Generator vs. iterator in Python

An iterator in Python serves as a holder for objects so that they can be iterated over; a generator facilitates the creation of a custom iterator.

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The main differences

Apart from the obvious syntactic differences, the following are some noteworthy differences:

Generator Iterator
Implemented using a function. Implemented using a class.
Uses the yield keyword. Does not use the yield keyword.
Usage results in a concise code. Usage results in a relatively less concise code.
All the local variables before the yield statements are stored. No local variables are used.

Examples

Let’s create a generator and an iterator to generate all the non-negative numbers up to the specified positive number.


1. Generator

Remember that a return statement terminates the execution of a function entirely, whereas, the yield statement saves the function state and resumes its execution, from this point, upon subsequent calls.


Implementation

# Function to generate all the non-negative numbers
# up to the given non-negative number
def upto(n):
  for i in range(n+1):
    # The yield statement is what makes a function 
    # a generator
    yield i
for number in upto(5):
  print(number)

2. Iterator

The __iter__() method returns the iterator object. This is required to allow an iterator to be used with the for and in statements.

The __next__() method returns the next element in the sequence. In the case of a finite iterator, once it reaches the end (defined by the termination condition), all of the subsequent calls to this method should should raise an exception.


Implementation

# Function to generate all the non-negative numbers
# up to the given non-negative number.
class UpTo:
    # giving the parameter a default value of 0
    def __init__(self, max = 0):
        self.max = max
    def __iter__(self):
        self.n = 0
        return self
    def __next__(self):
        # The next method will throw an
        # exception when the termination condition is reached.
        if self.n > self.max:
            raise StopIteration
        else:
            result = self.n
            self.n += 1
            return result
for number in UpTo(5):
    print(number)

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