Dynamically-scoped variables for Python
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stackful

stackful provides a variant on Lisp-style dynamic variables for Python. They're akin to Perl’s local variables but with less tendency to degrade into spaghetti. Use it with care, and you can quickly defeat many common cases of coupling without having to refactor the world.

Introduction

“It’s hard to make predictions—especially about the future.”—Markus Ronner

Any software framework will inevitably have holes. There will be points of customization you’ll wish you had, assumptions you’ll wish you hadn’t, and chains of dependency you wish you could just poke through without the maintenance drawbacks of globals. stackful gives you a new kind of variable which, used prudently, lets you overcome some of the common instances of framework shortsightedness without having to refactor your (or worse, somebody else’s) framework.

stackful is a one-trick pony. It lets you transform a global variable into one whose value is set on a per-call-stack basis. You can say...

from stackful import stackful

with stackful('some_global', 8):
    foo()

...and some_global will, for the extent of the with block, be converted into a so-called “stackful” variable which has the value 8. That new value will be seen only on this thread and only until the with exits: foo(), if it reads the some_global variable, will see the 8, as will anything foo() calls. However, other threads will continue to see the old value, if any. When the with exits, even this thread will see the variable go back to its original value.

What could this possibly be good for?

Globals can pass data around easily, but it's equally easy for that cross-cutting of layers to descend into chaos. stackful lets you add temporal limits to globals, making them easier to reason about. Here are a few examples.

Slicing singletons

Let’s say you’re deep in a web application. You have a database connection string in a global configuration variable, and you’re using a model framework like Django 1.1’s that supports only a single database at a time. But you also have a second database: an archival one which you still sometimes need to hit from production. You can’t just change the global variable and then invoke your ORM; you’ll screw up the other threads. And if you try to replace the global with a threading.local() object, all the code that looks at it it will have to change to get the threadlocal value out of the object. You’re stuck.

But stackful can save you! A simple transformation of the global variable to a stackful one during the archival operations is all you need:

from stackful import stackful

with stackful('db', 'postgres://archival.example.com'):
    do_archival_stuff()

For the length of the with block, the whole call stack from here down sees the db variable point to the archival database. There’s no need to pick through the entire ORM, retrofitting every method to take a connection string parameter. After the with exits, everything goes back to normal. And the whole time, other threads continue to see the original value unperturbed.

Tunneling parameters

It’s happened to us all—we find ourselves deep in a chain of function calls and, at in the innermost one, find that we’re missing some bit of data. What to do? We could change the signature of every function from here to the very top level to make sure the needed arg is passed in, but sometimes that’s not the best course of action. Those with a penchant for the grotesque might add a global variable, but that’s thread-unsafe and leaves open the possibility of forgetting to clear it when we’re done, leaking oddball parameters into the next invocation of the innermost function.

stackful to the rescue again! By making a global, giving it some obviously invalid value, and then making it stackful, we make forgetting to initialize it obvious while keeping its value private to the call stack where it was set. Here’s how it looks:

from stackful import stackful

rogue_param = None

def outer():
    global rogue_param
    with stackful('rogue_param', 9):
        middle()

def middle():
    inner()

def inner():
    global rogue_param
    print rogue_param + 1

outer()

The above prints 10. Again, everybody’s thread-safe, and once the with exits, rogue_param goes back to being None.

Incidentally, we don’t do it above, but we can even stack new values on top of the old for multiple concurrent invocations of our inner function with different values. Just call stackful() again on the same global.

Comparison with Perl’s local variables

Our “stackful” variables are a little different in that we don’t actually introduce any new symbols into called functions’ frames. Instead, we simply arrange call-stack-dependent values for symbols they already reference. This provides the useful properties of Perl’s local scoping but makes client code easier to reason about statically.

How it works

As the closest thing to an arbitrary block in Python, the with statement is a natural fit for setting the scope of stackful variables within a lexical scope. In the context handler implemented by stackful(), we walk up to the calling stack frame (using inspect) and replace the reference to the specified global with a special Proxy object. The main purpose of the Proxy is to make sure each thread (and thus each call stack) sees its own separate copy of the variable. However, since Python’s threading.local object requires an attribute access to get at its threadlocal value, we need to find a point at which to perform that access. Python gives us no way to do this upon simply reading a module-level symbol. Perhaps we could have registered a trace handler which examined the nearby instructions and interposed its magic at the right moment, but I assumed that would be an unacceptable performance hit. And code in running stack frames is immutable, so inserting cleanup instructions was out. So, instead, we harness the fact that almost everything you can do with a value involves operating on it, and Python lets us override the behavior of almost every operation. Proxy overrides __add__, __eq__, __getattribute__, and every other special method to present a convincing front of being identical to the thread-appropriate value of the stackful global. There are a few holes if you’re doing heavy-duty introspection, but, in practice, it works very well.

Caveats

Since this is a pure-Python implementation, there were some limits to the kinds of lies we could tell. Here are stackful’s constraints:

  • It works only with globals at the moment (which may not be such a bad thing from a static reasoning standpoint).
  • If someone rebinds a stackful global, it will cease to be stackful; Python gives us no opportunity to intercept the rebinding. Thus, it's best to stick to read-only values and ones that get mutated in place.
  • There are a few introspections we can’t paper over:
    • The obj is other_obj object identity test. Understandably, the interpreter goes straight to pointer comparison here for speed.
    • type(obj). There’s just no escaping this, but code should be using isinstance() for type testing, and other uses are pretty niche. isinstance() looks at __class__, and we do fake that.
  • I haven't even thought about wrapping old-style classes. Maybe it works, and maybe it doesn't.

Genesis

This started as a bit of a stunt during a "hack day" at Open Source Bridge 2011. I'd found myself reaching for Lisp-style dynamic vars from time to time for a few years and decided, more as a technical challenge than because it was a good idea, to try hacking them onto Python. Please keep this in mind if you decide to use stackful.

Version history

1.0
  • Initial release upon an unsuspecting world. Doubtless full of horrible bugs.