Python Cookbook

Using Random-Access Input/Output

Credit: Luther Blissett

Problem

You want to read a binary record from somewhere inside a large file of fixed-length records, without reading a record at a time to get there.

Solution

The byte offset of the start of a record in the file is the record size multiplied by the record number (counting from 0). So just seek, then read:

thefile = open('somebinfile', 'rb')
thefile.seek(record_size * record_number)
buffer = thefile.read(record_size)

Discussion

This approach works only on files (generally binary ones) defined in terms of records that are all the same, fixed size; it doesn’t on normal text files. For clarity, the recipe shows the file being opened for reading as a binary file, by passing 'rb' as the second argument to open. Of course, you don’t need to open the file just before performing the first seek on it. As long as the file object is open for reading as a binary file, you can perform as many seek and read operations as you want before eventually closing the file again.

See Also

The section of the Library Reference on file objects; Perl Cookbook Recipe 8.12.