Confused by python file mode w

Question

From the doc       Modes  r     w   and  a   open the file for updating  note that  w   truncates the file   Append  b  to the mode to open the file in binary mode  on systems that differentiate between binary and text files  on systems that don   t have this distinction  adding the  b  has no effect    and here     w     Opens a file for both writing and reading  Overwrites the existing file if the file exists  If the file does not exist  creates a new file for reading and writing    But  how to read a file open with w

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Here is a list of the different modes of opening a file    r     Opens a file for reading only  The file pointer is placed at the beginning of the file  This is the default mode   rb     Opens a file for reading only in binary format  The file pointer is placed at the beginning of the file  This is the default mode   r      Opens a file for both reading and writing  The file pointer will be at the beginning of the file   rb      Opens a file for both reading and writing in binary format  The file pointer will be at the beginning of the file   w     Opens a file for writing only  Overwrites the file if the file exists  If the file does not exist  creates a new file for writing   wb     Opens a file for writing only in binary format  Overwrites the file if the file exists  If the file does not exist  creates a new file for writing   w      Opens a file for both writing and reading  Overwrites the existing file if the file exists  If the file does not exist  creates a new file for reading and writing   wb      Opens a file for both writing and reading in binary format  Overwrites the existing file if the file exists  If the file does not exist  creates a new file for reading and writing   a     Opens a file for appending  The file pointer is at the end of the file if the file exists  That is  the file is in the append mode  If the file does not exist  it creates a new file for writing   ab     Opens a file for appending in binary format  The file pointer is at the end of the file if the file exists  That is  the file is in the append mode  If the file does not exist  it creates a new file for writing   a      Opens a file for both appending and reading  The file pointer is at the end of the file if the file exists  The file opens in the append mode  If the file does not exist  it creates a new file for reading and writing   ab      Opens a file for both appending and reading in binary format  The file pointer is at the end of the file if the file exists  The file opens in the append mode  If the file does not exist  it creates a new file for reading and writing

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I suspect there are two ways to handle what I think you r trying to achieve   1  which is obvious  is open the file for reading only  read it into memory then open the file with t  then write your changes   2  use the low level file handling routines     Open file in RW   create if it doesn t exist   Don t  pass O TRUNC  fd   os open filename  os O RDWR   os O CREAT    Hope this helps

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Both seems to be working same but there is a catch  r   -  Open the file for Reading and Writing Once Opened in the beginning file pointer will point to 0 Now if you will want to Read then it will start reading from beginning if you want to Write then start writing  But the write process will begin from pointer 0  So there would be overwrite of characters  if there is any In this case File should be present  either will FileNotFoundError will be raised   w   -  Open the file for Reading and Writing If file exist  File will be opened and all data will be erased  If file does not exist  then new file will be created In the beginning file pointer will point to 0  as there is not data  Now if you want to write something  then write File pointer will be Now pointing to end of file  after write process  If you want to read the data now  seek to specific point   for beginning seek 0    So  Overall saying both are meant to open the file to read and write but difference is whether we want to erase the data in the beginning and then do read write or just start as it is  abc txt - in beginning 1234567 abcdefg 0987654 1234  Code for r  with open  abc txt    r    as f         abc txt should exist before opening     print f tell                        Should give    gt  0     f write  abcd                          print f read                        Pointer is pointing to index 3   gt  4th position     f write  Sunny                      After read pointer is at End of file  Output 0 567 abcdefg 0987654 1234  abc txt - After Run  abcd567 abcdefg 0987654 1234Sunny  Resetting abc txt as initial Code for w  with open  abc txt    w    as f           print f tell                        Should give    gt  0     f write  abcd                          print f read                        Pointer is pointing to index 3   gt  4th position     f write  Sunny                      After read pointer is at End of file  Output 0    abc txt - After Run  abcdSunny

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All file modes in Python   r for reading r  opens for reading and writing  cannot truncate a file  w for writing     w  for writing and reading  can truncate a file   rb for reading a binary file  The file pointer is placed at the beginning of the file  rb  reading or writing a binary file wb  writing a binary file a  opens for appending ab  Opens a file for both appending and reading in binary  The file pointer is at the end of the file if the file exists  The file opens in the append mode  x open for exclusive creation  failing if the file already exists  Python 3

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Actually  there s something wrong about all the other answers about r  mode   test in file s content   hello1 ok2 byebye3   And the py script s    with open  test in    r   as f      f readline       f write  addition     Execute it and the test in s content will be changed to    hello1 ok2 byebye3 addition   However  when we modify the script to    with open  test in    r   as f      f write  addition     the test in also do the respond   additionk2 byebye3   So  the r  mode will allow us to cover the content from the beginning if we did t do the read operation  And if we do some read operation  f write  will just append to the file   By the way  if we f seek 0 0  before f write write content   the write content will cover them from the positon 0 0

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The file is truncated  so you can call read    no exceptions raised  unlike when opened using  w   but you ll get an empty string

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Let s say you re opening the file with a with statement like you should be  Then you d do something like this to read from your file   with open  somefile txt    w    as f        Note that f has now been truncated to 0 bytes  so you ll only       be able to read data that you write after this point     f write  somedata n       f seek 0     Important  return to the top of the file before reading  otherwise you ll just read an empty string     data   f read     Returns  somedata n    Note the f seek 0  -- if you forget this  the f read   call will try to read from the end of the file  and will return an empty string

User · Answer

As mentioned by h4z3  For a practical use  Sometimes your data is too big to directly load everything  or you have a generator  or real-time incoming data  you could use w  to store in a file and read later

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r for read  w for write  r  for read write without deleting the original content if file exists  otherwise raise exception  w  for delete the original content then read write if file exists  otherwise create the file  For example    gt  gt  gt  with open  file1 txt    w   as f        f write  ab n         gt  gt  gt  with open  file1 txt    w    as f        f write  c           cat file1 txt  c     gt  gt  gt  with open  file2 txt    r    as f        f write  ab n        Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  IOError   Errno 2  No such file or directory   file2 txt      gt  gt  gt  with open  file2 txt    w   as f        f write  ab n         gt  gt  gt  with open  file2 txt    r    as f        f write  c           cat file2 txt  cb

[python] Confused by python file mode "w+"

Here is a list of the different modes of opening a file:

r

rb

r+

rb+

w

wb

w+

wb+

a

ab

a+

ab+

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