python - Why do GeneratorExit and StopIteration have different base classes? -

i taking @ hierarchy of built-in python exceptions, , noticed stopiteration , generatorexit have different base classes:

baseexception  +-- systemexit  +-- keyboardinterrupt  +-- generatorexit  +-- exception       +-- stopiteration       +-- standarderror       +-- warning 

or in code:

>>> generatorexit.__bases__ (<type 'exceptions.baseexception'>,) >>> stopiteration.__bases__ (<type 'exceptions.exception'>,) 

when go specific description of each exception, can read following:

https://docs.python.org/2/library/exceptions.html#exceptions.generatorexit

exception generatorexit

raised when generator‘s close() method called. directly inherits baseexception instead of standarderror since technically not error.

https://docs.python.org/2/library/exceptions.html#exceptions.stopiteration

exception stopiteration

raised iterator‘s next() method signal there no further values. derived exception rather standarderror, since not considered error in normal application.

which not clear me. both similar in sense not notify errors, "event" change flow of code. so, not technically errors, , understand should separated rest of exceptions... why 1 subclass of baseexception , other 1 subclass of exception?.

in general considered exception subclasses errors, , when write blind try: except: (for instance calling third party code), tried catch exception, maybe wrong , should catching standarderror.

it quite common use try: ... except exception: ... blocks.

if generatorexit inherit exception following issue:

def get_next_element(alist):     element in alist:         try:             yield element         except baseexception:  # except exception             pass  element in get_next_element([0,1,2,3,4,5,6,7,8,9]):     if element == 3:         break     else:         print(element)  0 1 2 exception ignored in: <generator object get_next_element @ 0x7fffed7e8360> runtimeerror: generator ignored generatorexit 

this example quite simple imagine in try block more complex operation which, in case of failure, ignore issue (or print message) , next iteration.

if catch generic exception, end preventing user of generator breaking loop without getting runtimeerror.

a better explanation here.

edit: answering here long comment.

i'd rather opposite. generatorexit should inherit exception rather baseexception. when catch exception want catch everything. baseexception pep-352 states, exceptions need "excepted" in order allow user escape code otherwise catch them. in way can, example, still ctrl-c running code. generatorexit falls category in order break loops. interesting conversation on comp.lang.python.