Overview

iterate means to repeat something several times

exactly what we've done in loops.

We can iterate over objects, too. If we implement the __iter__ method.

What an iter method should do, is defined in "The Iterator Protocol" - a concept, present in most modern programming languages.

iterable and the __iter__ method

an object, implementing the __iter__ method is called itarable

The __iter__ method returns an iterator.

iterator is any object with a method called __next__, which is callable without any arguments.

The iterator protocol is changed a bit in Python 3. In the old protocol, iterator objects should have a method called next rather than __next__

iterator and __next__ method

an object, implementing the __next__ method is called iterator

When __next__ method is called, the iterator should return its "next value".

If the iterator has no more values to return, it should raise a StopIteration exception

There is a built-in function called next, wrapping around __next__ that you can use for convenience.

next(it) is equivalent to it.__next__()

Get iterator from iterable

The built-in function iter() can be used to get an iterator from an iterable object.

			iterable = [1,2,3]
			iterator = iter(iterable)


			print( next(iterator) )
			print( next(iterator) )
			print( next(iterator) )

			#1
			#2
			#3
		

Example

			class Fibs:
			  def __init__(self, limit):
			    self.a = 0
			    self.b = 1
			    self.limit = limit
			  def __next__(self):
			    self.a, self.b = self.b, self.a + self.b
			    if ( self.a < self.limit):
			      return self.a
			    else:
			      raise StopIteration
			  def __iter__(self):
			    return self

			fib_numbers = FibsIterable(30)

			for i in fib_numbers:
			  print( i )
		

Resources

Iterator Types @Python3 docs

Overview

A generator is a kind of iterator that is defined with normal function syntax

Or more formally: a Python generator is a function which returns a generator iterator by calling yield

Any function that contains a yield statement can be called a generator!

Making a Generator

Creating generator is like creating a normal function, but using yield instead of return

Once the function yields, the function is paused and the control is transferred to the caller.

			def simple_generator():
			  yield 1
		

Example - even numbers generator

			#define the generator function:
			def numbers_generator(start,end):
			  num  = start
			  while num<=end:
			  	# yield is almost like return, but it freezes the execution
			  	yield num
			  	num += 1
			 
			my_numbers = numbers_generator(1,10)
			# iterate over our generator:
			for i in my_numbers:
				print(i)
			

Example - nested list flatten generator

			nested = [[1, 2], [3, 4], [5]]

			def flatten(nested):
			  for sublist in nested:
			    for element in sublist:
			      yield element

			print( list(flatten(nested)) )
		

Resources

Generators @python wiki

The Task

Write a generator function, such that will yield an even number in pre-given interval

			def even_numbers_generator(start,end):
			  # write your code here
			 
			for i in even_numbers_generator(1,10):
				print(i)

			### expected result
			#2
			#4
			#6
			#8
			#10
		

The Task

Define a generator function which will yield a Cyrillic letter, starting from 'А', to 'Я'

You can get a letter form its code, using the chr() built-in function, as shown in next example

			print( chr(1040) )
			# 'А''

			print( chr(1041) )
			# 'Б'

			print( chr(1070) )
			# 'Ю'
			print( chr(1071) )
			# 'Я'