I've looked at a online courses, and they have examples like the following:
from itertools import count
# creates a count iterator object
iterator =(count(start = 0, step = 2))
# prints an even list of integers
print("Even list:",
list(next(iterator) for _ in range(5)))
... which you could write using range
or np.arange
. Here's another example:
# list containing some strings
my_list =["x", "y", "z"]
# count spits out integers for
# each value in my list
for i in zip(count(start = 1, step = 1), my_list):
print(i)
... which is basically just enumerate
. So my question is: can you give an example of itertools.count
and itertools.islice
that can't be done (or has to be done much more clunkily) using range
?
Here's a situation where the count
instance is used sporadically, not immediately in a single loop.
class Foo:
_x = count() # Infinite supply of unique integer values
def __init__(self):
self._id = f'Foo #{next(self._x)}'
Here's a case where islice
is used to prevent O(n) memory usage:
def is_sorted(some_list):
return all(i <= j for i, j in zip(some_list, islice(some_list, 1, None)))
If you had written that instead as
def is_sorted(some_list):
return all(i <= j for i, j in zip(some_list, some_list[1:]))
you would have had to make nearly a full copy of some_list
before even testing the first pair, which is a huge waste with a large list like [2, 1] + [3] * 10000
.
Neither one is necessary, in the sense that each is trivially definable:
def count(start=0, step=1):
while True:
yield start
start += step
# A more accurate translation would be more complicated than necessary for our purposes here.
# The real version would have to be able to handle stop=None
# and choose 1 and -1 as default values for step, depending
# on whether stop is less than or greater than start.
def islice(itr, start, stop, step):
for _ in range(start):
next(itr)
while start < stop:
yield next(itr)
start += step