How to read first N lines of a file

Question

We have a large raw data file that we would like to trim to a specified size  I am experienced in  net c   however would like to do this in python to simplify things and out of interest  How would I go about getting the first N lines of a text file in python  Will the OS being used have any effect on the implementation

User · Answer

fname   input  Enter file name     num lines   0  with open fname   r   as f   lines count     for line in f          num lines    1  num lines input   int  input  Enter line numbers       if num lines input  lt   num lines      f   open fname   r       for x in range num lines input           a   f readline           print a   else      f   open fname   r       for x in range num lines input           a   f readline           print a          print  Don t have   num lines input    lines print as much as you can     print  Total lines in the text  num lines

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N   10 with open  file txt    a   as file     the a opens it in append mode     for i in range N           line   next file  strip           print line

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most convinient way on my own   LINE COUNT   3 print  s for  i  s  in enumerate open  test txt    if i  lt  LINE COUNT    Solution based on List Comprehension The function open   supports an iteration interface  The enumerate   covers open   and return tuples  index  item   then we check that we re inside an accepted range  if i  lt  LINE COUNT  and then simply print the result   Enjoy the Python

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What I do is to call the N lines using pandas  I think the performance is not the best  but for example if N 1000  import pandas as pd yourfile   pd read csv  path to your file csv  nrows 1000

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Based on gnibbler top voted answer  Nov 20  09 at 0 27   this class add head   and tail   method to file object   class File file       def head self  lines 2find 1           self seek 0                              Rewind file         return  self next   for x in xrange lines 2find        def tail self  lines 2find 1             self seek 0  2                           go to end of file         bytes in file   self tell                        lines found  total bytes scanned   0  0         while  lines 2find 1  gt  lines found and                bytes in file  gt  total bytes scanned                byte block   min 1024  bytes in file-total bytes scanned              self seek - byte block total bytes scanned   2              total bytes scanned    byte block             lines found    self read 1024  count   n           self seek -total bytes scanned  2          line list   list self readlines            return line list -lines 2find     Usage   f   File  path to file    r   f head 3  f tail 3

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If you have a really big file  and assuming you want the output to be a numpy array  using np genfromtxt will freeze your computer  This is so much better in my experience   def load big file fname maxrows      only works for well-formed text file of space-separated doubles     rows         unknown number of lines  so use list  with open fname  as f      j 0             for line in f          if j  maxrows              break         else              line    float s  for s in line split                rows append np array line  dtype   np double               j  1 return np vstack rows     convert list of vectors to array

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usr bin python  import subprocess  p   subprocess Popen   tail    -n 3    passlist    stdout subprocess PIPE   output  err   p communicate    print  output   This Method Worked for me

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This works for Python 2  amp  3   from itertools import islice  with open   tmp filename txt   as inf      for line in islice inf  N  N M           print line

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For first 5 lines  simply do   N 5 with open  data file    r   as file      for i in range N          print file next

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If you want something that obviously  without looking up esoteric stuff in manuals  works without imports and try except and works on a fair range of Python 2 x versions  2 2 to 2 6    def headn file name  n          Like  x head -N command        result          nlines   0     assert n  gt   1     for line in open file name           result append line          nlines    1         if nlines  gt   n              break     return result  if   name         main         import sys     rval   headn sys argv 1   int sys argv 2        print rval     print len rval

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Python 2  with open  quot datafile quot   as myfile      head    next myfile  for x in xrange N   print head  Python 3  with open  quot datafile quot   as myfile      head    next myfile  for x in range N   print head   Here s another way  both Python 2  amp  3   from itertools import islice  with open  quot datafile quot   as myfile      head   list islice myfile  N   print head

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This worked for me   f   open  history export csv    r   line  5 for x in range line       a   f readline       print a

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There is no specific method to read number of lines exposed by file object    I guess the easiest way would be following    lines     with open file name  as f      lines extend f readline   for i in xrange N

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If you want to read the first lines quickly and you don t care about performance you can use  readlines   which returns list object and then slice the list   E g  for the first 5 lines   with open  pathofmyfileandfileandname   as myfile      firstNlines myfile readlines   0 5   put here the interval you want      Note  the whole file is read so is not the best from the performance point of view but it   is easy to use  fast to write and easy to remember so if you want just perform   some one-time calculation is very convenient   print firstNlines   One advantage compared to the other answers is the possibility to select easily the range of lines e g  skipping the first 10 lines  10 30  or the lasts 10   -10  or taking only even lines    2

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The two most intuitive ways of doing this would be    Iterate on the file line-by-line  and break after N lines  Iterate on the file line-by-line using the next   method N times   This is essentially just a different syntax for what the top answer does     Here is the code     Method 1  with open  fileName    r   as f      counter   0     for line in f          print line         counter    1         if counter    N  break    Method 2  with open  fileName    r   as f      for i in xrange N           line   f next           print line   The bottom line is  as long as you don t use readlines   or enumerateing the whole file into memory  you have plenty of options

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Starting at Python 2 6  you can take advantage of more sophisticated functions in the IO base clase   So the top rated answer above can be rewritten as       with open  datafile   as myfile         head   myfile readlines N      print head    You don t have to worry about your file having less than N lines since no StopIteration exception is thrown

[python] How to read first N lines of a file?

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