How to read a file line-by-line into a list

Question

How do I read every line of a file in Python and store each line as an element in a list    I want to read the file line by line and append each line to the end of the list

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You could simply do the following  as has been suggested   with open   your path file   as f      my lines   f readlines     Note that this approach has 2 downsides   1  You store all the lines in memory  In the general case  this is a very bad idea  The file could be very large  and you could run out of memory  Even if it s not large  it is simply a waste of memory   2  This does not allow processing of each line as you read them  So if you process your lines after this  it is not efficient  requires two passes rather than one    A better approach for the general case would be the following   with open   your path file   as f      for line in f          process line    Where you define your process function any way you want  For example   def process line       if  save the world  in line lower             superman save the world      The implementation of the Superman class is left as an exercise for you    This will work nicely for any file size and you go through your file in just 1 pass  This is typically how generic parsers will work

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Another option is numpy genfromtxt  for example   import numpy as np data   np genfromtxt  yourfile dat  delimiter   n     This will make data a NumPy array with as many rows as are in your file

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Introduced in Python 3 4  pathlib has a really convenient method for reading in text from files  as follows   from pathlib import Path p   Path  my text file   lines   p read text   splitlines      The splitlines call is what turns it from a string containing the whole contents of the file to a list of lines in the file    pathlib has a lot of handy conveniences in it  read text is nice and concise  and you don t have to worry about opening and closing the file  If all you need to do with the file is read it all in in one go  it s a good choice

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See Input and Ouput   with open  filename   as f      lines   f readlines     or with stripping the newline character   with open  filename   as f      lines    line rstrip   for line in f

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In case that there are also empty lines in the document I like to read in the content and pass it through filter to prevent empty string elements  with open myFile   r   as f      excludeFileContent   list filter None  f read   splitlines

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You could also use the loadtxt command in NumPy  This checks for fewer conditions than genfromtxt  so it may be faster   import numpy data   numpy loadtxt filename  delimiter   n

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According to Python s Methods of File Objects  the simplest way to convert a text file into a list is  with open  file txt   as f      my list   list f        my list    x rstrip   for x in f    remove line breaks   Demo   If you just need to iterate over the text file lines  you can use  with open  file txt   as f      for line in f               Old answer  Using with and readlines     with open  file txt   as f      lines   f readlines    If you don t care about closing the file  this one-liner works  lines   open  file txt   readlines    The traditional way  f   open  file txt     Open file on read mode lines   f read   splitlines     List with stripped line-breaks f close     Close file

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I would try one of the below mentioned methods  The example file that I use has the name dummy txt  You can find the file here  I presume  that the file is in the same directory as the code  you can change fpath to include the proper file name and folder path     In both the below mentioned examples  the list that you want is given by lst   1   First method   fpath    dummy txt  with open fpath   r   as f  lst    line rstrip   n  t   for line in f   print lst  gt  gt  gt   THIS IS LINE1     THIS IS LINE2     THIS IS LINE3     THIS IS LINE4      2   In the second method  one can use csv reader module from Python Standard Library   import csv fpath    dummy txt  with open fpath  as csv file      csv reader   csv reader csv file  delimiter            lst    row 0  for row in csv reader    print lst  gt  gt  gt   THIS IS LINE1     THIS IS LINE2     THIS IS LINE3     THIS IS LINE4      You can use either of the two methods  Time taken for the creation of lst is almost equal in the two methods

