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生成器与迭代器

Fluent Python Chapter 14读书笔记

可迭代对象

An iterable is an object that has an __iter__ method which returns an iterator, or which defines a __getitem__ method that can take sequential indexes starting from zero. So an iterable is an object that you can get an iterator from.

Python中的对象，只要它定义了可以返回一个迭代器的__iter__方法，或者定义了可以支持下标索引的__getitem__方法，那么它就是一个可迭代对象(Iterable)。简单说，可迭代对象就是能提供迭代器的任意对象。

It’s important to be clear about the relationship between iterables and iterators: Python obtains iterators from iterables.

The most accurate way to check whether an object x is iterable is to call iter(x) and handle a TypeError exception if it isn’t. 

Is Sentence(hello) iterable? True.

The iter built-in function:

  1. Checks whether the object implements __iter__, and calls that to obtain an iterator.
  2. If __iter__ is not implemented, but __getitem__ is implemented, Python creates an iterator that attemps to fetch items in order, starting from index 0 (zero).
  3. If that fails, Python raises TypeError, usually saying "C object is not iterable", where C is the class if the target object.

迭代器

An iterator is an object with a __next__ and __iter__ method. 迭代器是定义了__next__ 和__iter__方法的对象。

The standard interface for an iterator has two methods:

  • __next__: Returns the next available item, raising StopIteration when there are no more items.
  • __iter__: Returns self; this allows iterators to be used where an iterable is expected, for example, in a for loop.

Since iterators also implement the__iter__ method, they are iterable as well. 

The
time
has
come

The best way to check if an object x is an iterator is to call isinstance(x, abc.Iterator). 

True

Note

可迭代对象不能把自己作为迭代器。也就是说，可迭代对象有实现__next__方法。这遵循了迭代器设计模式：如果可迭代对象实现了__next__，它不能满足多次迭代的需要。正确的做法是将可迭代对象和迭代器分离。

生成器

And Python function that has the yield keyword in its body is a generator function(生成器函数): a function which called, return a generator(生成器) object. The __iter__ method below is a generator function which, when called, builds a generator object that implements the iterator interface, So the SentenceIterator class is no longer needed. A generator expression(生成器表达式) can be understood as a lazy version of a list comprehension: It does not eagerly build a list, but returns a generator that will lazily produce the items on demand. 

<generator object <genexpr> at 0x107a4bb10>


hello
world