GeneratorExit in Python generator

Question

I wrote a test program about Python generator. But I got an error that is not expected. And I don\'t know how to explain it. Let me show you the code:

def co         


        

Answer 1

Maybe It's because your's yield value n is inside try blocks which always returns new n reference which make the last n value is garbage collected automatically. It's also state in PEP 342 :

"Add support to ensure that close() is called when a generator iterator is garbage-collected"

"Allow yield to be used in try/finally blocks, since garbage collection or an explicit close() call would now allow the finally clause to execute."

Since close method in generator is equivalent to throw a GeneratorExit and catched by yours exception, then logging.error('GeneratorExit') expression is executed.

"RunTimeError" is raised because the generator is yield next value n(9), it's state in python documentation https://docs.python.org/3.6/reference/expressions.html#generator-iterator-methods :

"Raises a GeneratorExit at the point where the generator function was paused. If the generator function then exits gracefully, is already closed, or raises GeneratorExit (by not catching the exception), close returns to its caller. If the generator yields a value, a RuntimeError is raised. If the generator raises any other exception, it is propagated to the caller. close() does nothing if the generator has already exited due to an exception or normal exit"

May be the code should like this :

#pygen.py

import sys
import logging
logging.basicConfig(level=logging.DEBUG, format='%(asctime)s \
        %(levelname)s - %(message)s', datefmt='[%Y/%m/%d %H:%M:%S]')

def genwrapper(func):
    #makes gen wrapper
    #which automatically send(None) to generator
    def wrapper(n=None):
        f = func(n)
        f.send(None)
        return f
    return wrapper

@genwrapper
def countdown(n=None):
    logging.debug('Counting Down')
    while True:
        try:
            n = yield(n)
        except GeneratorExit as e:
            logging.error('GeneratorExit')
            raise e

if __name__ == '__main__':
    n = int(sys.argv[1])
    c = countdown() #avoid function call in loop block (avoid new reference to c)
     while n > 0:
         a = c.send(n)
         logging.debug('Value: %d', a)
         n -= 1

then in yours terminal :

guest@xxxxpc:~$ python pygen.py 5

will result :

[2018/12/13 16:50:45]         DEBUG - Counting Down
[2018/12/13 16:50:45]         DEBUG - Value: 5
[2018/12/13 16:50:45]         DEBUG - Value: 4
[2018/12/13 16:50:45]         DEBUG - Value: 3
[2018/12/13 16:50:45]         DEBUG - Value: 2
[2018/12/13 16:50:45]         DEBUG - Value: 1
[2018/12/13 16:50:45]         ERROR - GeneratorExit

Sorry for my bad english or my suggestion if not clear enough, thanks