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In the previous post we saw an example usage of generators in Python.

In this post, we will take another look at generators, in particular some of the background behind them, namely iterables and iterators.

We will then introduce coroutines, syntactically identical to generators apart from some additional features.

These extra features can be used to achieve single-threaded concurrency, the subject of the last section.

Iterables and iterators

We mentioned before how generators are sort of virtual, on-demand sequences, in contrast to physically stored sequences such as lists, tuples, and strings.

Both kinds of sequences, however, are members of a more general group, iterable objects.

An iterable object is any object with an __iter__() method returning an iterator when called.

An iterator, by definition, has a __next__() method. The first time it is called, the first item is returned, the second time it is called, the second item, etc.; if no items are left, a StopIteration exception is raised. It also has an __iter__() method that returns itself, i.e. every iterator is iterable.

NB: Python provides a couple of builtin shorthands, next() for __next__() and iter() for __iter__() .


Physical (real) sequences

Because physical sequences are stored in memory, they must be finite. Lists, tuples, strings are common examples.

You can call iter() on any of the above, e.g.

x = [1, 2, 3]
it = iter(x)  # list_iterator
# consume the iterator
while True:
    except StopIteration:
        print('No more items left')

Consuming the iterator in this way is actually what happens under the hood when you use a for loop, i.e. it is equivalent to

for item in x:

You can also use a for loop on the iterator directly

it = iter(x)  # once an iterator is exhausted, need to recreate it
for item in it:

You can replace x = [1, 2, 3] with x = (1, 2, 3) or x = '123', the code works in the same way.

Virtual (on-demand) sequences

This group basically contains any iterable that is not a physical sequence.

Because such sequences return items in the sequence one at a time when requested, and do not store all the items in memory, they can be infinite.

There are various builtin iterables that are not physical sequences. Common examples are

  • File, enumerate, and range objects
  • Dictionaries
  • Sets

File and enumerate objects are iterators as well as iterables

x = enumerate(('a', 'b'))  # enumerate object (not a physical sequence)
it = iter(x)
it is x  # True
y = open('')  # file object (not a physical sequence)
next(y)  # `y` is an iterator; gets first line of file

Of course, this works because both enumerate and file objects have order.

Dictionaries and sets, on the other hand, do not have order. It is not surprising then that they are not iterators.

d = {'a': 1, 'b': 2}
next(d)  # TypeError: 'dict' object is not an iterator
s = {1, 2, 3}
next(s)  # TypeError: 'set' object is not an iterator

But if they do not have order, how can they be iterable?

An arbitrary order is used, which is whatever Python decides (thus the order might be different each time the program is run).

s = {1, 2, 3}
it = iter(s)  # set_iterator
next(it)  # no guarantee to be same value next time script is run
d = {'a': 1, 'b': 2}
it1 = iter(d)  # dict_keyiterator
list(it1)  # no guarantee to be same value next time script is run
it2 = iter(d.keys())  # dict_keyiterator
list(it2)  # no guarantee to be same value next time script is run

d.values() and d.items() are also iterable.

However, just because an iterable object has a predfined, fixed order does not mean it is an iterator, e.g. a list is not an iterator

x = [1, 2]
next(x)  # TypeError: 'list' object is not an iterator

This is because you might want to create multiple iterators from the same iterable and iterate through them independently, e.g. in a nested for loop

x = enumerate('a', 'b', 'c')  # x is an iterator
for i, _ in x:
    for j, _ in x:
        print(i, j)
# 0 1
# 0 2
x = [0, 1, 2]  # x is not an iterator
it = iter(x)
it is x  # False
for i in x:
    for j in x:
        print(i, j)
# 0 0
# 0 1
# 0 2
# 1 0
# 1 1
# 1 2
# 2 0
# 2 1
# 2 2

Remember, the sequences in this section are not physical - their values are not stored in memory like the values of a string, list, tuple, etc. are.


Generators are iterables. They are also iterators, which is why they are sometimes referred to as generator iterators.

They are like the virtual sequences discussed above except they are not builtins that come with the Python language, but iterables created by the user.

