I recently wanted to implement a small event system where events can have different priorities. So for example the event with highest priority (lowest value) should be handled first.
Python comes with a heapq module which can transform a list into a heap in a way that it stays a list, but fulfills all heap constraints. Nevertheless you might want to wrap the heap like this, so you can do nice stuff:
import heapq
class Heap(object):
""" A neat min-heap wrapper which allows storing items by priority
and get the lowest item out first (pop()).
Also implements the iterator-methods, so can be used in a for
loop, which will loop through all items in increasing priority order.
Remember that accessing the items like this will iteratively call
pop(), and hence empties the heap! """
def __init__(self):
""" create a new min-heap. """
self._heap = []
def push(self, priority, item):
""" Push an item with priority into the heap.
Priority 0 is the highest, which means that such an item will
be popped first."""
assert priority >= 0
heapq.heappush(self._heap, (priority, item))
def pop(self):
""" Returns the item with lowest priority. """
item = heapq.heappop(self._heap)[1] # (prio, item)[1] == item
return item
def __len__(self):
return len(self._heap)
def __iter__(self):
""" Get all elements ordered by asc. priority. """
return self
def next(self):
""" Get all elements ordered by their priority (lowest first). """
try:
return self.pop()
except IndexError:
raise StopIteration
With this one could do the following:
h = Heap()
# add some nonsense:
h.push(10, "I'm a large one")
h.push(20, "Actually I'm larger")
h.push(5, "size is not everything")
h.push(0, "which size?")
# get it out in a cool way:
for item in h:
print item
# prints:
# which size?
# size is not everything
# I'm a large one
# Actually I'm larger
When I try running your code, this is the error I get:
"TypeError: iter() returned non-iterator of type 'Heap'"
How can I correct this?
Which python version do you use and how did you input my code sample? There seems to be a white-space problem with empty lines in WordPress and the code highlighting plugin I use, so maybe the whole problem is that your class definition wasn’t complete? (Should’ve given a syntax error though.)
I tested the code on py2.6 and 2.7, don’t know about 3.x yet.
Found your problem:
seriously? NO!
If you do it like that, you just iterate over the list backend of the heap which will not be sorted correctly!