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I am new to generator in python. I have a simple enough code which I am playing with but I can not understand the output I am getting out of it. Here is my code :

def do_gen(): for i in range(3): yield i def incr_gen(y): return y + 1 def print_gen(x): for i in x: print i x = do_gen() y = (incr_gen(i) for i in x) print_gen(x) print_gen(y)

I expected my output to be like this :

0 1 2 1 2 3

But I am seeing only : 0 1 2

I do not understand this output. Can anyone please help me sort out my lack of understanding? Thanks in advance.

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  • your generator is exhausted when you reach the last line. Commented Apr 23, 2014 at 16:17

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Generators (like all iterables) can only be iterated over once. By the time print_gen(x) is done, so is x. Any further attempts to get new values from x will result in StopIteration being raised.

This works:

x = do_gen() y = (incr_gen(i) for i in do_gen()) print_gen(x) print_gen(y)

as that creates two separate independent generators. In your version, the generator expression you assigned to y expects x to yield more values.

It is easier to see that the x generator is shared with y when you use the next() function on them in turn:

>>> def do_gen(): ... for i in range(3): ... yield i ... >>> def incr_gen(y): ... return y + 1 ... >>> x = do_gen() >>> y = (incr_gen(i) for i in x) >>> next(x) # first value for the range 0 >>> next(y) # second value from the range, plus 1 2 >>> next(x) # third value from the range 2 >>> next(y) # no more values available, generator done Traceback (most recent call last): File "<stdin>", line 1, in <module> StopIteration

Note the StopIteration raised by next(y) as well here.

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2 Comments

Or rather: generators return an iterator, which is the thing that can only be iterated over once.
@AlexThornton: generally speaking, generators are a specialized iterator.

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