您快要写完 Iterator 类了(我在答案末尾显示了Generator),但是__next__
每次使用调用对象时都会被调用next
,并返回一个generator对象。相反,要使您的代码以最少的更改和最少的代码行工作,请使用__iter__
,这使您的类实例化为可迭代的 (从技术上讲,它不是 生成器 ):
class Fib:
def __init__(self):
self.a, self.b = 0, 1
def __iter__(self):
while True:
yield self.a
self.a, self.b = self.b, self.a+self.b
当我们向传递一个iterable时iter()
,它为我们提供了一个 迭代器 :
>>> f = iter(Fib())
>>> for i in range(3):
... print(next(f))
...
0
1
1
为了使类本身成为 迭代器 ,它确实需要一个__next__
:
class Fib:
def __init__(self):
self.a, self.b = 0, 1
def __next__(self):
return_value = self.a
self.a, self.b = self.b, self.a+self.b
return return_value
def __iter__(self):
return self
现在,由于iter
只返回实例本身,因此我们无需调用它:
>>> f = Fib()
>>> for i in range(3):
... print(next(f))
...
0
1
1
这是带有我的注释的原始代码:
class Fib:
def __init__(self):
self.a, self.b = 0, 1
def __next__(self):
yield self.a # yield makes .__next__() return a generator!
self.a, self.b = self.b, self.a+self.b
f = Fib()
for i in range(3):
print(next(f))
因此,每次调用next(f)
时,都会生成__next__
返回的生成器对象:
<generator object __next__ at 0x000000000A3E4F68>
<generator object __next__ at 0x000000000A3E4F68>
<generator object __next__ at 0x000000000A3E4F68>
您仍然需要实现一个send和throw方法 Generator
from collections.abc import Iterator, Generator
import unittest
class Test(unittest.TestCase):
def test_Fib(self):
f = Fib()
self.assertEqual(next(f), 0)
self.assertEqual(next(f), 1)
self.assertEqual(next(f), 1)
self.assertEqual(next(f), 2) #etc...
def test_Fib_is_iterator(self):
f = Fib()
self.assertisinstance(f, Iterator)
def test_Fib_is_generator(self):
f = Fib()
self.assertisinstance(f, Generator)
现在:
>>> unittest.main(exit=False)
..F
======================================================================
FAIL: test_Fib_is_generator (__main__.Test)
----------------------------------------------------------------------
Traceback (most recent call last):
File "<stdin>", line 7, in test_Fib_is_generator
AssertionError: <__main__.Fib object at 0x00000000031A6320> is not an instance of <class 'collections.abc.Generator'>
----------------------------------------------------------------------
Ran 3 tests in 0.001s
Failed (failures=1)
<unittest.main.TestProgram object at 0x0000000002CAC780>
因此,让我们实现一个生成器对象,并利用Generator
collections模块中的抽象基类(请参见其实现的源代码),这意味着我们只需要免费实现send
并throw
-免费给我们close
,__iter__
(返回自身)和__next__
(与.send(None)
) (请参阅协程的Python数据模型):
class Fib(Generator):
def __init__(self):
self.a, self.b = 0, 1
def send(self, ignored_arg):
return_value = self.a
self.a, self.b = self.b, self.a+self.b
return return_value
def throw(self, type=None, value=None, traceback=None):
raise StopIteration
并使用上述相同的测试:
>>> unittest.main(exit=False)
...
----------------------------------------------------------------------
Ran 3 tests in 0.002s
OK
<unittest.main.TestProgram object at 0x00000000031F7CC0>
美国广播公司Generator
仅在Python 3.要做到这一点没有Generator
,我们需要至少写close
,__iter__
以及__next__
除了我们上面定义的方法。
class Fib(object):
def __init__(self):
self.a, self.b = 0, 1
def send(self, ignored_arg):
return_value = self.a
self.a, self.b = self.b, self.a+self.b
return return_value
def throw(self, type=None, value=None, traceback=None):
raise StopIteration
def __iter__(self):
return self
def next(self):
return self.send(None)
def close(self):
"""Raise GeneratorExit inside generator.
"""
try:
self.throw(GeneratorExit)
except (GeneratorExit, StopIteration):
pass
else:
raise RuntimeError("generator ignored GeneratorExit")