Python iterator, iterable有什么区别

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An ITERABLE is:

• anything that can be looped over (i.e. you can loop over a string or file)
• anything that can appear on the right-side of a for-loop:for x in iterable: ...
• anything you can call with iter() have it return an ITERATOR:iter(obj)
• an object that defines __iter__ that returns a fresh ITERATOR, or it may have a __getitem__ method suitable for indexed lookup.

An ITERATOR is:

• an object with state that remembers where it is during iteration
• an object with a __next__ method (Python 3; next before) that:
  • returns the next value in the iteration
  • updates the state to point at the next value
  • signals when it is done by raising StopIteration
• an object that is self-iterable (meaning that it has an __iter__ method that returns self). (一个iterator有必要有这个方法吗？那它不就是iterable?最令人困惑的是一个iterator只能在for循环中使用一次。)

The builtin function next calls the __next__ (Python 3) or next (Python 2) method on the object passed to it.

For example:

>>> s = 'cat'      # s is an ITERABLE
    	           # s is a str object that is immutable
            	   # s has no state
               	   # s has a __getitem__() method 

>>> t = iter(s)    # t is an ITERATOR
       		   # t has state (it starts by pointing at the "c"
               	   # t has a next() method and an __iter__() method

>>> next(t)        # the next() function returns the next value and advances the state
'c'
>>> next(t)        # the next() function returns the next value and advances
'a'
>>> next(t)        # the next() function returns the next value and advances
't'
>>> next(t)        # next() raises StopIteration to signal that iteration is complete
Traceback (most recent call last):
...
StopIteration

>>> iter(t) is t   # the iterator is self-iterable