Misunderstanding Laziness in Python



Laziness in programming languages is something that has been thrown around a fair amount recently. Certainly it seems like a lot of people want it implemented in their favourite language. This is often after having used it in Haskell, where it is extremely useful, easy to understand, and seemingly without any serious pitfalls.

I'd been playing around with decorators in Python, and laziness seemed like something that would make a good experiment. The actual concept of lazy evaluation isn't really that difficult; I'd already looked at it in ML, and understood the nature of delayed computation. Seeing as python treats functions as high order constructs, i.e values, it isn't difficult to implement the functionality for a function that returns a function, ready to be evaluated when needed.

But this wasn't really enough. I wanted a way for the laziness to be truly transparent, preferably with just the use of a decorator and that's all. For this, the solution wasn't immediately apparent, but python is a flexible language, so I thought there might be a chance. With all this on my mind I spent a long time thinking about lazy evaluation in python and in the end decided on a few things.

  1. It wouldn't be possible to add transparent laziness without manipulating the bytecode extensively or the serious internals of the compiler. Both things I didn't want to do.
  2. Transparent lazy evaluation isn't really a good idea in an imperative language.

I'll start with the first point. Lets look at some examples, starting with my lazy decorator.


def lazy(func):
    def lazyfunc(*args, **kwargs):
        wrapped = lambda x : func(*args, **kwargs)
        wrapped.__name__ = "lazy-" + func.__name__
    return wrapped
return lazyfunc


This is a fairly simple thing - the typical way of emulating lazy evaluation. Given a function, it returns a new function, which when called, evaluates to the original passed function.

We can use this as a decorator in the following.


def hello_add_normal(x, y):
	print "Hello I am Normal!"
	return x + y

def hello_add_lazy(x, y):
	print "Hello I am Lazy!"
	return x + y


Executing the first function will evaluate it, but the second one takes an extra call, used when we require the actual value of the evaluation. In this way we can evaluate the result when we want (or when it is needed, in the case of laziness).


$ hello_add_normal(1, 2)
Hello I am Normal!
$ hello_add_lazy(1, 2)
<function lazy-hello_add_lazy at 0x021E9470>
$ myval = hello_add_lazy(1, 2)
$ myval()
Hello I am Lazy!


As this point, it's perfectly possible for the user programmer to add in this extra call (or an "force" function) whenever they want the evaluation, but I was looking for some way for python to know when it is needed, and to do it automatically.

So really the trick here is to add this extra call at any point when the value of this function is requested. Unfortunately this requires code analysis. Something that is possible in the python runtime, but only at the bytecode level.

Still, perhaps this is enough, all in the name of lazy evaluation!? In the example above it is fairly simple. We'd want to add an extra call before the assignment to myval - after we'd just looked at a symbol we know is lazy. As python code is stack based, it looks like you could just add in an extra "call" bytecode, after the appearance of any "lazy" symbol at the top of the stack. In reality though, things get a bit harder. It is hard to monitor what is on top levels of the stack, and there are various other complications. Take a look at this example.


mylist = [hello_add_lazy(1,2), hello_add_lazy(2,5), hello_add_lazy(6,1)]
hello = mylist[0]


At this point, the only place we have referenced a "lazy" symbol in the bytecode is on the list construction, no where near the place we want to actually do the automatic evaluation. So in this case we'd have to keep track of this "list of lazy things", and look at that too. 

Sure, this example is still possible to solve, and reason about, but with arbitrarily complicated data structures, build in functions, high order functions and various other complicated python constructs, I wasn't feeling like it was worth it.

Bytecode is just too far removed from the semantic model of the language for this kind of analysis. If you wanted to do lazy evaluation seriously it would have to be done in the compiler, which brings me to my second point. Perhaps we don't even want automatic lazy evaluation.

Python has two semantic expressions that are perhaps the strongest in the language, namely assignment "=" and calling "()". Needless to say, messing with these is probably not the best idea. Unfortunately lazy evaluation does both. Lets take a look at another example.


def sleepy(x):
	return x
$ my_other_list = [sleepy(1), sleepy(2), sleepy(3)]


When run, this code sleeps for three seconds then return the list of [1,2,3]. This isn't lazy in the slightest, but would you expect it to do anything else? Perhaps sleeping is a bad example, but list any other number of common side effects including IO and you quickly get an idea of where things could go terribly wrong. The brackets after "sleepy" mean the function is being called, you expect it to be evaluated. Lazy evaluation messes with this notion.

Assignment is made confused too, lets take a look at another example.


the_time = time.time()
next_time = time.time()

if the_time == next_time: print "BOOM"


In this example, I would not want the code to ever print out "BOOM". With evaluation as lazy as possible, it would. Fine, you say, but what if we evaluate on assignment to the variable instead, to keep the ordering correct. Well this is kind of like wanting a mix of the pure and non pure, and in the end, following this argument means that you end up having to evaluate on assignment to ANY data structure, not just variables, and this just isn't lazy at all! Assignment can't mean equivalance and assignment at the same time.

In a language like Haskell, this kind of code would not be possible without monads. This is because in Haskell there are no side effects in functions, no sense of one thing then another. Sleeping, printing, all of that wouldn't be possible to declare in a function, and so with lazy evaluation it isn't an issue. It doesn't confuse when things happen.

This isn't the case in python. Programming without side effects is hard, and not always the preferred method (for me at least). There is nothing wrong with side effects providing you know what assignment and function calling actually mean semantically. And anyway, isn't it clearer and far nicer to know when you're functions are being called?

Lazy evaluation is great in Haskell, but it isn't the kind of "language feature" that can and should be added to languages on a whim. Most of the behaviours of lazy evaluation can be emulated via other means, that I think are essential in their clarity when not dealing with a pure, no side effect, language such as Haskell.


args = [1, (2, 4), 3]
funcs = [sleepy, hello_add_normal, time.sleep]
results = [f(*x) for f,x in zip(funcs, args)]