[Feature] Added priority queue based on python docs!

Author: exterkamp

README.md

@@ -45,6 +45,7 @@ $ coverage report -m
   * [Lose/Lose Hash Function](structures/hash_map.py)
 * [Heap](structures/heap.py)
 * [Trie](structures/trie.py)
+* [Priority Queue](structures/priority_queue.py)
 
 ### Algorithms
 

structures/priority_queue.py

@@ -0,0 +1,32 @@
+import itertools
+from heapq import heappush, heappop
+
+class PriorityQueue():
+
+    pq = []
+    mapper = {}
+    REMOVED = '__removed-task__'
+    counter = itertools.count()
+
+    def __init__(self):
+        pass
+    
+    def add_task(self, task, priority=0):
+        if task in self.mapper:
+            self.remove_task(task)
+        count = next(self.counter)
+        entry = [priority, count, task]
+        self.mapper [task] = entry
+        heappush(self.pq, entry)
+    
+    def remove_task(self, task):
+        entry = self.mapper.pop(task)
+        entry[-1] = self.REMOVED
+
+    def pop_task(self):
+        while self.pq:
+            _, _, task = heappop(self.pq)
+            if task is not self.REMOVED:
+                del self.mapper[task]
+                return task
+        raise KeyError('Pop from empty priority queue.')

tests/test_priority_queue.py

@@ -0,0 +1,24 @@
+import unittest
+from structures.priority_queue import PriorityQueue
+
+class TestPriorityQueue(unittest.TestCase):
+
+    def test_pq_simple(self):
+        pq = PriorityQueue()
+        pq.add_task('drive to work', 2)
+        pq.add_task('get keys')
+        pq.add_task('load car', 1)
+        pq.add_task('check gas', 1)
+        pq.add_task('turn on car', 1)
+        pq.add_task('park at work', 3)
+        pq.remove_task('load car')
+        pq.add_task('check gas', 1)
+
+        self.assertEqual('get keys',pq.pop_task())
+        self.assertEqual('turn on car', pq.pop_task())
+        self.assertEqual('check gas', pq.pop_task())
+        self.assertEqual('drive to work', pq.pop_task())
+        self.assertEqual('park at work', pq.pop_task())
+
+        with self.assertRaises(KeyError):
+            pq.pop_task()