Python Generator(what,yield and example)

What??

Generators simplifies creation of iterators.

A generator is a function that produces a sequence of results instead of a single value.

def yrange(n):

    i = 0

    while i < n:

        yield i

        i += 1

Each time the yield statement is executed the function generates a new value.

>>> y = yrange(3)

>>> y

<generator object yrange at 0x401f30>

>>> y.next()

0

>>> y.next()

1

>>> y.next()

2

>>> y.next()

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

StopIteration

So a generator is also an iterator. 

You don’t have to worry about the iterator protocol.

The word “generator” is confusingly used to mean both the function that generates and what it generates. 

 I’ll use the word “generator” to mean the generated object and “generator function” to mean the function that generates it.

Can you think about how it is working internally?

When a generator function is called, it returns an generator object without even beginning execution of the function. 

When next` method is called for the first time, the function starts executing until it reaches yield statement. 

The yielded value is returned by the next call.

The following example demonstrates the interplay between yield and call to next method on generator object.

>>> def foo():

...     print "begin"

...     for i in range(3):

...         print "before yield", i

...         yield i

...         print "after yield", i

...     print "end"

...

>>> f = foo()

>>> f.next()

begin

before yield 0

0

>>> f.next()

after yield 0

before yield 1

1

>>> f.next()

after yield 1

before yield 2

2

>>> f.next()

after yield 2

end

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

StopIteration

>>>