For example, we can use a generator to create an infinite sequence, e.g. Fibonacci

# generator function
def fib():
    a, b = 0, 1
    while 1:
      yield a + b
      tmp = b
      b = a + b
      a = tmp

gen = fib()  # generator object
it = iter(gen)
it is gen  # True
next(gen)  # 1
next(gen)  # 2
next(gen)  # 3
next(gen)  # 5
next(gen)  # 8
next(gen)  # 13
# infinite for loop!
for x in gen:

or a finite sequence

# generator function
def g():
    for i in range(10):
        yield i ** 2

gen = g()  # generator object
while 1:
    except StopIteration:
        print('No more values')

Generator objects are created from generator functions (like in the above two examples) or generator expressions.

In generator functions, yield is used to produce the values sent when next() is called on the generator iterator.


In Python, from a language point of view, coroutines are basically the same thing as generators. Any distinction between the two is typically somewhat arbitrary and user defined.

However, we cannot begin to discuss coroutines and start understanding them until some new syntax is introduced.

But before doing so, let’s remind ourselves that coroutines are not specific to Python. They are a general concept in computer science:

“Coroutines are computer program components that generalize subroutines for nonpreemptive multitasking, by allowing multiple entry points for suspending and resuming execution at certain locations”. That’s a rather technical way of saying, “coroutines are functions whose execution you can pause” Tall, Snarky Canadian

This pausing of a function’s execution is something we have already seen. It’s what happens when a yield is reached and is how a generator produces values on demand.

So far, we have only seen values pulled from a generator using next().

One of the extra features we need to consider in order to better understand coroutines is pushing values.

The other main extra features are closing the generator, getting the generator to return a value, and exception handling.

Typically, whenever one or more of these extra features is present, we no longer refer to the object as a generator but as a coroutine.


Pushing values, closing the coroutine

# compute running average
def averager():
    total = 0.0
    count = 0
    average = None
    while True:
        term = yield average  # assigning an expression containing `yield`
        total += term
        count += 1
        average = total / count

coro = averager()  # generator object, just like before
next(coro)  # None
coro.send(10)  # 10
coro.send(20)  # 15
coro.send(6)  # 12
import inspect
inspect.getgeneratorstate(coro)  # 'GEN_SUSPENDED'
inspect.getgeneratorstate(coro)  # 'GEN_CLOSED

The way to understand next(coro) is to ignore the assignment and only consider the expression to the right of the equal sign, i.e.

total = 0.0
count = 0
average = None
while True:
    yield average  # ignore everything to the left of `yield`

Thus next(coro) returns None and the coroutine is paused at yield.

averager() computes a running average. As we haven’t given it any values yet, no average can be computed thus next(coro) returning None is coherent.

The way to pass a value x to averager() is coro.send(x).

So why bother with next(coro) at all, and not coro.send(10) straight away?

A coroutine can only accept values when it is suspended. next(coro) primes the coroutine and puts it in a suspended state meaning it is ready to start receiving values (another way to prime it is coro.send(None)).

Once the coroutine is primed, coro.send(10) resumes the coroutine where it was paused at yield average. Before, when yield was at the start of the line, we just moved to the next line in the body of the function definition.

Now, there is an assignment to take care of. What Python does is to take the value sent in, here 10, and assign it to term (you can think of 10 as replacing yield average).

average now evaluates correctly to 10, and because we are in a while loop, we yield average just like we did before when calling next(coro), which is what coro.send(10) returns.

The process is repeated when coro.send(20) is run. The average is updated to take into account the new value of 20, and the updated average is returned by coro.send(20).

Thus we can keep sending values into the coroutine with coro.send(), and get back each time the updated average.

Once we have sent in our last value, we could just carry on with the rest of our program. However, this will leave our coroutine paused. The correct thing to do is to close the coroutine once the last value has been sent in.

This is done using coro.close()

Returning a value

def coro():
    yield 5
    return 10

c = coro()
next(c)  # 5
next(c)  # fall off the end of the function body (no more `yield`)
# Raise `StopIteration` exception just like before
# However, the value returned 10 is available in the exception
c = coro()  # coroutine is exhausted it, make new instance of it
while 1:
        next(c)  # 5
    except StopIteration as e:
        print(f'Value returned by coroutine is {e.value}')  # 'Value returned by coroutine is 10'

