Properly sort the sa(8) accounting records by time stamp

sa_times.py

#!/usr/bin/python

from __future__ import print_function
from datetime import datetime

'''
Display the account records gathered by the sa(8) facility.
from /usr/include/sys/acct.h
or on macOS, /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk/usr/include/sys/acct.h

/*
 * Accounting structures; these use a comp_t type which is a 3 bits base 8
 * exponent, 13 bit fraction ``floating point'' number.  Units are 1/AHZ
 * seconds.
 */
typedef u_int16_t comp_t;

struct acct {
    char      ac_comm[10];  /* command name */
    comp_t    ac_utime; /* user time */
    comp_t    ac_stime; /* system time */
    comp_t    ac_etime; /* elapsed time */
    u_int32_t ac_btime; /* starting time */
    uid_t     ac_uid;   /* user id */
    gid_t     ac_gid;   /* group id */
    u_int16_t ac_mem;   /* average memory usage */
    comp_t    ac_io;    /* count of IO blocks */
    dev_t     ac_tty;   /* controlling tty */

#define AFORK   0x01        /* fork'd but not exec'd */
#define ASU 0x02        /* used super-user permissions */
#define ACOMPAT 0x04        /* used compatibility mode */
#define ACORE   0x08        /* dumped core */
#define AXSIG   0x10        /* killed by a signal */
    u_int8_t  ac_flag;  /* accounting flags */
};
'''

def chunks(l, n):
    """Yield successive n-sized chunks from l."""
    for i in xrange(0, len(l), n):
        yield l[i:i + n]


def flipHex(i):
    return int("".join(i)[::-1].encode("hex"), 16) 


fh = open("/var/account/acct", 'r')
sa = fh.read()

tfmt = "%Y-%m-%d %H:%M:%S"
pfmt = "{: <20} {: <5} {: <5} {}"
print(pfmt.format("Start Time", "uid", "gid", "command"))
records = []

# entries from the acct file are each 40 chars long
for rec in chunks(sa, 40):
    r = {}
    r['cmd'] = rec[:9]
    r['start_time'] = flipHex(rec[16:20])
    r['uid'] = flipHex(rec[20:24])
    r['gid'] = flipHex(rec[24:28])
    records.append(r)

sorted_records = sorted(records, key=lambda k: k['start_time']) 

for r in sorted_records:
    print(pfmt.format(str(datetime.fromtimestamp(r['start_time'])), r['uid'], r['gid'], r['cmd']))