Iterator objects in python conform to the iterator protocol, which basically means they provide two methods: __iter__() and next(). The __iter__ returns the iterator object and is implicitly called at the start of loops. The next() method returns the next value and is implicitly called at each loop increment. next() raises a StopIteration exception when there are no more value to return, which is implicitly captured by looping constructs to stop iterating.
Here’s a simple example of a counter:
class Counter:
def __init__(self, low, high):
self.current = low
self.high = high
def __iter__(self):
return self
def next(self):
if self.current > self.high:
raise StopIteration
else:
self.current += 1
return self.current - 1
for c in Counter(3, 8):
print c
This will print:
[test@Master python]$ python counter.py
3
4
5
6
7
8
python3的版本:
class Counter:
def __init__(self, low, high):
self.current = low
self.high = high
def __iter__(self):
return self
def __next__(self):
if self.current > self.high:
raise StopIteration
else:
self.current += 1
return self.current - 1
for c in Counter(3, 8):
print(c)
运行结果:
[test@Master python]$ python3 counter.py
3
4
5
6
7
8
上面这个类虽然已经可以遍历了，但是却不能重复遍历
class Counter:
def __init__(self, low, high):
self.low = low
self.high = high
def __iter__(self):
self.current = self.low
return self
def __next__(self):
if self.current > self.high:
raise StopIteration
else:
self.current += 1
return self.current - 1
counter = Counter(3, 8)
for c in counter:
print(c)
print('==========')
for c in counter:
print(c)
我感觉我这个类改写得真好，但是实际上去不是这样的，因为这是一个iterator，而不是一个iterable,所以这样改写是错误的，如果你要改写，应该如下:
class Counter:
def __init__(self, low, high):
self.current = low
self.high = high
def __iter__(self):
return self
def __next__(self):
if self.current > self.high:
raise StopIteration
else:
self.current += 1
return self.current - 1
class IterableCounter:
def __iter__(self):
return Counter(3, 9)
for c in IterableCounter():
print(c)
print('==========')
for c in IterableCounter():
print(c)
This is easier to write using a generator:
def counter(low, high):
current = low
while current <= high:
yield current
current += 1
for c in counter(3, 8):
print(c)
The printed output will be the same. Under the hood, the generator object supports the iterator protocol and does something roughly similar to the class Counter.