We can now update our running average example to return a value (we won’t bother yielding the updated the average as we go along)

def averager2():
    total = 0.0
    count = 0
    average = None
    while True:
        term = yield
        if term is None:
            break  # in order to return a value, a coroutine must terminate
            # normally, i.e. cannot be stuck in an infinite loop
        total += term
        count += 1
        average = total / count
    return {'count': count, 'average': average}

coro = averager2()
next(averager2)  # prime the coroutine
except StopIteration as e:
      print(e.value)  # {'count': 4, 'average': 12.5}

Exception handling

class DemoException(Exception):

def demo_exc_handling():
    while 1:
            x = yield  # equivalent to `x = yield None`
        except DemoException:
            print('Demo exception handled. Continuing...')
            print(f'Coroutine received value of {x}')

coro = demo_exc_handling()
next(coro)  # None
coro.send(34) # 'Coroutine received value of 34'
coro.send(343444) # 'Coroutine received value of 343444'
# 'Demo exception handled. Continuing...'
coro.send('hi') # 'Coroutine received value of hi'

Concurrency and coroutines

Concurrency means doing multiple tasks at once, i.e. given a unit of time, multiple tasks make progress.

Concurrency can be single or multithreaded. Here, we will be looking at single threaded concurrency.

NB: When running tasks on multiple threads, and those threads are on different CPUs, this is parallelism. In Python, although multithreading is supported, because of the Global Interpreter Lock (GIL), multithreading (almost?) always happens on one CPU, thus parallelism is not possible. Only concurrency is possible in Python.

Let’s suppose we have two functions, func1 and func2.

func1 takes two seconds to run. func2 takes three seconds to run, on condition that func1 has finished running. If func1 has not finished running, func2 cannot finish.

The obvious solution is to call func1 first and then func2, leading to a total running time of five seconds.

If we called func2 first, it would never finish and our script would hang forever.

However, with concurrency, func1 and func2 can both make progress in the same time period.

With concurrency, we could start func2 and immediately after start func1.

func1 would finish first after about two seconds. About one second later, func2 would also finish (it has been running for three seconds and func1 has finished running). This gives a total running time of about three seconds.

We could achieve the above by running func1 and func2 in separate threads.

Alternatively, we could use a single thread and coroutines:

import time

# coroutine implementation of `func1`
def coroutine1():
    start = time.time()
    while time.time() - start < 2:
        yield 'Running...'
    yield 'Done'

# coroutine implementation of `func2`
def coroutine2():
    start = time.time()
    coro1_done = False
    while not (time.time() - start >= 3 and coro1_done):
        coro1_done = yield 'Running...'
    yield 'Done'

def run_loop():
    print('Starting event loop...')
    start = time.time()
    coro1 = coroutine1()
    coro2 = coroutine2()
    next(coro2)  # prime `coro2`
    coro1_done, coro2_done = False, False
    current_coro = 'coro1'
    while not (coro1_done and coro2_done):
        time.sleep(0.25)  # check progress every 0.25 seconds
        if current_coro == 'coro1' and not coro1_done:
            status = next(coro1)
            print(f'coroutine1 status: {status}')
            if status == 'Done':
                coro1_done = True
            if not coro2_done:
                current_coro = 'coro2'
        elif current_coro == 'coro2' and not coro2_done:
            status = coro2.send(coro1_done)
            print(f'coroutine2 status: {status}')
            if status == 'Done':
                coro2_done = True
            if not coro1_done:
                current_coro = 'coro1'
    print(f'Event loop finished in {time.time() - start:.2f}s')

Starting event loop...
coroutine1 status: Running...
coroutine2 status: Running...
coroutine1 status: Running...
coroutine2 status: Running...
coroutine1 status: Running...
coroutine2 status: Running...
coroutine1 status: Running...
coroutine2 status: Running...
coroutine1 status: Done
coroutine2 status: Running...
coroutine2 status: Running...
coroutine2 status: Done
Event loop finished in 3.02s

This is just the start of the concurrency story in Python using coroutines. In the next post, we will develop the ideas seen in the above code snippet and in this section further.

Disclaimer: In no way, shape, or form do I claim all the content in this post to be my own work / not copied, paraphrased, or derived in any other way from an external source.

To the best of my knowledge, all sources used are referenced. If you feel strongly about any of the content in this post from a plagarism, copyright, etc. point of view, please do not hesitate to get in touch to discuss and resolve the situation.