Trouble understanding python generators

 

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  • What does the “yield” keyword do? 36 answers

    • 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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