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Lazy Evaluation for Python

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Lazy Evaluation for Python

This package implements something like lazy evaluation for Python - and it does so in 100% Python code. Installation is standard:

easy_install lazypy

I test and work with it under Python 2.6, so that’s where I can say it works with newer versions. For older versions you lose forked futures, since multiprocessing is part of Python starting with 2.6. But it might be possible to still make it work with versions as far back as Python 2.3.

ATTENTION: if you used versions before 0.5, you had a different module name LazyEvaluation instead of of the current lazypy. I changed this with 0.5 but kept the LazyEvaluation package name around as a proxy for old code. This old package name is deprecated, though, and will go away in one of the future versions. One breaking change is that __version__ is only available through the lazypy package, not the old compatibility package.

What is lazy evaluation or promises?

Lazy evaluation is a way to encapsulate a calculation without actually computing it - it will only be computed when the result of that calculation is actually accessed. After the calculation is done, further access to the lazy calculation will just return the cached result.

Of course, since Python doesn’t support lazy evaluation natively and since there aren’t enough hooks in the interpreter to do something like this in Python at all, this is faked lazy evaluation. What it actually does, is wrapping function calls in objects that will force the function call result at the latest possible moment.

Regardless of what happens, a Promise should produce the same error message you would get without the Promise - just the code location should be different. This should be the case - but I can’t guarantee it yet. If you notice a situation where a Promise gives some different exception from the object itself in the same situation, please notify me.

There are several ways to get lazy evaluation in your code. The primary way is to use either the lazy/delay functions or to subclass LazyEvaluated or to use the LazyEvaluationMetaClass as a metaclass to your own class.

Using delay

>>> from LazyEvaluation import delay
>>> def func(a, b):
...     return a+b
>>> res = delay(func, (5, 6))
>>> print repr(res)
>>> print res

This will print out a Promise instance in the first print statement and a number in the second print statement. This happens because print calls the __str__ method on objects passed in (or actually it uses the str builtin or something much like that one - on some objects it will call __str__ and on others like strings it will just return the string itself).

Using lazy

>>> from LazyEvaluation import lazy
>>> def func(a, b):
...     return a+b
>>> func = lazy(func)
>>> print repr(func(5,6))
>>> print func(5,6)

This will print the Promise instance in the first print and the number 11 in the second print. The function ‘lazy’ turns any function into it’s lazy equivalent. It can be used as decorator in Python 2.4 and up.

Using LazyEvaluated

>>> from LazyEvaluation import LazyEvaluated
>>> class TestClass(LazyEvaluated):
...       def test(self, a, b):
...           return a+b
>>> print repr(TestClass().test(5,6))
>>> print TestClass().test(5,6)

This will print the result number. To use LazyEvaluated will turn your class into a lazy evaluated class. Only the direct attributes will be turned into lazy evaluated methods, though! It’s just a handy way if you don’t have a full class hierarchy but just a single class you want to turn into something that evaluates lazy. It’s probably not the best way to do this.

Using LazyEvaluatedMetaClass

>>> from LazyEvaluation import LazyEvaluatedMetaClass
>>> class TestClass(object):
...       __metaclass__ = LazyEvaluatedMetaClass
...       def test(self, a, b):
...           return a+b
>>> print repr(TestClass().test(5,6))
>>> print TestClass().test(5,6)

This will make all function attributes of your class into lazy evaluated methods. It’s mostly identical to the above sample, only that it doesn’t use inheritance. It might be usefull to build subclasses to already existing classes whose direct function attributes are evaluated lazy.

Some bits on the semantics

Lazy evaluation for Python behaves a bit different from what fully lazy languages behave: it’s not really full lazy evaluation but just deferred evaluation. Results are forced to be evaluated at the latest possible moment. To achieve this, the Promise class implements many standard magic methods and implement them by first forcing the deferred evaluations (so getting their real value) and applying the builtin method to those values.

It has special handling for binary operator methods in that it first tries the primary method that was called and if that either doesn’t exist or returns NotImplemented it will run the other method with reversed arguments. This should work for most situations where binary operators are used.

