How to read specific lines from a file by line number

Question

I m using a for loop to read a file  but I only want to read specific lines  say line  26 and  30  Is there any built-in feature to achieve this

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If you don t mind importing then fileinput does exactly what you need  this is you can read the line number of the current line

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A better and minor change for Alok Singhal s answer  fp   open  file   for i  line in enumerate fp 1       if i    26            26th line     elif i    30            30th line     elif i  gt  30          break fp close

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If your large text file file is strictly well-structured  meaning every line has the same length l   you could use for n-th line  with open file  as f      f seek n l      line   f readline        last pos   f tell     Disclaimer This does only work for files with the same length

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with open  quot test txt quot    quot r quot   as fp  lines   fp readlines   print lines 3    test txt is filename prints line number four in test txt

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If the file to read is big  and you don t want to read the whole file in memory at once   fp   open  file   for i  line in enumerate fp       if i    25            26th line     elif i    29            30th line     elif i  gt  29          break fp close     Note that i    n-1 for the nth line     In Python 2 6 or later   with open  file   as fp      for i  line in enumerate fp           if i    25                26th line         elif i    29                30th line         elif i  gt  29              break

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if you want line 7   line   open  file txt    r   readlines   7

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Reading from specific line  n   4     for reading from 5th line with open  quot write txt quot   r   as t       for i line in enumerate t            if i  gt   n                i    n-1 for nth line             print line

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You can do this very simply with this syntax that someone already mentioned  but it s by far the easiest way to do it   inputFile   open  lineNumbers txt    r   lines   inputFile readlines   print  lines 0   print  lines 2

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If you want to read specific lines  such as line starting after some threshold line then you can use the following codes  file    open  files txt   r   lines   file readlines      convert to list of lines datas    lines 11       raed the specific lines

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For the sake of offering another solution   import linecache linecache getline  Sample txt   Number of Line    I hope this is quick and easy

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Reading files is incredible fast  Reading a 100MB file takes less than 0 1 seconds  see my article Reading and Writing Files with Python   Hence you should read it completely and then work with the single lines   What most answer here do is not wrong  but bad style  Opening files should always be done with with as it makes sure that the file is closed again   So you should do it like this   with open  path to file txt   as f      lines   f readlines   print lines 26      or whatever you want to do with this line print lines 30      or whatever you want to do with this line   Huge files  If you happen to have a huge file and memory consumption is a concern  you can process it line by line   with open  path to file txt   as f      for i  line in enumerate f           pass    process line i

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To print desired line  To print line above below required line   def dline file no add sub 0       tf open file      for sno line in enumerate tf           if sno  no-1 add sub           print line      tf close     execute---- dline  D  dummy txt  6   i e dline  file path   line number  if you want upper line of the searched line give 1 for lower -1 this is optional default value will be taken 0

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Here s my little 2 cents  for what it s worth     def indexLines filename  lines  2 4 6 8 10 12 3 5 7 1        fp     open filename   r       src    fp readlines       data     index  line  for index  line in enumerate src  if index in lines      fp close       return data     Usage below filename    C   Your  Path  And  Filename txt  for line in indexLines filename     using default list  specify your own list of lines otherwise     print  Line   s nData   s n     line 0   line 1

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A fast and compact approach could be   def picklines thefile  whatlines     return  x for i  x in enumerate thefile  if i in whatlines    this accepts any open file-like object thefile  leaving up to the caller whether it should be opened from a disk file  or via e g a socket  or other file-like stream  and a set of zero-based line indices whatlines  and returns a list  with low memory footprint and reasonable speed   If the number of lines to be returned is huge  you might prefer a generator   def yieldlines thefile  whatlines     return  x for i  x in enumerate thefile  if i in whatlines    which is basically only good for looping upon -- note that the only difference comes from using rounded rather than square parentheses in the return statement  making a list comprehension and a generator expression respectively   Further note that despite the mention of  lines  and  file  these functions are much  much more general -- they ll work on any iterable  be it an open file or any other  returning a list  or generator  of items based on their progressive item-numbers   So  I d suggest using more appropriately general names -

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Fairly quick and to the point   To print certain lines in a text file   Create a  lines2print  list and then just print when the enumeration is  in  the lines2print list  To get rid of extra   n  use line strip   or line strip   n    I just like  list comprehension  and try to use when I can  I like the  with  method to read text files in order to prevent leaving a file open for any reason   lines2print    26 30    can be a big list and order doesn t matter   with open  filepath    r   as fp       print x strip    for ei x in enumerate fp  if ei in lines2print    or if list is small just type in list as a list into the comprehension   with open  filepath    r   as fp       print x strip    for ei x in enumerate fp  if ei in  26 30

