Prepend line to beginning of a file

Question

I can do this using a separate file  but how do I append a line to the beginning of a file   f open  log txt   a   f seek 0   get to the first position f write  text   f close     This starts writing from the end of the file since the file is opened in append mode

User · Answer

If the file is the too big to use as a list  and you simply want to reverse the file  you can initially write the file in reversed order and then read one line at the time from the file s end  and write it to another file  with file-read-backwards module

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In modes  a  or  a    any writing is done at the end of the file  even if at the current moment when the write   function is triggered the file s pointer is not at the end of the file  the pointer is moved to the end of file before any writing  You can do what you want in two manners   1st way  can be used if there are no issues to load the file into memory   def line prepender filename  line       with open filename   r    as f          content   f read           f seek 0  0          f write line rstrip   r n       n    content    2nd way   def line pre adder filename  line to prepend       f   fileinput input filename  inplace 1      for xline in f          if f isfirstline                print line to prepend rstrip   r n       n    xline          else              print xline    I don t know how this method works under the hood and if it can be employed on big big file  The argument 1 passed to input is what allows to rewrite a line in place  the following lines must be moved forwards or backwards in order that the inplace operation takes place  but I don t know the mechanism

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The clear way to do this is as follows if you do not mind writing the file again  with open  a txt    r    as fp      lines   fp readlines         lines is list of line  each element      n      lines insert 0  one line     you can use any index if you know the line index     fp seek 0                    file pointer locates at the beginning to write the whole file again     fp writelines lines          write whole lists again to the same file   Note that this is not in-place replacement  It s writing a file again   In summary  you read a file and save it to a list and modify the list and write the list again to a new file with the same filename

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Different Idea    1  You save the original file as a variable    2  You overwrite the original file with new information    3  You append the original file in the data below the new information   Code   with open  lt filename gt   r   as contents        save   contents read   with open  lt filename gt   w   as contents        contents write  lt  New Information  gt   with open  lt filename gt   a   as contents        contents write save

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In all filesystems that I am familiar with  you can t do this in-place  You have to use an auxiliary file  which you can then rename to take the name of the original file

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There s no way to do this with any built-in functions  because it would be terribly inefficient  You d need to shift the existing contents of the file down each time you add a line at the front   There s a Unix Linux utility tail which can read from the end of a file  Perhaps you can find that useful in your application

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To put code to NPE s answer  I think the most efficient way to do this is   def insert originalfile string       with open originalfile  r   as f          with open  newfile txt   w   as f2               f2 write string              f2 write f read        os rename  newfile txt  originalfile

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with open  fruits txt    r    as file      file write  bab111y       file seek 0      content   file read       print content

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num    1  2  3   List containing Integers  with open  ex3 txt    r    as file      readcontent   file read      store the read value of exe txt into                                    readcontent      file seek 0  0   Takes the cursor to top line     for i in num            writing content of list One by One          file write str i      n    convert int to str since write   deals                                       with str     file write readcontent   after content of string are written  I return                                back content that were in the file

[python] Prepend line to beginning of a file

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