How to find out whether a file is at its eof

Question

fp   open  a txt    do many things with fp  c   fp read   if c is None      print  fp is at the eof    Besides the above method  any other way to find out whether is fp is already at the eof

User · Answer

f   open filename  r   f seek -1 2        go to the file end  eof   f tell       get the end of file location f seek 0 0         go back to file beginning  while f tell      eof        lt body gt    You can use the file methods seek   and tell   to determine the position of the end of file  Once the position is found  seek back to the file beginning

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You can use below code snippet to read line by line  till end of file   line   obj readline   while line               Do Something     line   obj readline

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This code will work for python 3 and above file open  quot filename txt quot      f file readlines      reads all lines from the file EOF -1    represents end of file temp 0 for k in range len f -1 -1 -1       if temp  0          if f k    quot  n quot               EOF k         else              temp  1 print  quot Given file has quot  EOF  quot lines quot   file close

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If file is opened in non-block mode  returning less bytes than expected does not mean it s at eof  I d say  NPE s answer is the most reliable way      f tell      os fstat f fileno    st size

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Here is a way to do this with the Walrus Operator  new in Python 3 8  f   open  quot a txt quot    quot r quot    while  c    f read n        process c   f close    Useful Python Docs  3 8   Walrus operator  https   docs python org 3 whatsnew 3 8 html assignment-expressions Methods of file objects  https   docs python org 3 tutorial inputoutput html methods-of-file-objects

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Get the EOF position of the file   def get eof position file handle       original position   file handle tell       eof position   file handle seek 0  2      file handle seek original position      return eof position   and compare it with the current position  get eof position    file handle tell

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The Python read functions will return an empty string if they reach EOF

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As python returns empty string on EOF  and not  EOF  itself  you can just check the code for it  written here  f1   open  sample txt    while True      line   f1 readline       print line     if        line           print  file finished          break

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f open file name  for line in f     print line

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You can use tell   method after reaching EOF by calling readlines   method  like this   fp open  file name   r   lines fp readlines   eof fp tell     here we store the pointer                 indicating the end of the file in eof fp seek 0    we bring the cursor at the begining of the file if eof    fp tell      we check if the cursor      do something      reaches the end of the file

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I use this function     Returns True if End-Of-File is reached def EOF f       current pos   f tell       file size   os fstat f fileno    st size     return current pos  gt   file size

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I d argue that reading from the file is the most reliable way to establish whether it contains more data  It could be a pipe  or another process might be appending data to the file etc   If you know that s not an issue  you could use something like   f tell      os fstat f fileno    st size

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I really don t understand why python still doesn t have such a function  I also don t agree to use the following  f tell      os fstat f fileno    st size   The main reason is f tell   doesn t likely to work for some special conditions   The method works for me is like the following  If you have some pseudocode like the following  while not EOF f        line   f readline          do something with line    You can replace it with   lines   iter f readlines    while True       try          line   next lines            do something with line       except StopIteration          break   This method is simple and you don t need to change most of you code

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Reading a file in batches of BATCH SIZE lines  the last batch can be shorter    BATCH SIZE   1000    lines  with open   path to a file   as fin      eof   False     while eof is False            We use an iterator to check later if it was fully realized  This           is a way to know if we reached the EOF            NOTE  file tell   can t be used with iterators          batch range   iter range BATCH SIZE           acc    line for     line  in zip batch range  fin              DO SOMETHING WITH  acc             If we still have something to iterate  we have read the whole           file          if any batch range               eof   True

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Although I would personally use a with statement to handle opening and closing a file  in the case where you have to read from stdin and need to track an EOF exception  do something like this   Use a try-catch with EOFError as the exception   try      input lines          for line in sys stdin readlines            input lines    line              except EOFError as e      print e

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read returns an empty string when EOF is encountered  Docs are here

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You can compare the returned value of fp tell   before and after calling the read method  If they return the same value  fp is at eof   Furthermore  I don t think your example code actually works  The read method to my knowledge never returns None  but it does return an empty string on eof

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The  for-else  design is often overlooked  See  Python Docs  Control Flow in Loop    Example  with open  foobar file    rb   as f      for line in f          foo        else            No more lines to be read from file         bar

User · Answer

Python doesn t have built-in eof detection function but that functionality is available in two ways  f read 1  will return b   if there are no more bytes to read  This works for text as well as binary files  The second way is to use f tell   to see if current seek position is at the end  If you want EOF testing not to change the current file position then you need bit of extra code    Below are both implementations   Using tell   method  import os  def is eof f     cur   f tell        save current position   f seek 0  os SEEK END    end   f tell        find the size of file   f seek cur  os SEEK SET    return cur    end   Using read   method  def is eof f     s   f read 1    if s    b         restore position     f seek -1  os SEEK CUR    return s    b     How to use this  while not is eof my file       val   my file read 10    Play with this code

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fp read   reads up to the end of the file  so after it s successfully finished you know the file is at EOF  there s no need to check  If it cannot reach EOF it will raise an exception   When reading a file in chunks rather than with read    you know you ve hit EOF when read returns less than the number of bytes you requested  In that case  the following read call will return the empty string  not None   The following loop reads a file in chunks  it will call read at most once too many   assert n  gt  0 while True      chunk   fp read n      if chunk                break     process chunk    Or  shorter   for chunk in iter lambda  fp read n            process chunk

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When doing binary I O the following method is useful   while f read 1       f seek -1 1        whatever   The advantage is that sometimes you are processing a binary stream and do not know in advance how much you will need to read

[python] How to find out whether a file is at its `eof`?

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