Trouble understanding python generators [duplicate]

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What does the "yield" keyword do in Python? (51 answers)

Closed 9 years ago.

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.