Some thoughts on Python

25Sep2008 Filed under: Uncategorized Author: osfameron

I've never studied Python, but I'd quite like to at some point in the future. It seems very much like Perl with a different set of design criteria. Yeah, there are differences, but compare with C, Lisp, COBOL, and you can see why Perl and Python are clustered together in the "P languages" (Perl/Python/P^HRuby etc.)

Actually, that's one of the main reasons I've not expended signifant effort into learning Python - I can hurt my brain far more by trying to learn C or Haskell. But I'm sure it would be a good thing to do at some point. Here are a few notes on first impressions: these are just that, impressions, and may not be factually accurate: if you want to know full and correct details about Python, there are many excellent blogs that will clarify.

2008-09-25 Thanks for the excellent comments, I'll fold in corrections below!

Whitespace

This is the one wart that Perl programmers fairly consistently bring up. I'm coming around to the idea that it's a good idea, but I do think that:

it's not entirely necessary, as most good code is indented anyway
Haskell does it better, with some excellent whitespace inferencing rules, but the possibility to drop back to an explitly { braces; and; semicolons } style if you need it

Things that you miss out on include multiline strings (2008-09-25: Bill Mill points that """syntax""" does indeed allow multiline strings, thanks), and multiline well, anything, for example lambdas, but that's OK because...

Functional Programming

Python has likened to an easy version of Lisp, and it has the same kind of functional goodies as Perl. At some point Guido wanted to remove (I think) reduce, grep, map from the core language, and there was much gnashing of teeth. I think he backed down, but the idea that he suggested it is interesting. He also wanted to get rid of lambdas, which seems utterly crazy, as even the Java people are now thinking about adding them. But lambdas are still deprecated, and are considered not the "pythonic" way of doing things. Hence they are discouraged and restricted to single lines. Hmmmm.

(2008-09-25: Bill Mill points out that map remains; reduce got moved to Standard Lib, as comprehensions already do the right thing; and grep doesn't exist. Is that called "filter" in Python? Of course comprehensions can already to map/grep too)

On the other hand, python has list comprehensions which work equally well on lazy generators! Very nice indeed. It also has a much fuller destructuring bind than Perl's (which only works on single-level lists). So some good, some bad, and some interesting syntax and features. I doubt there's a clear "winner", some more discussion on Perl/Python (via Lisp of course) in Paul Graham's Revenge of the Nerds post.

Oh, and Python has continuations! With a builtin yield command. Perl has the Coro module, which is a fantastic piece of work, but rather complicated with confusing documentation. The yield command itself is in Coro::Generator but it has a number of limitations, like not being able to clone a continuation. There was a lovely post on creating monadic "do-notation" using continuations only, and I ported it to Perl with much swearing only to find that though it worked great for simple monads, it failed utterly for the List monad (important as it's what list comprehensions are built on) precisely because of this. Apparently the Perl internals don't really mesh with this, so it's unlikely that we'll be getting it for Perl 5 any time soon. Point to Python.

(2008-09-25: dolio points out I'm confusing generators and coroutines. Yup, I do that, sorry! I think one can be implemented in terms of each other (and vice-versa?), and that some literature does also blur the distinction, but could be wrong about that too? And yes, Ruby has callcc, but apparently the Monad example I was reading can't be easily ported to Ruby because the continuations/coroutines/thingies generated by yield aren't clonable)

OO

Python's OO looks serviceable enough, and method prototypes and so on are nice. Perl is really catching up here though with projects like Moose, which truly does deliver on being a "postmodern object system". Sure, the syntax could do with some work in places, but the new buzz around Devel::Declare and Method::Signatures means that this is coming on well too.

Will on Geekup posted some nice examples of "declarators on attributes", which I think are similar to lvalue tied methods, but look rather better designed. (From Damian Conway's talk Contextual::Return may fix the problems Perl has with this kind of nice syntax. I say may as with Damian's talks it's hard to tell whether they truly are a massive advance in usability and functionality or crack-fueled insanity). Actually, declarators deserve their own section:

@Declarators

These are really nice syntax for modifying subroutine declarations. Of course, there's nothing you can't do in Perl, but it's prettier than direct symbol table hacking. And less hackish than bloody :attributes. Still, nothing a bit of Devel::Declare or similar sugar couldn't fix :-)

But talking about declarators, I have to ask:

Is Python really more readable?