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Here is a Python 3  helper library class that I use to simplify file I O   import os    handle files using a callback method  prevents repetition def  FileIO  file handler file path  mode  callback   lambda f  None     f   open file path  mode    try      return callback f    except Exception as e      raise IOError  Failed to  s file      write to    read from   mode lower   in  r rb r   split          finally      f close     class FileIO      return the contents of a file   def read file path  mode    r        return   file handler file path  mode  lambda rf  rf read         get the lines of a file   def lines file path  mode    r   filter fn   lambda line  len line   gt  0       return  line for line in FileIO read file path  mode  strip   split   n   if filter fn line        create or update a file  NOTE  can also be used to replace a file s original content    def write file path  new content  mode    w        return   file handler file path  mode  lambda wf  wf write new content        delete a file  if it exists    def delete file path       return os remove   if os path isfile file path  else None   You would then use the FileIO lines function  like this   file ext lines   FileIO lines    path to file ext    for i  line in enumerate file ext lines     print  Line         format i   1  line     Remember that the mode   r  by default  and filter fn  checks for empty lines by default  parameters are optional   You could even remove the read  write and delete methods and just leave the FileIO lines  or even turn it into a separate method called read lines

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Having a Text file content  line 1 line 2 line 3  We can use this Python script in the same directory of the txt above  gt  gt  gt  with open  quot myfile txt quot   encoding  quot utf-8 quot   as file          x    l rstrip  quot  n quot   for l in file   gt  gt  gt  x   line 1   line 2   line 3    Using append  x      with open  quot myfile txt quot   as file      for l in file          x append l strip     Or   gt  gt  gt  x   open  quot myfile txt quot   read   splitlines    gt  gt  gt  x   line 1    line 2    line 3    Or   gt  gt  gt  x   open  quot myfile txt quot   readlines    gt  gt  gt  x   linea 1 n    line 2 n    line 3 n    Or  def print output lines in textfile       print  quot lines in textfile   quot   lines in textfile   y    x rstrip   for x in open  quot 001 txt quot    print output y   with open  001 txt    r   encoding  utf-8   as file      file   file read   splitlines       print output file   with open  001 txt    r   encoding  utf-8   as file      file    x rstrip  quot  n quot   for x in file      print output file   output  lines in textfile     line 1    line 2    line 3   lines in textfile     line 1    line 2    line 3   lines in textfile     line 1    line 2    line 3

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Clean and Pythonic Way of Reading the Lines of a File Into a List    First and foremost  you should focus on opening your file and reading its contents in an efficient and pythonic way  Here is an example of the way I personally DO NOT prefer   infile   open  my file txt    r      Open the file for reading   data   infile read      Read the contents of the file   infile close      Close the file since we re done using it    Instead  I prefer the below method of opening files for both reading and writing as it is very clean  and does not require an extra step of closing the file once you are done using it  In the statement below  we re opening the file for reading  and assigning it to the variable  infile    Once the code within this statement has finished running  the file will be automatically closed     Open the file for reading  with open  my file txt    r   as infile       data   infile read      Read the contents of the file into memory    Now we need to focus on bringing this data into a Python List because they are iterable  efficient  and flexible   In your case  the desired goal is to bring each line of the text file into a separate element  To accomplish this  we will use the splitlines   method as follows     Return a list of the lines  breaking at line boundaries  my list   data splitlines       The Final Product     Open the file for reading  with open  my file txt    r   as infile       data   infile read      Read the contents of the file into memory     Return a list of the lines  breaking at line boundaries  my list   data splitlines     Testing Our Code    Contents of the text file         A fost odat   ca-n povesti       A fost ca niciodat         Din rude m  ri   mp  r  testi       O prea frumoas   fat       Print statements for testing purposes        print my list    Print the list         Print each line in the list      for line in my list          print line        Print the fourth element in this list      print my list 3     Output  different-looking because of unicode characters            A fost odat xc3 xa3 ca-n povesti     A fost ca niciodat xc3 xa3          Din rude m xc3 xa3ri  xc3 xaemp xc3 xa3r xc3 xa3testi     O prea      frumoas xc3 xa3 fat xc3 xa3          A fost odat   ca-n povesti  A fost ca niciodat    Din rude m  ri        mp  r  testi  O prea frumoas   fat          O prea frumoas   fat

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If you want the  n included   with open fname  as f      content   f readlines     If you do not want  n included   with open fname  as f      content   f read   splitlines

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This is more explicit than necessary  but does what you want   with open  file txt   as file in      lines          for line in file in          lines append line

