[Python] Iterator/Generator/Coroutine

Rex Chiang

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Jan 12, 2022

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Iterator

• An iterator is an object that can be iterated upon, meaning that you can traverse through all the values.
• An iterator is an object which implements the iterator protocol, which consist of the methods __iter__ and __next__.
• Strings, lists, tuples, dictionaries, and sets are all iterable objects.

L = ["A", "B", "C"]
my_iter = iter(L)

print(next(my_iter)) # A
print(next(my_iter)) # B
print(next(my_iter)) # C
print(next(my_iter)) # StopIteration

Generator

• Generator functions allow you to declare a function that behaves like an iterator.
• If a function contains at least one yield statement, it becomes a generator function.
• yield statement pauses the function, and saves all its states for later continues from there.

def my_generator(num):
  for n in range(num):
    yield n

  return "END!"

gen = my_generator(3)

print(next(gen)) # 0
print(next(gen)) # 1
print(next(gen)) # 2
print(next(gen)) # StopIteration: END!

Coroutine

• Coroutines have many entry points for suspending and resuming execution.
• Coroutine can suspend its execution, then transfer control to other coroutine and can resume again execution from the point it left off.
• Coroutine can input data, and send it to another coroutine for extra processing.

def my_coroutine1(a):
  print("Start : a = ", a)
  b = yield a
  print("Recieved : b = ", b)
  c = yield a + b
  print("Received : c = ", c)

  return "END!"

coro = my_coroutine1(1)

print(next(coro))   # Start : a = 1
                    # 1

print(coro.send(2)) # Recieved : b = 2
                    # 3

print(coro.send(3)) # Recieved : c = 3
                    # StopIteration: END!

def my_coroutine2(n):
  ret = yield from my_coroutine1(n)
  print("Return From Coroutine1 : ", ret)

  return ret

coro = my_coroutine2(1)

print(next(coro))   # Start : a = 1
                    # 1

print(coro.send(2)) # Recieved : b = 2
                    # 3

print(coro.send(3)) # Recieved : c = 3
                    # Return From Coroutine1 : END!
                    # StopIteration: END!