Read and overwrite a file in Python

Question

Currently I m using this   f   open filename   r    text   f read   text   re sub  foobar    bar   text  f seek 0  f write text  f close     But the problem is that the old file is larger than the new file  So I end up with a new file that has a part of the old file on the end of it

User · Accepted Answer

If you don t want to close and reopen the file  to avoid race conditions  you could truncate it  f   open filename   r    text   f read   text   re sub  foobar    bar   text  f seek 0  f write text  f truncate   f close    The functionality will likely also be cleaner and safer using open as a context manager  which will close the file handler  even if an error occurs  with open filename   r    as f      text   f read       text   re sub  foobar    bar   text      f seek 0      f write text      f truncate

User · Answer

Honestly you can take a look at this class that I built which does basic file operations  The write method overwrites and append keeps old data  class IO      def read self  filename           toRead   open filename   quot rb quot            out   toRead read           toRead close                    return out          def write self  filename  data           toWrite   open filename   quot wb quot            out   toWrite write data          toWrite close        def append self  filename  data           append   self read filename          self write filename  append data

User · Answer

Probably it would be easier and neater to close the file after text   re sub  foobar    bar   text   re-open it for writing  thus clearing old contents   and write your updated text to it

User · Answer

The fileinput module has an inplace mode for writing changes to the file you are processing without using temporary files etc   The module nicely encapsulates the common operation of looping over the lines in a list of files  via an object which transparently keeps track of the file name  line number etc if you should want to inspect them inside the loop  from fileinput import FileInput for line in FileInput  quot file quot   inplace 1       line   line replace  quot foobar quot    quot bar quot       print line

User · Answer

I find it easier to remember to just read it and then write it  For example  with open  file   as f      data   f read   with open  file    w   as f      f write  hello

User · Answer

Try writing it in a new file    f   open filename   r    f2  open filename2  a    text   f read   text   re sub  foobar    bar   text  f seek 0  f close   f2 write text  fw close

[python] Read and overwrite a file in Python

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