如何编写生成器类?
你已经接近成功了,正在编写一个迭代器类(我在答案末尾展示一个生成器),但是__next__
每次使用next
调用对象时都会被调用,返回一个生成器对象。相反,为了使你的代码能够以最少的更改和最少的代码行数工作,请使用__iter__
,它使你的类实例化一个可迭代对象(这不是严格意义上的生成器):
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
当我们将可迭代对象传递给
iter()
函数时,它会返回一个迭代器。
>>> f = iter(Fib())
>>> for i in range(3):
... print(next(f))
...
0
1
1
为了使类本身成为一个迭代器,确实需要一个
__next__
方法:
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
仅返回实例本身,我们不需要调用它:
>>> f = Fib()
>>> for i in range(3):
... print(next(f))
...
0
1
1
“为什么self.a的值没有被打印出来?”
以下是您的原始代码及我的注释:
class Fib:
def __init__(self):
self.a, self.b = 0, 1
def __next__(self):
yield self.a
self.a, self.b = self.b, self.a+self.b
f = Fib()
for i in range(3):
print(next(f))
每次调用
next(f)
,你都会得到生成器对象,它是由
__next__
返回的:
<generator object __next__ at 0x000000000A3E4F68>
<generator object __next__ at 0x000000000A3E4F68>
<generator object __next__ at 0x000000000A3E4F68>
还有,我如何为生成器编写单元测试?
您仍需要为 Generator
实现 send 和 throw 方法。
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)
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,并利用集合模块中的Generator
抽象基类(请参见其实现源代码),这意味着我们只需要实现send
和throw
- 就可以免费获得close
、__iter__
(返回self)和__next__
(与.send(None)
相同)(请参见Python协程数据模型):
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>
Python 2
ABC Generator
只适用于 Python 3。如果没有 Generator
,我们需要至少编写 close
、__iter__
和 __next__
,以及上面定义的方法。
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")
请注意,我直接从Python 3
标准库复制了
close
,没有进行修改。