Dict Destroyer!

Let's make a class that returns a random number for its hash.

>>> import random
>>> class DictDestroyer(object):
...    def __hash__(self):  return random.randint(0, 2**31)
...
>>> d = DictDestroyer()
>>> hash(d)
520880499
>>> hash(d)
793941724

Now, DictDestroyer lives up to its name:

>>> a = {d:1, d:2, d:3}
>>> import pprint
>>> pprint.pprint(a)
{<__main__.DictDestroyer object at 0x00CE8E90>: 1,
 <__main__.DictDestroyer object at 0x00CE8E90>: 2,
 <__main__.DictDestroyer object at 0x00CE8E90>: 3}

What is this?  One key has multiple values?  What happens when data is fetched?

>>> a[d]
1
>>> a[d]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: <__main__.DictDestroyer object at 0x00CE8E90>

Inconsistent behavior.  Some times it returns one of the values, other times it gives a key error.
The C source for python dictionaries holds the answer.
I'll have to update later when I figure it out.

cron-like functionality (sched module)

The Python standard library includes a handy little scheduler, with which you can do cron-like functionality.  The following code will print the current time every 5 seconds forever.

>>> import sched
>>> import datetime
>>> import time
>>> s = sched.scheduler(time.time, time.sleep)
>>> def print_time():
...    print datetime.datetime.now()
...    s.enter(5, 1, print_time, ())
...
>>> print_time()
2011-06-22 17:56:41.262000
>>> s.run()
2011-06-22 17:56:46.668000
2011-06-22 17:56:51.669000
2011-06-22 17:56:56.669000
2011-06-22 17:57:01.669000

This is a simple example of continuation passing style -- where functions "schedule" each other to run at a later time rather than calling directly.