Recipe2.7.Using Random-Access InputOutput

Recipe 2.7. 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 size of a record, in bytes, multiplied by the progressive number of the record (counting from 0). So, you can just seek right to the proper spot, then read the data. For example, to read the seventh record from a binary file where each record is 48 bytes long:

thefile = open('somebinfile', 'rb') record_size = 48 record_number = 6 thefile.seek(record_size * record_number) buffer = thefile.read(record_size)

Note that the record_number of the seventh record is 6: record numbers count from zero!

Discussion

This approach works only on files (generally binary ones) defined in terms of records that are all the same fixed size in bytes; it doesn't work 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, just before the seek. 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 need, before eventually closing the file againyou don't necessarily open the file just before performing a seek on it.

See Also

The section of the Library Reference and Python in a Nutshell on file objects; Perl Cookbook recipe 8.12.