Understand the iterator and iterable

Preface:

Before iterator iterable this section it has been some confusion, with some information, to express my understanding of these concepts may not have to, but for the beginning of my friends to start from scratch to understand

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Straight to the point first introduced iterables popular understanding and iterators

　　An iterator is just be next () call to get the next iteration of the value of the object, the iterator does not directly save the sequence value iteration, and iteration of the algorithm stored value obtained under

　　Iterables is to be iter () method call to get iterable objects, only iterables available for a for loop

 

The principle underlying the for loop:

　　The following is a for loop format

 

　　for I in iterables: 
    loop

　　The method essence is to call a built-ITER () obtained iterator object, then the iteration value i is obtained by each next (iterator) call

　　For example the following code:

for i in x:
    print(i)

　　It can be rewritten as:

iterator = iter(x)
while True:
    i = next(iterator)
    print(i)

 

Strictly speaking, or underlying principle

　　Iterator or iterator object: implements the __next __ () method Magic (class instantiated) object, the method returns the next value iterator

　　Iterables: __ implements the __iter magic methods (class instantiated) object (), which returns an iterator object

　　

In fact, next and iter is a further package __next__ and __iter__ or say next and iter when they are called to perform the features within __next__ and __iter__ in order to achieve the iterator and iterable and function

 

In summary, we can understand the for loop, iterables, iterators three contact:

　　The for loop iterations can only be objects, iterables they need to implement the iterator

Thus, we can implement a custom iterators and iterator object, in order to achieve the effect of the cycle for custom　

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Example:

Fibonacci series to achieve cut through the iterator and iterable

They are defined iterator class and object belongs iteration:

class On Feb: 
　　'' 'may be defined in the object's class iteration' ''
     DEF  the __init__ (Self, NUM): indicates the length of the number of the #NUM column 
    　　self.a =. 1 
    　　self.b, = 2 
　　 　　self.current = self.a 
　　 　　self.num NUM =
     DEF  the __iter__ (Self):
         return FebIterator (Self)
 class FebIterator (Self): 
　　'' 'defined iterator class' ''
     DEF  the __init__ (Self, Source): 
        self.source = Source
     DEF  __next__ (Self): 
　　 　　
　　 　　IF ( . 1-self.num> = 0): 
　　　　 　　self.num = self.num. 1- 
    　　　　self.current =  self.a
    　　　　Self.a = Self.b, 
    　　　　self.b, = + self.current self.b,    # above two intermediate variable assignment may be omitted to directly write self.a, self.b, self.b, =, + self.a Self, B 
　　 　　　　return Self. curent 
　　 　  the else: The raise StopIteration

By calling these two classes to instantiate objects can be achieved circulation print Fibonacci number

feb = Feb()

for i in feb:
    print(i)

In fact, for simplicity and convenience, can simultaneously achieve the __iter __ () and the __next __ () method in a class, the class that is instantiated objects is only one iteration iterator objects, as follows:

class Feb:
    def __init__():
        self.a = 1
        self.b = 2
　　 　　sele.current = self.a
    def __iter__(self):
        return self
    def __next__():
　　 　　if(self.num-1>=0):

　　　　 　　= self.num. 1-self.num 
    　　　　self.current = self.a 
    　　　　self.a = self.b,  　　　　self.b, self.current # + = self.b, above two intermediate variable assignment may be omitted to directly write self.a , self.b, self.b, =, + self.a Self, B 
　　 　　　　return self.curent 
　　 　 the else: The raise the StopIteration

Prior to its use and there is no difference, but more simple, provided there is a clear understanding and awareness of the iterator and iterable, or would confuse the two concepts