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Use this   import pandas as pd data   pd read csv filename    You can also add parameters such as header  sep  etc  array   data values   data is a dataframe type  and uses values to get ndarray  You can also get a list by using array tolist

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The easiest ways to do that with some additional benefits are  lines   list open  filename     or lines   tuple open  filename     or lines   set open  filename     In the case with set  we must be remembered that we don t have the line order preserved and get rid of the duplicated lines  Below I added an  important supplement from  MarkAmery   Since you re not calling  close on the file object nor using a with statement  in some Python implementations the file may not get closed after reading and your process will leak an open file handle  In CPython  the normal Python implementation that most people use   this isn t a problem since the file object will get immediately garbage-collected and this will close the file  but it s nonetheless generally considered best practice to do something like   with open  filename   as f  lines   list f     to ensure that the file gets closed regardless of what Python implementation you re using

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Just use the splitlines   functions  Here is an example   inp    file txt  data   open inp  dat   data read   lst   dat splitlines   print lst   print lst    for python 3   In the output you will have the list of lines

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The simplest way to do it  A simple way is to    Read the whole file as a string Split the string line by line   In one line  that would give   lines   open  C  path file txt   read   splitlines     However  this is quite inefficient way as this will store 2 versions of the content in memory  probably not a big issue for small files  but still    Thanks Mark Amery    There are 2 easier ways    Using the file as an iterator   lines   list open  C  path file txt          or if you want to have a list without EOL characters lines    l rstrip   for l in open  C  path file txt       If you are using Python 3 4 or above  better use pathlib to create a path for your file that you could use for other operations in your program    from pathlib import Path file path   Path  C  path file txt    lines   file path read text   split lines         or      lines    l rstrip   for l in file path open

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This will yield an  array  of lines from the file   lines   tuple open filename   r      open returns a file which can be iterated over  When you iterate over a file  you get the lines from that file  tuple can take an iterator and instantiate a tuple instance for you from the iterator that you give it  lines is a tuple created from the lines of the file

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If you d like to read a file from the command line or from stdin  you can also use the fileinput module     reader py import fileinput  content      for line in fileinput input        content append line strip     fileinput close     Pass files to it like so     python reader py textfile txt    Read more here  http   docs python org 2 library fileinput html

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Command line version     bin python3 import os import sys abspath   os path abspath   file    dname   os path dirname abspath  filename   dname   sys argv 1  arr   open filename  read   split   n    print arr    Run with   python3 somefile py input file name txt

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If you want to are faced with a very large   huge file and want to read faster  imagine you are in a Topcoder Hackerrank coding competition   you might read a considerably bigger chunk of lines into a memory buffer at one time  rather than just iterate line by line at file level   buffersize   2  16 with open path  as f       while True          lines buffer   f readlines buffersize          if not lines buffer              break         for line in lines buffer              process line

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with open filename  as f      content   f readlines     you may also want to remove whitespace characters like   n  at the end of each line content    x strip   for x in content

