Python and the need for speed

Chris Angelico rosuav at gmail.com
Sun Apr 9 05:05:35 EDT 2017


On Sun, Apr 9, 2017 at 5:39 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> On Sun, 09 Apr 2017 13:57:28 +1000, Chris Angelico wrote:
>>>From that page:
>>
>>> Other candidates for banishment from TurboPython include eval and exec.
>>
>> Bye bye namedtuple.
>
> All that would mean is that the implementation of namedtuple would have
> to change. It would probably require some sort of support from the
> compiler, but that's okay, the compiler can do (nearly) anything.
>
> Something as important and useful as namedtuple would not be dropped from
> this hypothetical TurboPython. It would just shift the implementation
> from pure-Python to something else.
>
> exec() is actually only needed for a *tiny* bit of namedtuple. The
> original implementation by Raymond Hettinger takes the easy way out by
> using exec on the entire class definition, but it is trivially easy to
> restrict that to just the class __new__ method:
>
> https://code.activestate.com/recipes/578918-yet-another-namedtuple
>
>
> but even that could be avoided with a minimal amount of help from the
> compiler.

When people talk about making a restricted optimizable subset of
Python, the implication (if not the explicit declaration) is that it's
done strictly by removing, not by rewriting. A couple more quotes from
the article:

> It should be possible to define a subset of the Python language,
> uninspiredly dubbed “TurboPython”, that excludes those features
> that stand in the way of high-performance JIT execution (or
> compilation). Not using some of these features coincides with
> good design practices, so it doesn’t necessarily have to be all bad.
...
> Since TurboPython is a subset of Python, it will also run on Python
> interpreters, albeit slower.

Both of these imply that the standard library of TurboPython is
*exactly* the standard library of CPython, minus the bits that aren't
supported. We just cut a few of those nasty dynamic features out of
the language, and voila! it becomes faster. (Or in this case,
JITable.) There's nothing suggested here about reimplementing existing
features in a different way, with the consequent possibility of having
slightly different behaviour - a Python script is guaranteed to have
the same semantics on TurboPython as on CPython. This is the kind of
description used by Asm.js (http://asmjs.org), defined as a strict
subset of JavaScript that can be implemented very efficiently, but
with an absolute guarantee that the semantics will be identical.

>> And compile() is going to have to go,
>
> Indeed.
>
>
>> since you
>> could implement eval/exec by creating a new function:
>>
>>>>> runme = r"""
>>   print("Hello, world!")
>>   import sys sys.stdout.write("I can import modules.\n")
>> """
>>>>> type(lambda: 1)(compile("def f():" + runme, "exec",
>>>>> "exec").co_consts[0], globals())()
>> Hello, world!
>> I can import modules.
>>
>> So if compile() goes, you also lose ast.parse,
>
> Probably.

The only way to *not* lose ast.parse() is to completely reimplement it
or compile(), which is not a good idea IMO (unless you can somehow
dump a pure-Python compile() out of an existing source), and which is
not at all implied to be part of the proposal.

>> which means you lose introspection tools,
>
> *Some* introspection tools. Python has many that don't rely on compile or
> ast.parse.

Yes, that's what I meant - that there will be introspection tools that
you lose. Sorry for the unclear language. Obviously there's a lot of
introspection that's based on pre-existing features, like the
attributes on functions and code objects.

>> plus you lose literal_eval and friends.
>
> I don't know what "friends" you are referring to, but again, if
> literal_eval is important, the compiler can support it. If you can
> support an entire Python interpreter and compiler in the form of
> compile(), then you can support more restricted subset of the language.
> Writing a parser to evaluate strings, integers, and a few other data
> types is not exactly brain surgery.

Other tools built on top of literal_eval. Though I had the feeling
there were more of them than a 'git grep' has shown up - it's actually
only used in a couple of places in the stdlib. My bad.

But again, you certainly COULD reimplement literal_eval, but then you
have to keep your implementation accurate and up-to-date, else you
risk bugs creeping in. It's a non-trivial task to rewrite these kinds
of things and maintain parallel versions; by calling on compile(), the
current implementation *for free* is kept up-to-date with changes in
CPython's grammar. It wouldn't have needed any changes when the u"..."
prefix was re-added to Python 3.3, for instance, because the AST
didn't change.

