Iterators

Provide a construct to yield successive items
Also called enumerators

for i in range(10):
  print(i)

for i in [1,2,3]:
  print(i)

for character in "Hello, World!":
  print(character)

“True” Iterators

Many languages provide a way to iterate over objects
A true iterator provides a way to iterate without storing intermediate data structures

# Never creates a list of items in memory
for i in range(10):
  print(i)

# Creates a list of items in memory, then iterates over them
for i in [0,1,2,3,4,5,6,7,8,9]:
  print(i)

Python Iterator

Includes an __iter__ method that returns an iterator
Includes a __next__ method that returns the next item

Duck Typing

We can identify the type of an object by its properties
If it walks like a duck and quacks like a duck, it is a duck
We can implement and iterator without inheriting from an iterator class

def range(start, stop=None, step=1):
    if stop == None:
        stop = start
        start = 0

    current = start

    results = []

    while current < stop:
        results.append(current)
        current += step

    return results

Generators

Provide a generalized construct for creating iterators
We can yield successive values

def range(start, stop=None, step=1):
    if stop == None:
        stop = start
        start = 0
    
    current = start
    
    while current < stop:
        yield current
        current += step

def squares():
  i = 1
  while(True):
    yield i*i
    i += 1

results = squares()
for i in range(20):
  print(next(results))

def primes():
  current = 2

  while True:
    if is_prime(current):
      yield current
    current += 1