python binary file patch script

#!/usr/bin/python
"""
simple script to patch a binary file
"""
import sys,os
from binascii import *
import re
def read_bytes(filename,start_address,number_of_bytes):
    fh = open(filename,'rb')
    fh.seek(start_address)
    data = fh.read(number_of_bytes)
    fh.close()
    return hexlify(data)
def replace_bytes(filename,search,replace):
    output = filename + ".patched"
    o = open(output,wb)
    data = open(filename).read() 
    o.write(re.sub(a2b_hex(search),a2b_hex(replace),data) )
    o.close()
def write_bytes(filename,start_address,newbytes):
    output = filename + ".patched"   
    fh = open(output,'wb')
    newbytes_hex = a2b_hex(newbytes)
    bytesize  = len(newbytes_hex)/2  #read data up to the start address
    end_address = start_address + bytesize
    for i in open(filename,'rb').read():
        t = fh.tell()
        if t < start_address or t > end_address: 
            fh.write(i)
            #print fh.tell()
        else:
            fh.write(newbytes_hex)
            #print  "patched " + output +" with " + newbytes + " starting at " + hex(t) 
    fh.flush()
    fh.close()
def main(name,address,bytes):
    size = len(bytes)/2
    print "before " + read_bytes(name,address,size)
    write_bytes(name,address,bytes)
    print "after  " + read_bytes(name+".patched",address,size)
if __name__ == "__main__":
    f = "test.exe"   
    a = 0x0003113
    w = "DEADBEEF"  
    main(f,a,w)