queue with array python

1class ArrayQueue:
2	def __init__(self):
3		self._data = []
4		self._size = 0
5		self._front = 0
6
7	def __len__(self):
8		return self._size()
9
10	def is_empty(self):
11		return self._size == 0
12
13	def enqueue(self, e):
14		self._data.append(e)
15		self._size = self._size + 1
16
17	def deque(self):
18		if self.is_empty():
19			raise Empty('queue is empty')
20		else:
21			value = self._data[self._front]
22			self._data[self._front] = None
23			self._front = self._front + 1
24			self._size = self._size - 1
25			return value
26
27	def first(self):
28		if self.is_empty():
29			raise Empty('queue is empty')
30		else:
31			return self._data[self._front]
32 
33
34class Empty(Exception):
35	pass
36