Trouble understanding python generators [duplicate]

Question

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.

Answer 1

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.