有人可以告诉我如何理解下面的python程序（我们使用__iter __（）做什么？）

Question

can someone tell me how that can happen?

I have a program like this:

class Fib(object):

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

    def __iter__(self):
        return self 

    def next(self):
        self.a, self.b = self.b, self.a + self.b 
        if self.a > 100000:
            raise StopIteration();
        return self.a 

if I enter :

>>> for n in Fib():
...     print n

the output is:

1
1
2
3
5
...
46368
75025

the question is that I have no idea how __iter__ is related to next(self), and how computer read the program?can someone help me explain this program?

Answer 1

Check this out

 class Fib:                                        ①
     def __init__(self, max):                      ②
         self.max = max

     def __iter__(self):                           ③
         self.a = 0
         self.b = 1
         return self

     def next(self):                           ④
         fib = self.a
         if fib > self.max:
             raise StopIteration                   ⑤
         self.a, self.b = self.b, self.a + self.b
         return fib                                ⑥

Thoroughly confused yet? Excellent. Let's see how to call this iterator:

from fibonacci2 import Fib
for n in Fib(1000): 
    print n,

0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987

Here's what happens:

The for loop calls Fib(1000), as shown. This returns an instance of the Fib class. Call this fib_inst. Secretly, and quite cleverly, the for loop calls iter(fib_inst), which returns an iterator object. Call this fib_iter. In this case, fib_iter == fib_inst, because the iter () method returns self, but the for loop doesn't know (or care) about that. To “loop through” the iterator, the for loop calls next(fib_iter), which calls the next () method on the fib_iter object, which does the next-Fibonacci-number calculations and returns a value. The for loop takes this value and assigns it to n, then executes the body of the for loop for that value of n. How does the for loop know when to stop? I'm glad you asked! When next(fib_iter) raises a StopIteration exception, the for loop will swallow the exception and gracefully exit. (Any other exception will pass through and be raised as usual.) And where have you seen a StopIteration exception? In the next () method, of course!

Answer 2

In Python, there is the concept of iterables and iterators. Loosely, an iterable is anything with an iter method. An example is a list. When a for loop is ran on this list, say:

for i in range(5):
        print i

Python returns an iterator, which automagically calls produces all the result.

Now an iterator(not to be confused with iterables) by itself is any object with an iter function and a next method. When this iterator loops, it calls the next method on every loop, which basically produces the next output, much like a for loop produces outputs within the range.

When the next call reaches the last element, a StopIteration error is raised, ending the loop. This is very analogous to how a for loop works behind the scenes.

In a nutshell, what your code is doing is that it is creating an iterator, and defining what happens every time the iterator wants to fetch the next value ie the next() method.

Answer 3

Python has iterators, similar to Java's Iterator and C#'s IEnumerable(T).

Read this:

https://docs.python.org/2/library/stdtypes.html#iterator-types

https://www.python.org/dev/peps/pep-0234/