Continue reading file from the position where it was left

Question

I have to read a file multiple times in which some error info is appended everyday. Is there a way to start reading the file from the point it was left previous day instead

Answer 1

You can use the python tell file method to see what position you are in a file before you close it and the seek method to return to that position after you open it again.

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Example:

Given a file foo with the contents

edas
agfa
agf
fgfgfg

You can return to a given position as follows:

>>> f = open('foo')
>>> f.tell()
0
>>> f.readline()
'edas\n'
>>> f.tell()
5
>>> f.close()
>>> f = open('foo')
>>> f.tell()
0
>>> f.seek(5)
>>> f.readline()
'agfa\n'

Answer 2

Yes - it's possible. I believe - you've a file whose size is pretty big enough that you don't want to read from start every time. So here's something you can do - write to a file in your home directory .fileinfo or something and in that write the last offset of the file you read in that file. And then your program can do something like following

if not os.path.exists('/home/.fileinfo'):
     seek_from = 0
else:
     of = open('/home/.fileinfo', 'r')
     seek_from = int(of.readline().strip())

with open('/path/to/your/file', 'r') as f:
    f.seek(seek_from, 0)

    # do whatever you want
    f.seek(0,-1) # Go to the end of the file
    end = f.tell() # Get the position
    of = open('/home/.fileinfo', 'w')
    of.write(str(end))
    of.close()

Something along these lines.

Hope that helps