Trouble understanding python generators -

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• what “yield” keyword do? 33 answers

i new generator in python. have simple enough code playing can not understand output getting out of it. here code :

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

i expected output :

0  1  2  1  2  3 

but seeing : 0 1 2

i not understand output. can please me sort out lack of understanding? in advance.

generators (like iterables) can iterated on once. time print_gen(x) done, x. further attempts new values x result in stopiteration being raised.

this works:

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

as creates 2 separate independent generators. in version, generator expression assigned y expects x yield more values.

it easier see x generator shared y when use next() function on them in turn:

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

note stopiteration raised next(y) here.