Subscriptable/Indexable generator

Question

I'm not a Python developper, but I enjoy programming with it, and for a project I wanted to have generators that I can easily index. Using python's slice model is obviously the way to go, and here's the solution I've come up with.

class _SubscriptableGenerator():
 def __init__(self, generator, *args):
 self.gen = generator(*args)
 def __getitem__(self, key):
 try:
 if isinstance(key, int):
 self.ignore(key)
 yield next(self.gen)
 else:
 step = key.step if key.step else 1
 start = key.start if key.start else 0
 i = start
 self.ignore(start)
 while i < key.stop:
 yield next(self.gen)
 i = i + step
 self.ignore(step-1)
 except Exception:
 self.raiseInvalidSlice(key)
 def raiseInvalidSlice(self, key):
 raise KeyError("{0} is not a valid key (only int and [x:y:z] slices are implemented.".format(key))
 def ignore(self, n):
 for i in range(n):
 next(self.gen)

It is not intended to be called by the user of the module, that's internal code. I for example define my generators like so

def _myGen(arg1, arg2):
 while 1:
 yield something

and provide them wrapped in my class

def myGen(*args):
 return _SubscriptableGenerator(_myGen, *args)

I'd like to know what a more pythonic solution would be, if there are things to fix, etc. I am not sure about the way to handle exceptions on the key.

Answer 1

The most Pythonic solution would be to use itertools.islice from the standard library. For example, like this:

from itertools import islice
class Sliceable(object):
 """Sliceable(iterable) is an object that wraps 'iterable' and
 generates items from 'iterable' when subscripted. For example:
 >>> from itertools import count, cycle
 >>> s = Sliceable(count())
 >>> list(s[3:10:2])
 [3, 5, 7, 9]
 >>> list(s[3:6])
 [13, 14, 15]
 >>> next(Sliceable(cycle(range(7)))[11])
 4
 >>> s['string']
 Traceback (most recent call last):
 ...
 KeyError: 'Key must be non-negative integer or slice, not string'
 """
 def __init__(self, iterable):
 self.iterable = iterable
 def __getitem__(self, key):
 if isinstance(key, int) and key >= 0:
 return islice(self.iterable, key, key + 1)
 elif isinstance(key, slice):
 return islice(self.iterable, key.start, key.stop, key.step)
 else:
 raise KeyError("Key must be non-negative integer or slice, not {}"
 .format(key))

Note that I've given the class a better name, written a docstring, and provided some doctests.

• teh internets is made of catz: Nice, I didn't know about this itertools feature. Thanks !