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Iterables and Iteration

Iterables and Iteration

Iteration Tools

Batteries Included

  • powerful vocabularly for working with iterators, including enumerate() and sum()
  • itertools module contains many useful functions and generators for processing iterable streams of data

itertools.islice()

  • perform lazy slicing of any iterator
# sudocode: get first 1000 prime integers
from itertools import islice
islice(all_primes, 1000)

itertools.count()

  • unbounded arithmetic sequence of integers

islice and count

from itertools import count, islice

thousand_primes = islice((x for x in count() if is_prime(x)), 1000)
thousand_primes #=> <itertools.islice object at 0x10a0d8530>
list(thousand_primes)[-10:] #=> [7841, 7853, 7867, 7873, 7877, 7879, 7883, 7901, 7907, 7919]

sum(islice((x for x in count() if is_prime(x)), 1000)) #=> 3682913

Boolean Aggregation

  • any(): determines if any elements in a series are true
  • all(): determines if all elements in a series are true

any and all

any([False, False, True]) #=> True
all([False, False, True]) #=> False

# are there any prime numbers in the inclusive range 1328 to 1360
any(is_prime(x) for x in range(1328, 1361)) #=> False

# all of city names are proper nouns with initial uppercase letters
all(name == name.title() for name in ['London', 'Paris', 'Tokyo', 'New York', 'Sydney', 'Kuala Lumpur']) #=> True

zip

  • synchronize iteration across two or more iterables
  • when iterated, yields tuples
sunday = [12, 14, 15, 15, 17, 21, 22, 22, 23, 22, 20, 18]
monday = [13, 14, 14, 14, 16, 20, 21, 22, 22, 21, 19, 17]

for item in zip(sunday, monday):
    print(item)

#=> (12, 13)
#=> (14, 14)
#=> (15, 14)
#=> (15, 14)
#=> (17, 16)
#=> (21, 20)
#=> (22, 21)
#=> (22, 22)
#=> (23, 22)
#=> (22, 21)
#=> (20, 19)
#=> (18, 17)


sunday = [12, 14, 15, 15, 17, 21, 22, 22, 23, 22, 20, 18]
monday = [13, 14, 14, 14, 16, 20, 21, 22, 22, 21, 19, 17]
tuesday = [2, 2, 3, 7, 9, 10, 11, 12, 10, 9, 8, 8]

for temps in zip(sunday, monday, tuesday):
    print(f"min = {min(temps):4.1f}, max={max(temps):4.1f}, average={sum(temps) / len(temps):4.1f}")


# one, long series for sunday, monday, and tuesday
### can lazily concatenate iterables using `itertools.chain`
from itertools import chain
temperatures = chain(sunday, monday, tuesday)

# can then check that all temperatures are above freezing point without memory impact of data duplication
all(t < 0 for t in temperatures) #=> True
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