Stateful Callbacks in Python

What's a Callback?

If you're unfamiliar with what a callback is, don't worry, we can sort that out quickly. If callbacks are old hat for you you might want to skip to the interesting bit.

Simply put, a callback is a function that is passed as an argument to another function which may execute it.

Take, for example, these functions:

def bar():
    return "I'm the callback!"

def foo(func):
    return func()

If we call foo like this

>>> foo(bar)
"I'm the callback!"

then bar is a callback.

Why Should I Use A Callback?

There are many reasons to use callbacks. For me, the most compelling is customization. Let's take a look at a Python built-in as an example. Say we have a list of users as dictionaries with a name and an age:

users = [
    dict(age=77, name='John Cleese'),
    dict(age=74, name='Eric Idle'),
]

Imagine that we want to sort our users. If we had just a list of ages or a list of names we could easily do this with the built-in sorted function, but by default Python has no idea how to compare our dictionaries during sorting.

>>> sorted(users)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unorderable types: dict() < dict()

Should it sort by age? By name? We need to tell Python how this should be done. Fortunately Python provides and the sorted function has a keyword argument called key that takes, you guessed it, a callback. Let's create some of our own!

def by_age(user):
    return user['age']

def by_name(user):
    return user['name']

Armed with these callbacks we can sort our users.

>>> sorted(users, key=by_age)
[{'age': 74, 'name': 'Michael Palin'}, {'age': 77, 'name': 'John Cleese'}]
>>> sorted(users, key=by_name)
[{'age': 77, 'name': 'John Cleese'}, {'age': 74, 'name': 'Michael Palin'}]

Since the sorted function takes a callback for the key argument we are free to customize its behavior. All we have to do is define a function that returns the key we intend to sort by and as long as that's an orderable type Python will take care of the rest.

What Does It Mean to Have State?

So, by now we have something of an idea of what callbacks are, how we can use them, and why, but what's the point of state? State is most easily described as a memory of prior events. This is the core of what every program does and we use it all the time, even if we don't realize it. Heck, even saving a variable involves keeping track of state.

>>> baz = 1  # The Python interpreter is now tracking the state of 'baz'.
>>> print(baz)  # We can recall that state at a later point.
1

Basically, we need state if we care to remember what happened previously so that we can make decisions about what to do next.

What Normally Happens to State Inside a Callback?

In our first callback function we didn't define any names. To demonstrate what typically happens to state inside the scope of a callback let's make a function that creates some state.

def quux():
    plugh = "xyzzy"
    return plugh

When we execute this function we get the expected result.

>>> quux()
'xyzzy'

After the function is executed we can see that the plugh name is not defined.

>>> plugh
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'plugh' is not defined

This is because when the function is finished executing its frame is removed from the call stack along with any locally defined variables. By itself our callback can't remember anything.

Stateful Callbacks

Alright, so we know what callbacks are, we know what state is. How can we combine the two to make a callback that retains its state? As we saw above we can't rely on any state that we define inside our callback. The trick to making a stateful callback is to rely on names bound to an external scope.

To motivate creating a stateful callback let's say that we still want to sort users like we did above, only now we have 1 Million users. It's going to take a while to sort those users, so it would be nice to have a progress report so we know something is still happening, maybe once per 10,000 users.

Using Functions

To use names bound to an external scope with a plain ol' function as our callback we'll need to take advantage of closures (which could be an entirely separate post). Here's a function that allows us to use our original by_age and by_name sorters while still giving us progress.

def sort_reporter(func):
    state = dict(count=0)  # We can't just call this 'count'...

    def _sort_reporter(user):
        state['count'] += 1  # Because we'd get an UnboundLocalError here.
        if state['count'] % 10000 == 0:
            print("Sorted {count} users.".format(count=state['count']))
        return func(user)

    return _sort_reporter

We can use it like so.

>>> sorted_users = sorted(users, key=sort_reporter(by_name))
Sorted 10000 users.
Sorted 20000 users.
# Lots more of this...

How does it work? The key is the state dictionary. It lets us keep a mutable reference to a name defined outside the scope of the actual reporter function, _sort_reporter. As the sorted built-in is processing our users each new call to _sort_reporter still gets to refer to the original state.

Note: We could avoid having a state dictionary by using Python 3's nonlocal keyword, but then I'd miss an opportunity in the bonus section.

Using Classes

If the functional approach doesn't suit you we can also tackle this problem from an object-oriented angle. Python lets classes define a __call__ method which makes them callable. This isn't strictly necessary for an OOP approach, but when we're making callbacks it's nice to be able to treat our instances as functions.

class SortReporter:
    def __init__(self, func):
        self.func = func
        self.count = 0

    def __call__(self, user):
        self.count += 1
        if self.count % 10000 == 0:
            print("Sorted {count} users.".format(count=self.count))
        return self.func(user)

Just as easy to use as our functional option.

>>> sorted_users = sorted(users, key=SortReporter(by_age))
Sorted 10000 users.
Sorted 20000 users.
# One eternity later...

Conceptually, this works for much the same reason that the functional approach does. The SortReporter instance and all its associated state lives on because the sorted built-in is carrying around a reference to it and it just pretends to be a plain ol' function whenever sorted needs it to be one.

Which Should I Use?

Neither approach is any more or less valid than the other. For this particular example there isn't much more code or complexity either way. I generally regard functions as being simpler than classes, so I prefer those when possible, but classes also provide good structure for more complex callbacks. Try them both!

Bonus

As homework a bonus, try instantiating a SortReporter and examining its __dict__ attribute. Meditate on what you find there and how it relates to the state dictionary in the functional approach.

If you get really bold and want to try for extra credit assign the return value of the sort_reporter function to some variable and examine its __closure__ attribute. This may help you explain why the state dictionary doesn't disappear after the sort_reporter function is called.