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Outline and Summary  With a filename  handling the file from a Path filename  object  or directly with open filename  as f  do one of the following    list fileinput input filename   using with path open   as f  call f readlines    list f  path read text   splitlines   path read text   splitlines keepends True  iterate over fileinput input or f and list append each line one at a time pass f to a bound list extend method use f in a list comprehension   I explain the use-case for each below      In Python  how do I read a file line-by-line    This is an excellent question  First  let s create some sample data   from pathlib import Path Path  filename   write text  foo nbar nbaz     File objects are lazy iterators  so just iterate over it   filename    filename  with open filename  as f      for line in f          line   do something with the line   Alternatively  if you have multiple files  use fileinput input  another lazy iterator  With just one file   import fileinput  for line in fileinput input filename        line   process the line   or for multiple files  pass it a list of filenames   for line in fileinput input  filename  2        line   process the line   Again  f and fileinput input above both are return lazy iterators  You can only use an iterator one time  so to provide functional code while avoiding verbosity I ll use the slightly more terse fileinput input filename  where apropos from here      In Python  how do I read a file line-by-line into a list    Ah but you want it in a list for some reason  I d avoid that if possible  But if you insist    just pass the result of fileinput input filename  to list   list fileinput input filename     Another direct answer is to call f readlines  which returns the contents of the file  up to an optional hint number of characters  so you could break this up into multiple lists that way     You can get to this file object two ways  One way is to pass the filename to the open builtin   filename    filename   with open filename  as f      f readlines     or using the new Path object from the pathlib module  which I have become quite fond of  and will use from here on    from pathlib import Path  path   Path filename   with path open   as f      f readlines     list will also consume the file iterator and return a list - a quite direct method as well   with path open   as f      list f    If you don t mind reading the entire text into memory as a single string before splitting it  you can do this as a one-liner with the Path object and the splitlines   string method  By default  splitlines removes the newlines   path read text   splitlines     If you want to keep the newlines  pass keepends True   path read text   splitlines keepends True       I want to read the file line by line and append each line to the end of the list    Now this is a bit silly to ask for  given that we ve demonstrated the end result easily with several methods  But you might need to filter or operate on the lines as you make your list  so let s humor this request   Using list append would allow you to filter or operate on each line before you append it   line list      for line in fileinput input filename       line list append line   line list   Using list extend would be a bit more direct  and perhaps useful if you have a preexisting list   line list      line list extend fileinput input filename   line list   Or more idiomatically  we could instead use a list comprehension  and map and filter inside it if desirable    line for line in fileinput input filename     Or even more directly  to close the circle  just pass it to list to create a new list directly without operating on the lines   list fileinput input filename     Conclusion  You ve seen many ways to get lines from a file into a list  but I d recommend you avoid materializing large quantities of data into a list and instead use Python s lazy iteration to process the data if possible   That is  prefer fileinput input or with path open   as f

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Read and write text files with Python 2 and Python 3  it works with Unicode     usr bin env python3   - - coding  utf-8 - -    Define data lines          A first string               A Unicode sample                  German                Write text file with open  file txt    w   as fp      fp write   n  join lines      Read text file with open  file txt    r   as fp      read lines   fp readlines       read lines    line rstrip   n   for line in read lines   print lines    read lines    Things to notice    with is a so-called context manager  It makes sure that the opened file is closed again  All solutions here which simply make  strip   or  rstrip   will fail to reproduce the lines as they also strip the white space    Common file endings   txt  More advanced file writing reading   CSV  Super simple format  read  amp  write  JSON  Nice for writing human-readable data  VERY commonly used  read  amp  write  YAML  YAML is a superset of JSON  but easier to read  read  amp  write  comparison of JSON and YAML  pickle  A Python serialization format  read  amp  write  MessagePack  Python package   More compact representation  read  amp  write  HDF5  Python package   Nice for matrices  read  amp  write  XML  exists too  sigh   read  amp  write    For your application  the following might be important    Support by other programming languages Reading writing performance Compactness  file size    See also  Comparison of data serialization formats  In case you are rather looking for a way to make configuration files  you might want to read my short article Configuration files in Python

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f   open  your file txt   r   out   f readlines     will append in the list out   Now variable out is a list  array  of what you want  You could either do   for line in out      print  line    Or   for line in f      print  line    You ll get the same results

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Here s one more option by using list comprehensions on files   lines    line rstrip   for line in open  file txt      This should be more efficient way as the most of the work is done inside the Python interpreter

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I like to use the following  Reading the lines immediately   contents      for line in open filepath   r   readlines        contents append line strip      Or using list comprehension   contents    line strip   for line in open filepath   r   readlines

