leejunkim

Iterator vs Iterable

3 min read

  • Iterable
    • An object that has an __iter__ method which returns an iterator, or which defines a __getitem__ method that can take indexes
      • lists, strings, tuples, files, etc
    • So an iterable is an object that you can get an iterator from
  • Iterator
    • an object with state that remembers where it is during iteration
    • object that defines how to actually do the iteration—specifically what is the next element
    • has a __next__ method that
      • returns the next value in the iteration
      • updates the state to point at the next value
      • keeps returning next item until it raises a StopIteration error
    • an object used to iterate over an iterable
  • Example
    • "HELLO" is an iterable, not an iterator
    • iter("HELLO") returns an iterator

Example

Look, a for loop. Looks like a job for an iterator… Let’s get one. … There’s this obj guy, so let’s ask him.

“Mr. obj, do you have your iterator?” (… calls iter(obj), which calls obj.__iter__(), which happily hands out a shiny new iterator _i.)

OK, that was easy… Let’s start iterating then. (x = _i.next() … x = _i.next()…)

  • Basically, when you have a for loop like for c in "helloworld", the string "helloworld is an iterable, and Python automatically gets an iterator from it by calling the __iter__() method
    • on each iteration of the loop, Python calls the __next__() method of the iterator, which keeps track of the character in the string

Custom for loop

 
""" 1st ver """
 
def my_for(iterable):
    it = iter(iterable) # getting the iterator
    # for i in range(len(iterable)):
    #     print(next(it))
    while True:
        try:
            print(next(it))
        except StopIteration:
            print("end of iterator!")
            break
 
my_for("hello")
# h
# e 
# l
# l
# o
# end of iterator!
 
""" 2nd ver - with func """
# we add a function that we'd like to do
def my_for(iterable, func):
	it = iter(iterable) # getting the iterator
	while True:
		try:
			i = next(it)
		except StopIteration:
			break
		else:
			func(i)
my_for([1,2,3], print)
# 1
# 2
# 3
 
def square(x):
	print(x*x)
 
my_for([1,2,3], square)
# 1
# 4 
# 9

Custom iterable/iterator

 
""" Making a custom Iterable Class - using iter"""
 
class Counter:
    def __init__(self, low, high):
        self.low = low
        self.high = high
 
    # iter(Counter) will return a iterator, which will have a 'next'
    def __iter__(self):
        return iter(range(self.low, self.high))
    
for n in Counter(50,55):
    print(n)
 
# lkim@fedora:~/Desktop/python_practice$ python iterators.py 
# 50
# 51
# 52
# 53
# 54
 
""" Making a custom Iterable Class - using custom class"""
 
class Counter:
    def __init__(self, low, high):
        self.low = low
        self.high = high
 
    # iter(Counter) will return a iterator, which will have a 'next'
    def __iter__(self):
        return CountIterator(self.low, self.high)
 
class CountIterator:
	def __init__(self, low, high):
		self.current = low
		self.high = high
		
	def __next__(self):
		if self.current < self.high:
			cur = self.current
			self.current += 1
			return cur
		raise StopIteration
    
for n in Counter(50,55):
    print(n)
 
# lkim@fedora:~/Desktop/python_practice$ python iterators.py 
# 50
# 51
# 52
# 53
# 54
 
""" Making a custom Iterable/Iterator Class - Counter"""
# we can have BOTH by letting __iter__ return self
 
class Counter:
	# both classes have the same init method, so ig we can merge them
    def __init__(self, low, high):
        self.current = low
        self.high = high
 
	# the iter function returns the object, which refers to the same instance in memory, which ALSO has __next__ defined
    def __iter__(self):
        return self 
        
	def __next__(self):
		if self.current < self.high:
			cur = self.current
			self.current += 1
			return cur
		raise StopIteration
    
for n in Counter(50,55):
    print(n)
 
# lkim@fedora:~/Desktop/python_practice$ python iterators.py 
# 50
# 51
# 52
# 53
# 54

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