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Doing without

XXX - intro text goes here

The joy of order

When you write Python code, one prevailing, deep, unassailled assumption is that a line of code within a block is only ever executed after the preceding line is finished.

pod_bay_doors.open()
pod.launch()

The pod bay doors open, and only then does the pod launch. That's wonderful. One-line-after-another is a built-in mechanism in the language for encoding the order of execution. It's clear, terse, and unambiguous.

Exceptions make things more complicated. If pod_bay_doors.open() raises an exception, then we cannot know with certainty that it completed, and so it would be wrong to proceed blithely to the next line. Thus, Python gives us try, except, finally and else, which together model almost every conceivable way of handling a raised exception, and tend to work really well.

Function application is the other way we encode order of execution:

pprint(sorted(x.get_names()))

First x.get_names() gets called, then sorted is called with its return value, and then pprint with whatever sorted returns. It can also be written as:

names = x.get_names()
sorted_names = sorted(names)
pprint(sorted_names)

Sometimes it leads us to encode the order when we don't need to, as in this example::

total = 0
for account in accounts:
    total += account.get_balance()
print "Total balance $%s" % (total,)

But that's normally not such a big deal.

All in all, things are pretty good, and all of the explanation above is laboring familiar and obvious points. One line comes after another and one thing happens after another, and both facts are inextricably tied.

But what if we had to do it differently?

A hypothetical problem

What if we could no longer rely on the previous line of code being finished (whatever that means) before we started to interpret & execute the next line of code? What if pod_bay_doors.open() returned immediately, triggering something somewhere else that would eventually open the pod bay doors, recklessly sending the Python interpreter plunging into pod.launch()?

That is, what would we do if the order of execution did not match the order of lines of Python? If 'returning' no longer meant 'finishing'?

In other words, how can we deal with asynchronous operations?

How would we prevent our pod from hurtling into the still-closed doors? How could we respond to a potential failure to open the doors at all? What if opening the doors gave us some crucial information that we needed in order to launch the pod? How would we get access to that information?

And, crucially, since we are writing code, how can we write our code so that we can build other code on top of it?

The components of a solution

We would still need a way of saying "do this only when that has finished".

We would need a way of distinguishing between successful completion and interrupted processing, normally modeled with try, expect, else, and finally.

We need a mechanism for getting return failures and exception information from the thing that just executed to the thing that needs to happen next.

We need somehow to be able to operate on results that we don't have yet. Instead of acting, we need to make and encode plans for how we would act if we could.

Unless we hack the interpreter somehow, we would need to build this with the Python language constructs we are given: methods, functions, objects, and the like.

Perhaps we want something that looks a little like this:

placeholder = pod_bay_doors.open()
placeholder.when_done(pod.launch)

One solution: Deferred

Twisted tackles this problem with Deferreds, a type of object designed to do one thing, and one thing only: encode an order of execution separately from the order of lines in Python source code.

It doesn't deal with threads, parallelism, signals, or subprocesses. It doesn't know anything about an event loop, greenlets, or scheduling. All it knows about is what order to do things in. How does it know that? Because we explicitly tell it the order that we want.

Thus, instead of writing:

pod_bay_doors.open()
pod.launch()

We write:

d = pod_bay_doors.open()
d.addCallback(lambda ignored: pod.launch())

That introduced a dozen new concepts in a couple of lines of code, so let's break it down. If you think you've got it, you might want to skip to the next section.

Here, pod_bay_doors.open() is returning a Deferred, which we assign to d. We can think of d as a placeholder, representing the value that open() will eventually return when it finally gets around to finishing.

To say "do this next", we add a 'callback' to d. A callback is a function that will be called with whatever open() eventually returns. In this case, we don't care, so we make a function with a single, ignored parameter that just calls pod.launch().

So, we've replaced the "order of lines is order of execution" with a deliberate, in-Python encoding of the order of execution, where d represents the particular flow and d.addCallback replaces "new line".

A catalog of failures

Of course, programs generally consist of more than two lines, and exceptions add a delicate and pleasing spice to the whole experience.

In what follows, we are going to take each way of expressing order of operations in Python using sequences of lines and try/except and translate them into an equivalent built with Deferred.

This is going to be a bit painstaking, but if you want to really understand how to use Deferreds and how to understand and maintain code that uses Deferreds, it is worth understanding each of these examples.

One thing, then another, then another

Recall our example from earlier:

pprint(sorted(x.get_names()))

Also written as:

names = x.get_names()
sorted_names = sorted(names)
pprint(sorted_names)

What if neither get_names nor sorteds can be relied on to finish before they return? That is, if both are asynchronous operations?