One of the Python slogans is something like "executable pseudocode", as if you can just write out readable summaries of code which are readable but compile. From the python examples I was looking at (admittedly, these may have been complex, with features like @declarators) I couldn't make head or tail of it. Now I'm not trying to suggest that Python is naturally unreadable, or that Perl is naturally readable. But I would say that

If you know language X but not language Y, then you may find X more readable than Y.
This may hold even if X happens to be Perl, or Y happens to Python :-)

This may be a good time to pimp my Readable Perl slides up, for those people that insist on joking that the phrase is oxymoronic.

Community

Both Perl and Python communities seem to be vibrant, productive (of modules, apps, blogs, and conferences), and to have a mix of helpful and friendly, and snobbish and trollish. So pretty much situation normal for perhaps underrated "scripting" languages :-)

Some differences I've heard mentioned include (with no figures or such to back them up...):

Python conferences are apparently full of people talking about cool projects they've done. At Perl conferences, people often talk about syntax and technical problems and... oh, yeah, I used this to do this project but I don't have time to talk about it now...
I've very rarely heard Perl programmers talk badly about Python. Well OK, we always whine about the whitespace, but I know that jerakeen and Dean have said generally positive things about it. I've had Python people telling me bad things about Perl, despite not knowing anything about modern Perl, which I think is fairly silly, unless they are doing it only for postmodern ironic reasons (which is an excuse apparently).
Of course this may be because Perl programmers pick on PHP and Java instead. But that's OK because we're being postmodern and ironic.
I have an (utterly uninformed) hunch that Python conferences are better attended, but that this may be in part due to there being fewer of them.
Python has Guido. Perl has @Larry (in the plural). (I'm not sure that this isn't overrated, $Larry probably gets the deciding vote).

There is a constant meme that Python is somehow more "modern" than Perl. This is almost never qualified, I remember in this interesting book on programming language design that it was mentioned that the fact that Python's regular expressions lived in an external module was a more modern design that Perl's, which had them as builtins. Perhaps Perl not building in support for declarators and list comprehensions is more modern too? I'm sure if I say this often enough, it'll be a good substitute for actually making sense :-)

But Python does seem to get more buzz, with things like Google's AppEngine supporting Python before Perl. Hey ho. I still think that there are many more Perl jobs going, for what that's worth.

Modules

CPAN is a big advantage for the Perl community, and I believe that Python's module ecosystem is somewhat less complete. But it looks very impressive nonetheless:

Off the top of my head, there's Twisted, Python's equivalent to POE; Django (Catalyst); and PyGame (possibly comparable to SDL_Perl?) as fairly high-profile complex libraries that show, to be honest, that both languages are pretty healthy with regards to module support.

(2008-09-25: Bill Mill points out PyPI link, and admits it's "still no CPAN, but it's coming along", which is more or less what I was trying to suggest ;-)

So...

Python looks like a great language, with many of the same strengths of Perl. If you've looked at Perl and it didn't really click, try Python. And vice-versa. If you've not used a dynamic language and want to try, then use one. Honestly, you'll love it. (And if you don't, try the other one. Or Ruby. Or go crazy and learn Haskell.)

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11 Responses to “Some thoughts on Python”

dolio
September 25th, 2008 at 2:54 am

Are you sure Python has continuations? I was under the impression that it had generators (which your stuff about the 'yield' keyword seems to indicate you're talking about), but those are not full first-class continuations (and Coro sounds like coroutines, which are also not continuations).

It's possible that things have changed without my hearing about it, though (I don't watch Python very closely).

Ruby, for instance, has continuations (see Kernel#callcc or something of that sort).
Bill Mill
September 25th, 2008 at 5:30 am

> Things that you miss out on include multiline strings

errmmm... no you don't.

def foobar(): return """this is a long string that is triple-quoted therefore it is a multiline string"""
Bill Mill
September 25th, 2008 at 5:36 am

> Guido wanted to remove (I think) reduce, grep, map from the core language, and there was much gnashing of teeth.