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To read a file into a list you need to do three things    Open the file Read the file Store the contents as list   Fortunately Python makes it very easy to do these things so the shortest way to read a file into a list is   lst   list open filename     However I ll add some more explanation   Opening the file  I assume that you want to open a specific file and you don t deal directly with a file-handle  or a file-like-handle   The most commonly used function to open a file in Python is open  it takes one mandatory argument and two optional ones in Python 2 7    Filename Mode Buffering  I ll ignore this argument in this answer    The filename should be a string that represents the path to the file  For example   open  afile       opens the file named afile in the current working directory open  adir afile                relative path  relative to the current working directory  open  C  users aname afile      absolute path  windows  open   usr local afile          absolute path  linux    Note that the file extension needs to be specified  This is especially important for Windows users because file extensions like  txt or  doc  etc  are hidden by default when viewed in the explorer   The second argument is the mode  it s r by default which means  read-only   That s exactly what you need in your case    But in case you actually want to create a file and or write to a file you ll need a different argument here  There is an excellent answer if you want an overview   For reading a file you can omit the mode or pass it in explicitly   open filename  open filename   r     Both will open the file in read-only mode  In case you want to read in a binary file on Windows you need to use the mode rb   open filename   rb     On other platforms the  b   binary mode  is simply ignored     Now that I ve shown how to open the file  let s talk about the fact that you always need to close it again  Otherwise it will keep an open file-handle to the file until the process exits  or Python garbages the file-handle     While you could use   f   open filename        do stuff with f f close     That will fail to close the file when something between open and close throws an exception  You could avoid that by using a try and finally   f   open filename    nothing in between  try        do stuff with f finally      f close     However Python provides context managers that have a prettier syntax  but for open it s almost identical to the try and finally above    with open filename  as f        do stuff with f   The file is always closed after the with-scope ends    The last approach is the recommended approach to open a file in Python   Reading the file  Okay  you ve opened the file  now how to read it    The open function returns a file object and it supports Pythons iteration protocol  Each iteration will give you a line   with open filename  as f      for line in f          print line    This will print each line of the file  Note however that each line will contain a newline character  n at the end  you might want to check if your Python is built with universal newlines support - otherwise you could also have  r n on Windows or  r on Mac as newlines   If you don t want that you can could simply remove the last character  or the last two characters on Windows    with open filename  as f      for line in f          print line  -1     But the last line doesn t necessarily has a trailing newline  so one shouldn t use that  One could check if it ends with a trailing newline and if so remove it   with open filename  as f      for line in f          if line endswith   n                line   line  -1          print line    But you could simply remove all whitespaces  including the  n character  from the end of the string  this will also remove all other trailing whitespaces so you have to be careful if these are important   with open filename  as f      for line in f          print f rstrip      However if the lines end with  r n  Windows  newlines   that  rstrip   will also take care of the  r   Store the contents as list  Now that you know how to open the file and read it  it s time to store the contents in a list  The simplest option would be to use the list function   with open filename  as f      lst   list f    In case you want to strip the trailing newlines you could use a list comprehension instead   with open filename  as f      lst    line rstrip   for line in f    Or even simpler  The  readlines   method of the file object by default returns a list of the lines   with open filename  as f      lst   f readlines     This will also include the trailing newline characters  if you don t want them I would recommend the  line rstrip   for line in f  approach because it avoids keeping two lists containing all the lines in memory   There s an additional option to get the desired output  however it s rather  suboptimal   read the complete file in a string and then split on newlines   with open filename  as f      lst   f read   split   n     or   with open filename  as f      lst   f read   splitlines     These take care of the trailing newlines automatically because the split character isn t included  However they are not ideal because you keep the file as string and as a list of lines in memory   Summary   Use with open      as f when opening files because you don t need to take care of closing the file yourself and it closes the file even if some exception happens  file objects support the iteration protocol so reading a file line-by-line is as simple as for line in the file object   Always browse the documentation for the available functions classes  Most of the time there s a perfect match for the task or at least one or two good ones  The obvious choice in this case would be readlines   but if you want to process the lines before storing them in the list I would recommend a simple list-comprehension

[python] How to read a file line-by-line into a list?

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