10.2. File positioning for random-access devices

For random-access devices such as disk files, there are methods that let you find your current position within a file, and move to a different position.

F.tell() returns your current position in file F.
F.seek(N) moves your current position to N, where a position of zero is the beginning of the file.
F.seek(N, 1) moves your current position by a distance of N characters, where positive values of N move toward the end of the file and negative values move toward the beginning.
For example, F.seek(80, 1) would move the file position 80 characters further from the start of the file.
F.seek(N, 2) moves to a position N characters relative to the end of the file. For example, F.seek(0, 2) would move to the end of the file; F.seek(-200, 2) would move your position to 200 bytes before the end of the file.

>>> treeFile = open ( "trees" )
>>> treeFile.tell()
0L
>>> treeFile.read(6)
'yew\noa'
>>> treeFile.tell()
6L
>>> treeFile.seek(1)
>>> treeFile.tell()
1L
>>> treeFile.read(5)
'ew\noa'
>>> treeFile.tell()
6L
>>> treeFile.seek(1, 1)
>>> treeFile.tell()
7L
>>> treeFile.seek(-3, 1)
>>> treeFile.tell()
4L
>>> treeFile.seek(0, 2)
>>> treeFile.tell()
26L
>>> treeFile.seek(-3, 2)
>>> treeFile.tell()
23L
>>> treeFile.read()
'er\n'

A Python 2.7 programming tutorial

10.2. File positioning for random-access devices