardized data format  so non-python programs can easily read and understand the json files json files are human-readable Any nested or non-nested list dictionary structure can be saved to a json file  as long as all the contents are serializable    Disadvantages   The data is stored in plain-text  ie it s uncompressed   which makes it a slow and bloated option for large amounts of data  ie probably a bad option for storing large numpy arrays  that s what hdf5 is for   The contents of a list dictionary need to be serializable before you can save it as a json  so while you can save things like strings  ints  and floats  you ll need to write custom serialization and deserialization code to save objects  classes  and functions  When to use json vs pickle   If you want to store something you know you re only ever going to use in the context of a python program  use pickle If you need to save data that isn t serializable by default  ie objects   save yourself the trouble and use pickle  If you need a platform agnostic solution  use json If you need to be able to inspect and edit the data directly  use json  Common use cases   Configuration files  for example  node js uses a package json file to track project details  dependencies  scripts  etc      Most REST APIs use json to transmit and receive data Data that requires a nested list dictionary structure  or requires variable length lists dicts Can be an alternative to csv  xml or yaml files

User · Answer

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

User · Answer

errorlist     aaaa    bbbb    cccc    dddd    f   open  quot filee txt quot    quot w quot   f writelines nthstring     n  for nthstring in errorlist   f   open  quot filee txt quot    quot r quot   cont   f read   contentlist   cont split   print contentlist

User · Answer

What I did not like with many answers is that it makes way too many system calls by writing to the file line per line  Imho it is best to join list with   n   line return  and then write it only once to the file   mylist     abc    def    ghi   myfile    file txt  with open myfile   w   as f      f write   n  join mylist     and then to open it and get your list again   with open myfile   r   as f      mystring   f read   my list   mystring split   n

User · Answer

I am using pandas    import pandas as pd x   pd Series  1 2 3 4 5   x to excel  temp xlsx   y   list pd read excel  temp xlsx   0   print y    Use this if you are anyway importing pandas for other computations

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

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