python 语法杂记 -- 迭代器、生成器、上下文管理器

>>> from collections import Iterable

>>> isinstance((), Iterable)        # 元组
True
>>> isinstance([], Iterable)        # 列表
True
>>> isinstance({}, Iterable)        # 字典
True
>>> isinstance('abc', Iterable)     # 字符串
True
>>> isinstance(100, Iterable)       # 数字
False

>>> hasattr((1, 2, 3), '__iter__')
True
>>> hasattr((1, 2, 3), 'next')  # 有 __iter__ 方法但是没有 next 方法，不是迭代器
False
>>>
>>> hasattr([1, 2, 3], '__iter__')
True
>>> hasattr([1, 2, 3], 'next')
False
>>>
>>> hasattr({'a': 1, 'b': 2}, '__iter__')
True
>>> hasattr({'a': 1, 'b': 2}, 'next')
False

>>> from collections import Iterator
>>> isinstance((), Iterator)
False
>>> isinstance([], Iterator)
False
>>> isinstance({}, Iterator)
False
>>> isinstance('', Iterator)
False
>>> isinstance(123, Iterator)
False

>>> from collections import Iterator
>>> isinstance(iter([1, 2, 3]), Iterator)  # 使用 iter() 函数，获得迭代器对象
True
>>> isinstance(iter('abc'), Iterator)
True

for x in [1, 2, 3]:
    print i

it = iter([1, 2, 3])

while True:
    try:
        x = next(it)
        print x
    except StopIteration:
        # 没有后续元素，退出循环
        break

class Fib(object):
    def __init__(self):
        self.a, self.b = 0, 1

    # 返回迭代器对象本身
    def __iter__(self):
        return self

    # 返回容器下一个元素
    def next(self):
        self.a, self.b = self.b, self.a + self.b
        return self.a

def main():
    fib = Fib()    
    for i in fib:
        if i > 10:
            break
        print i

>>> def generator_function():
...     print 'hello 1'
...     yield 1
...     print 'hello 2'
...     yield 2
...     print 'hello 3'
>>>
>>> g = generator_function()  # 函数没有立即执行，而是返回了一个生成器
>>> g.next()  # 当使用 next()(或 next(g))的时候开始执行，遇到 yield 暂停并返回
hello 1
1
>>> g.next()    # 从原来暂停的地方继续执行
hello 2
2
>>> g.next()    # 从原来暂停的地方继续执行，没有 yield，抛出异常
hello 3
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

>>> def fib():
...     a, b = 0, 1
...     while True:
...         a, b = b, a + b
...         yield a
...
>>> f = fib()
>>> for item in f:
...     if item > 10:
...         break
...     print item
...
1
1
2
3
5
8

In [2]: def generator_function():
   ...:     # test send()
   ...:     value1 = yield 0
   ...:     print('value1 is ', value1)
   ...:
   ...:     # test throw()
   ...:     try:
   ...:         yield 'Normal'
   ...:     except ValueError:
   ...:         yield 'Error'
   ...:
   ...:     # test close()
   ...:     yield 1
   ...:     yield 2
   ...:     yield 3
   ...:

In [3]: g = generator_function()

In [4]: next(g)
Out[4]: 0

In [5]: g.send(10)
value1 is  10
Out[5]: 'Normal'

In [6]: g.throw(ValueError)
Out[6]: 'Error'

In [7]: next(g)
Out[7]: 1

In [8]: g.close()

In [9]: next(g)
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-9-5f315c5de15b> in <module>()
----> 1 next(g)

StopIteration:

from math import sqrt, pow

class Point(object):
    def __init__(self, x, y):
        print 'initialize x and y'
        self.x, self.y = x, y

    def __enter__(self):
        print "Entering context"
        return self

    def __exit__(self, type, value, traceback):
        print "Exiting context"

    def get_distance(self):
        distance = sqrt(pow(self.x, 2) + pow(self.y, 2))
        return distance

with Point(3, 4) as pt:
    print 'distance: ', pt.get_distance()

# output
initialize x and y   # 调用了 __init__ 方法
Entering context     # 调用了 __enter__ 方法
distance:  5.0       # 调用了 get_distance 方法
Exiting context      # 调用了 __exit__ 方法

with Point(3, 4) as pt:
    pt.get_length()        # 访问了对象不存在的方法

# output
initialize x and y
Entering context
Exiting context
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-216-ab4a0e6b6b4a> in <module>()
      1 with Point(3, 4) as pt:
----> 2     pt.get_length()

AttributeError: 'Point' object has no attribute 'get_length'

def __exit__(self, type, value, traceback):
    print "Exception has been handled"
    print "Exiting context"
    return True

with Point(3, 4) as pt:
    pt.get_length()      # 访问了对象不存在的方法

# output
initialize x and y
Entering context
Exception has been handled
Exiting context

from contextlib import contextmanager

@contextmanager
def point(x, y):
    print 'before yield'
    yield x * x + y * y
    print 'after yield'

with point(3, 4) as value:
    print 'value is: %s' % value

# output
before yield
value is: 25
after yield