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Some of these are lovely  but it can be done much more simply   start   0   some starting index end   5000   some ending index filename    test txt    some file we want to use  with open filename  as fh      data   fin readlines   start end   print data    That will use simply list slicing  it loads the whole file  but most systems will minimise memory usage appropriately  it s faster than most of the methods given above  and works on my 10G  data files  Good luck

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def getitems iterable  items     items   list items    get a list from any iterable and make our own copy                         since we modify it   if items      items sort       for n  v in enumerate iterable         if n    items 0           yield v         items pop 0          if not items            break  print list getitems open   usr share dict words     25  29        Abelson n    Abernathy n     note that index 25 is the 26th item

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I think this would work   open file1   open  E   test txt   r    read it1   open file1 read    myline1       for line1 in read it1 splitlines     myline1 append line1   print myline1 0

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file     path to file to be read txt  with open file  as f      print f readlines   26      print f readlines   30    Using the with statement  this opens the file  prints lines 26 and 30  then closes the file  Simple

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OP  you can use enumerate  for n line in enumerate open  file         if n 1 in  26 30     or n in  25 29          print line rstrip

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f   open filename   r   totalLines   len f readlines    f close   f   open filename   r    lineno   1 while lineno  lt  totalLines      line   f readline        if lineno    26          doLine26Commmand line       elif lineno    30          doLine30Commmand line       lineno    1 f close

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I prefer this approach because it s more general-purpose  i e  you can use it on a file  on the result of f readlines    on a StringIO object  whatever   def read specific lines file  lines to read         file is any iterable  lines to read is an iterable containing int values       lines   set lines to read     last   max lines     for n  line in enumerate file         if n   1 in lines            yield line       if n   1  gt  last            return   gt  gt  gt  with open r c  temp words txt   as f           s for s in read specific lines f   1  2  3  1000      A n    a n    aa n    accordant n

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For the sake of completeness  here is one more option   Let s start with a definition from python docs      slice An object usually containing a portion of a sequence  A slice is created using the subscript notation     with colons between numbers when several are given  such as in variable name 1 3 5   The bracket  subscript  notation uses slice objects internally  or in older versions    getslice     and   setslice         Though the slice notation is not directly applicable to iterators in general  the itertools package contains a replacement function   from itertools import islice    print the 100th line with open  the file   as lines      for line in islice lines  99  100           print line    print each third line until 100 with open  the file   as lines      for line in islice lines  0  100  3           print line   The additional advantage of the function is that it does not read the iterator until the end  So you can do more complex things   with open  the file   as lines        print the first 100 lines     for line in islice lines  100           print line        then skip the next 5     for line in islice lines  5           pass        print the rest     for line in lines          print line   And to answer the original question     how to read lines  26 and  30 In  365   list islice xrange 1 100   25  30  4   Out 365    26  30

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You can do a seek   call which positions your read head to a specified byte within the file  This won t help you unless you know exactly how many bytes  characters  are written in the file before the line you want to read  Perhaps your file is strictly formatted  each line is X number of bytes   or  you could count the number of characters yourself  remember to include invisible characters like line breaks  if you really want the speed boost    Otherwise  you do have to read every line prior to the line you desire  as per one of the many solutions already proposed here

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How about this    gt  gt  gt  with open  a    r   as fin  lines   fin readlines    gt  gt  gt  for i  line in enumerate lines         if i  gt  30  break       if i    26  dox         if i    30  doy

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File objects have a  readlines   method which will give you a list of the contents of the file  one line per list item  After that  you can just use normal list slicing techniques   http   docs python org library stdtypes html file readlines

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To print line  3      line number   3  with open filename  r   as file  current line   1 for line in file      if current line    line number          print file readline            break     current line    1   Original author  Frank Hofmann

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The quick answer   f open  filename   lines f readlines   print lines 25  print lines 29    or   lines  25  29  i 0 f open  filename   for line in f      if i in lines          print i     i  1   There is a more elegant solution for extracting many lines  linecache  courtesy of  python  how to jump to a particular line in a huge text file    a previous stackoverflow com question    Quoting the python documentation linked above    gt  gt  gt  import linecache  gt  gt  gt  linecache getline   etc passwd   4   sys x 3 3 sys  dev  bin sh n    Change the 4 to your desired line number  and you re on  Note that 4 would bring the fifth line as the count is zero-based   If the file might be very large  and cause problems when read into memory  it might be a good idea to take  Alok s advice and use enumerate     To Conclude    Use fileobject readlines   or for line in fileobject as a quick solution for small files   Use linecache for a more elegant solution  which will be quite fast for reading many files  possible repeatedly  Take  Alok s advice and use enumerate   for files which could be very large  and won t fit into memory  Note that using this method might slow because the file is read sequentially

[python] How to read specific lines from a file (by line number)?

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