Sometimes you might have the need to force evaluation without using any of the normal ways to force evaluation - for example to store a forced value somewhere. You can use the force(value) function for this:

>>> from LazyEvaluation import force
>>> def anton(a,b):
...     return range(a,b)
>>> f = lazy(anton)
>>> l = f(1,10)
>>> l = force(l)
>>> print l

There is one speciality in the behaviour of promises that can produce problems in your code: getattr and setattr don’t force it’s object! calling setattr on the promise actually will setattr to the promise object itself, not to the forced value. So if you set an attribute value on a promise and later force it, that forced value won’t have the originally set attribute. The main reason for this is that overloading __setattr__ in promise classes is rather hairy - setattr handles all attribute setting and a promise does have some instance variables (== attributes). If you want to set attributes on values from a promise, you allways must force the value yourself.

How to have new behaviour

Sometimes the builtin promise class Promise isn’t what you need. If you want to build your own, you have two ways of doing it: either just subclass the Promise class and add your needed stuff or write your own class.

Your own promise class must adhere to the Promise interface. It must define a __init__ method that accepts a function, an argument list and an optional argument dictionary. And it must define a __force__ method that forces it’s evaluation. And it must use PromiseMetaClass as it’s __metaclass__ as that one will fill in all the needed magic methods. If it needs to change the way some magic method operates, it can just define that method locally - the metaclass will automatically skip that predefined method.

An example for a different behaviour is the module: this implements futures, a high-level-concurrency concept. Instead of just delaying the computation until the __force__ is called, a thread is started that computes the result in the background. If you try to force a Future, your call will block until the result is ready. Exceptions are catched and later reraised, too. This allows very easy parallism in your code - just push some computation into the background and if it is ready when you access it, your code will just go on. Only if it isn’t already fullfilled will you have to wait for it.

Starting with lazypy 0.3 there are ForkedFutures, too. Those behave much like the normal futures, but run in a separate process. This allows writing code that makes better use of multicore systems, since multiple processes allow getting around the global interpreter lock. These ForkedFutures make use of the multiprocessing module in Python 2.6, so won’t be available with older python versions (and their test cases will fail on older versions).

To make use of futures, you can just use the spawn/future pair of functions that behave exactly like delay/lazy - spawn is a parallel version of apply and future is a decorator that turns any callable into a parallel version of itself. To use ForkedFutures, just pass the ForkedFuture class as the class to be used for the future in those calls.

There is an additional pair of functions fork/forked that use those forked futures by default. Remember that they are all just syntactic sugar for the same concepts - you can use delay, spawn or fork interchangeably by passing the correct target class. Same goes for the decorators.

So what to use - lazy, future or forked?

Here is a very simple rundown on the three different variants on promises implemented in lazypy and when to use them:

  • delay/lazy is best used when you want to capture a state of computation but not run it at the time of capturing, but instead decide later when to actually compute it (or wether to not compute it at all). This could be some lenghty computation that only will make sense later in your code when you discover some special circumstances. You could get very similar results with closures, delay and lazy are just more uniform idioms for that.
  • spawn/future is best used when you have something that can run alongside the main thread to compute some lengthy stuff. Since this uses threads and since Python has the GIL, this is not really performance enhancing. But it can be useful if your main thread mostly waits on stuff and the computation can run in parallel to prepare the result that you later need.
  • fork/forked is best used when you need to make use of multiple cores and want some lengthy calculation run alongside the main process to prepare a result you will need later. Since full processes are used, you can make full use of your multiple cores to speed up some calculations that are parallelizable.

In general, all three promise variants could be replicated by using the underlying mechanisms directly (lazy evaluation can be simulated using closures, futures can be just implemented with the threading or the multiprocessing modules directly). lazypy only gives a nice syntactic wrapper around it and a way to add parallelism or lazy evaluation to code that is allready there - without the need of changing the consuming code. It can be especially useful if you work in a functional style with Python or if you want to annotate existing classes to get lazy or future semantics on some of it’s methods.

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