How do you read a file into a list in Python

Question

I want to prompt a user for a number of random numbers to be generated and saved to a file  He gave us that part  The part we have to do is to open that file  convert the numbers into a list  then find the mean  standard deviation  etc  without using the easy built-in Python tools   I ve tried using open but it gives me invalid syntax  the file name I chose was  numbers  and it saved into  My Documents  automatically  so I tried open numbers   r   and open C  name MyDocuments numbers   r   and neither one worked

User · Answer

hdl   open  C  name MyDocuments numbers    r   milist   hdl readlines   hdl close

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The pythonic way to read a file and put every lines in a list   from   future   import with statement  for python 2 5 with open  C  path numbers txt    r   as f      lines   f readlines     Then  assuming that each lines contains a number   numbers   int e strip    for e in lines

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If you have multiple numbers per line and you have multiple lines  you can read them in like this          usr bin env python      from os path import dirname      with open dirname   file        data path filename txt   as input data          input list   map int num split    for num in input data readlines

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Two ways to read file into list in python  note these are not either or  -   use of with - supported from python 2 5 and above use of list comprehensions    1  use of with  This is the pythonic way of opening and reading files    Sample 1 - elucidating each step but not memory efficient lines      with open  C  name MyDocuments numbers   as file      for line in file           line   line strip    or some other preprocessing         lines append line   storing everything in memory    Sample 2 - a more pythonic and idiomatic way but still not memory efficient with open  C  name MyDocuments numbers   as file      lines    line strip   for line in file    Sample 3 - a more pythonic way with efficient memory usage  Proper usage of with and file iterators   with open  C  name MyDocuments numbers   as file      for line in file          line   line strip    preprocess line         doSomethingWithThisLine line   take action on line instead of storing in a list  more memory efficient at the cost of execution speed    the  strip   is used for each line of the file to remove  n newline character that each line might have  When the with ends  the file will be closed automatically for you  This is true even if an exception is raised inside of it   2  use of list comprehension  This could be considered inefficient as the file descriptor might not be closed immediately  Could be a potential issue when this is called inside a function opening thousands of files    data    line strip   for line in open  C  name MyDocuments numbers    r      Note that file closing is implementation dependent  Normally unused variables are garbage collected by python interpreter  In cPython  the regular interpreter version from python org   it will happen immediately  since its garbage collector works by reference counting  In another interpreter  like Jython or Iron Python  there may be a delay

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f   open  file txt   lines   f readlines     Look over here  readlines   returns a list containing one line per element  Note that these lines contain the  n  newline-character  at the end of the line  You can strip off this newline-character by using the strip  -method  I e  call lines index  strip   in order to get the string without the newline character   As joaquin noted  do not forget to f close   the file   Converting strint to integers is easy  int  12

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with open  C  path numbers txt   as f      lines   f read   splitlines     this will give you a list of values  strings  you had in your file  with newlines stripped   also  watch your backslashes in windows path names  as those are also escape chars in strings   You can use forward slashes or double backslashes instead

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To summarize a bit from what people have been saying   f open  data txt    w     will make a new file or erase a file of that name if it is present f open  data txt    r     will open a file as read-only f open  data txt    a     will open a file for appending  appended data goes to the end of the file    If you wish have something in place similar to a try catch   with open  data txt   as f      for line in f          print line   I think  movieyoda code is probably what you should use however

User · Answer

You need to pass a filename string to open  There s an extra complication when the string has   in it  because that s a special string escape character to Python  You can fix this by doubling up each as    or by putting a r in front of the string as follows  r C  name MyDocuments numbers    Edit  The edits to the question make it completely different from the original  and since none of them was from the original poster I m not sure they re warrented  However it does point out one obvious thing that might have been overlooked  and that s how to add  My Documents  to a filename   In an English version of Windows XP  My Documents is actually C  Documents and Settings name My Documents  This means the open call should look like   open r C  Documents and Settings name My Documents numbers    r     I presume you re using XP because you call it My Documents - it changed in Vista and Windows 7  I don t know if there s an easy way to look this up automatically in Python

[python] How do you read a file into a list in Python?

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