>> I'm also
>> not sure whether the import machinery would have to be rewritten, but a
>> quick 'git grep' suggests that it would. Removing eval and exec is not
>> as simple as removing them from the standard library.
>
> Well of course not, but removing eval and exec is only a necessary, not
> sufficient, condition, for enabling a huge bunch of compiler optimizations
> and speeding up Python.
>
> This "TurboPython" would require a completely new implementation of the
> Python interpreter to be fast. It's not as if eval and exec are great
> heavy concrete weights chained to the leg of the compiler, and all you
> need do is remove the chain and the compiler suddenly becomes thirty
> times faster.

Right. But my point is that it _also_ has to be a completely new
implementation of large slabs of the standard library, too. At some
point, it's not "Python minus the bits we can't JIT", but it's "a
complete implementation of a Python-like language with a restricted
stdlib". RPython has already been mentioned.

>> In fact, extreme dynamism is baked deep into the language. You'd have to
>> make some fairly sweeping language changes to get any real benefits from
>> restricting things.
>
> Well, maybe. As is pointed out many, many times, 99% of Python code
> avoids the sorts of extreme dynamism that keeps things slow. Lots of
> people would be satisfied with a language *really close* to Python that
> was ten or twenty times faster, even if it meant that you couldn't write
> code like this:
>
>
> answer = input("What's your name?")
> exec("name = %r" % answer)
> print(name)

Sure. But would they also be happy that dunder methods for operators
have different behaviour? I rather suspect that they'll be a target
early on. And mandatory type hints don't help, because type hinting
guarantees that something is a valid subclass, but optimization
depends on the exact type.

rosuav at sikorsky:~/tmp$ cat demo.py
def add_two_integers(x: int, y: int):
    tot = x + y
    print("I am adding", x, "and", y, "to get", tot)
    return tot

# Simple example
add_two_integers(5, 7)

# Proof that non-integers are rejected
# add_two_integers(5.0, 7.0)
# Uncomment to get an error from MyPy

# Evil example
class Int(int):
    def __add__(self, other):
        return int(self) + other - 1
add_two_integers(Int(5), 7)
rosuav at sikorsky:~/tmp$ mypy demo.py
rosuav at sikorsky:~/tmp$ python3 demo.py
I am adding 5 and 7 to get 12
I am adding 5 and 7 to get 11


The only way to optimize this function is to somehow know that you're
working with actual integers - which probably means you're doing work
that can be handed off to numpy.

> Even better would be if the compiler was smart enough to use the
> optimized, fast runtime when the dynamic features aren't used, and fall
> back on a slower implementation only when needed to support the more
> dynamic features.

Yes. As long as it can know when the more dynamic features are being
used - and that's the hard part. Remember, as soon as you have a
single class implemented in Python, it could have a method injected
into it without your knowledge. Can you detect that statically, or can
you use the versioning of __dict__ to notice that something's been
broken? What makes you fall back to the default implementation?

The problem is the same thing that gives Python a lot of its beauty:
that there's very little difference between built-in types and
user-defined types. In CPython, types implemented in C are immutable,
but other than that, they're basically the same thing as anything you
make, and you can inspect them and stuff:

>>> int.__add__(5, 7)
12

In contrast, JavaScript has a fundamental difference between "stuff
implemented in JavaScript" and "stuff the interpreter gives you". For
example, you can create a constructor function and set its .prototype
attribute to be some object, which is broadly like subclassing that
object; but you can't do that with a Number (the JS floating-point
type). You don't get a subclass of Number that you can then tweak the
behaviour of; you get a completely different thing, one that doesn't
behave like a Number at all. I'm sure a JS expert could tell me how to
make an object that behaves like a Number, but it isn't as simple as
Python's way:

class MyInt(int):
    ...

Pike, too, manages to outperform Python by a notable factor (usually
around 3:1 or better) - and its native integer type is also distinctly
different from its object types. You can't say:

class MyInt
{
    inherit int;
}

You can inherit from the equivalent object form Gmp.mpz, for those
situations where you want an int-like object, but it's not going to
perform as well as the native int does.

Personally, I think that would be a far better avenue to go down. I'm
not sure it would be possible to retrofit this to Python (for example,
there are guarantees about object identity that apply to *all
objects*), but making the core immutable types into value-only
non-objects wouldn't break a lot of code, and might provide for some
significant performance improvements to string manipulation and
arithmetic operations. But at some point, it's not really Python any
more.

ChrisA


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