Well, in Twisted-speak they would return Deferreds and so we would write:

d = x.get_names()
d.addCallback(sorted)
d.addCallback(pprint)

Eventually, sorted will get called with whatever get_names finally delivers. When sorted finishes, pprint will be called with whatever it delivers.

We could also write this as:

x.get_names().addCallback(sorted).addCallback(pprint)

Since d.addCallback returns d.

Simple failure handling

We often want to write code equivalent to this:

try:
    x.get_names()
except Exception, e:
    report_error(e)

How would we write this with Deferreds?

d = x.get_names()
d.addErrback(report_error)

"errback" is the Twisted name for a callback that is called when an error is received.

This glosses over an important detail. Instead of getting the exception object e, report_error would get a Failure object, which has all of the useful information that e does, but is optimized for use with Deferreds.

We'll dig into that a bit later, after we've dealt with all of the other combinations of exceptions.

Handle an error, but do something else on success

What if we want to do something after our try block if it actually worked? That is, what if we wanted to do the equivalent of this generic code:

try:
    y = f()
except Exception, e:
    g(e)
else:
    h(y)

Well, we'd write it like this with Deferred:

d = f()
d.addCallbacks(h, g)

Where addCallbacks means "add a callback and an errback at the same time". h is the callback, g is the errback.

Now that we have addCallbacks along with addErrback and addCallback, we can match any possible combination of try, except, else and finally by varying the order in which we call them. Explaining exactly how it works is tricky (although Twisted's Deferred tutorial does rather a good job), but once we're through all of the examples it ought to be clearer.

Handle an error, then proceed anyway

What if we want to do something after our try/except block, regardless of whether or not there was an exception? Abandoning our contrived examples and reaching for generic variable names, we get:

try:
    y = f()
except Exception, e:
    y = g(e)
h(y)

And with Deferred:

d = f()
d.addErrback(g)
d.addCallback(h)

Because addErrback returns d, we can chain the calls like so:

f().addErrback(g).addCallback(h)

The order of addErrback and addCallback matters. In the next section, we can see what would happen when we them around.

Handle an error for the entire operation

What if we want to wrap up a multi-step operation in one exception handler?

try:
    y = f()
    z = h(y)
except Exception, e:
    g(e)

With Deferred, it would look like this:

d = ()
d.addCallback(h)
d.addErrback(g)

Or, more succinctly:

d = f().addCallback(h).addErrback(g)

Do something regardless

What about finally? How do we do something regardless of whether or not there was an exception? How do we translate this:

try:
    y = f()
finally:
    g()

Well, roughly we do this:

d = f()
d.addBoth(g)

This adds g as both the callback and the errback. It is equivalent to:

d.addCallbacks(g, g)

Why "roughly"? Because if f raises, g will be passed a Failure object representing the exception. Otherwise, g will be passed the asynchronous equivalent of the return value of f() (i.e. y).

Unprocessed notes & deleted stuff

Still to do

  • How to catch different types of exceptions
  • Link to Failure API docs
  • Introduction text

Exceptions

Exceptions add a little spice, but it's still pretty clear::

try:
    pod_bay_doors.open()
except:
    # We don't know if they are open or not.  Probably not.
    hal.send_message("Open the pod bay doors, HAL")
else:
    pod.launch()

If an exception is raised during pod_bay_doors.open(), Python itself doesn't give us a way of telling how far into open() our program actually got. All we know is it didn't finish and we need to take special action. We learn the rest from API docs & convention.

Let's dig into this by refining our "normal" example:

def unexpected_error(exception):
    print "Got unexpected exception", str(exception)
    sys.exit(3)

try:
    x.get_names()
except NoNamesFound:
    print "Could not find names"
except:
    e = sys.exc_info()[1]
    unexpected_error(e)

Here's the Deferred-using equivalent:

def no_names_found(failure):
    failure.trap(NoNamesFound)
    print "Could not find names"

def unexpected_error(failure):
    print "Got unexpected exception", failure.getErrorMessage()
    sys.exit(3)

d = x.get_names()
d.addErrback(no_names_found).addErrback(unexpected_error)

Notes

  • No secret ingredient, it's just you No threads No signals No subprocess No bytecode hacks
  • Forget for a moment about UNIX, threads, sockets, network, web
  • What if you couldn't be sure that the thing on one line would run and complete before the thing on the next line?
    • a) you'd need some way, in Python, of defining an order of execution
    • b) you'd need some way of using the result of one thing as input to another thing
    • c) you'd need to handle errors

Themes

  • order of execution vs order of lines
  • operation on values you don't have yet
  • functions are sources of return & raise events

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