Python Cookbook

Updating a Random-Access File

Credit: Luther Blissett

Problem

You want to read a binary record from somewhere inside a large file of fixed-length records, change the values, and write the record back.

Solution

Read the record, unpack it, perform whatever computations you need for the update, pack the fields back into the record, seek to the start of the record again, and write it back. Phew. Faster to code than to say:

import struct

thefile = open('somebinfile', 'r+b')
record_size = struct.calcsize(format_string)

thefile.seek(record_size * record_number)
buffer = thefile.read(record_size)
fields = list(struct.unpack(format_string, buffer))

# Perform computations, suitably modifying fields, then:

buffer = struct.pack(format_string, *fields)
thefile.seek(record_size * record_number)
thefile.write(buffer)

thefile.close(  )

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 work on normal text files. Furthermore, the size of each record must be that defined by a struct’s format string, as shown in the recipe’s code. A typical format string, for example, might be "8l", to specify that each record is made up of eight four-byte integers, each to be interpreted as a signed value and unpacked into a Python int. In this case, the fields variable in the recipe would be bound to a list of eight ints. Note that struct.unpack returns a tuple. Because tuples are immutable, the computation would have to rebind ...