Nope, reduce got moved to the standard library instead of the builtin functions because list comprehensions are now the built-in way to perform reduces. map() is still in the builtins and there's never been a grep() function, I'm not sure what you're talking about there.

> CPAN is a big advantage for the Perl community

PyPI is still no CPAN, but it's coming along.
Pawel Murias
September 25th, 2008 at 10:02 am

lambda's in python don't really mix with the indentation oriented syntax, the none-deprecated equivalent is def somename(): pass foo(somename)

if you can make up enough meaningfull names, it's better ;)
GTof
September 25th, 2008 at 12:37 pm
About Python, Ruby and the monads, i think it's the opposite. Monads on Ruby are very simple thanks to http://github.com/aanand/ruby-do-notation/tree/master and the blocks of Ruby. The impementation in http://www.valuedlessons.com/2008/01/monads-in-python-with-nice-syntax.html have two problem :

1 - he copies the generator by replaying it from the beginng, it's not very efficient. 2 - lambdas in python are restraint to only expression

Furthermore when trying to implement monad in Python i noticed some more issues. Imagine you want a function :

def foo(a): return do x >= (lamnda x : N(a))

'a' occrus in a lambda in an expression, that's bad !

I tried to use the same technique as in Ruby but cecause of the complexity of the Python bytecode i gaved up.

If you want you can use the code i've done to simulate do-notation. It works but code need to be in strings which are parsed, transformed and interpreded. it's not as beatiful as in Ruby.

#

THE DO NOTATION

#

import re import compiler

PARSE ASSIGNMENT "x <- m"

doasgn = re.compile(r"^\s(\w+)\s> (" + dolist(l[1:]) + ")")

TRANSORM A DO BLOCK INTO A BIND EXPRESSION

do = lambda s : dolist(dostmt.findall(s))

COMPILE A DO BLOCK INTO A CODE EXPRESSION

cdo = lambda s : compiler.compile(do(s),'compiledo.py','eval')

PYTHON HAS PROBLEMS TO SEE LOCAL VARIABLES WHEN EVALUATING :

#

def test(x) :

return eval(do("""

List.mreturn(x)

""")

#

print test(5).run()

#

Will give the good result but :

#

def test(x) :

return eval(do("""

y <- .....

List.mreturn(x)

""")

#

print test(5).run()

#

Will give the error : NameError: global name 'x' is not defined

To avoid that, we build a closure from the list of local variables

with (lambda x : ...)(locals()["x"])

So x is in the context of the expression with the good value in locals

def closure(v,m): l = v for x in m.keys() : l = "((lambda " + x + " : " + l + ")(m[\"" + x + "\"]))" return "(lambda m : " + l + ")"

JUST TO EVALUATE THE BLOCK

def rundo(g,l,s): return eval(closure(do(s),l),g,l)(l)

#

MONADES

#

class Monad: value=None def init(self,val): self.value=val
```
@staticmethod
def mreturn(val):
    raise NotImplementedError

@classmethod
def sequence(self,l):
   if len(l) == 0:
      return self.mreturn([])
   else :
      return rundo(globals(),locals(),"""
         x &lt;- l[0]
         q &lt;- self.sequence(l[1:])
         self.mreturn([x] + q)
        """)

@classmethod
def foldM(self,f,i,l):
  if len(l) == 0 :
    return self.mreturn(i)
  else :
    return rundo(globals(),locals(),"""
       x &lt;- f(i,l[0])
       self.foldM(f,x,l[1:])
      """)

def bind(self, func):
    raise NotImplementedError

def __ge__(self, bindee):
    return self.bind(bindee)

def __rshift__(self, bindee_without_arg):
    return self.bind(lambda _ : bindee_without_arg)
```
#

MONADE LISTE

#

def concat(x) : res = [] for y in x : res.extend(y) return res

class Liste(Monad): @staticmethod def mreturn(val) : return Liste([val])
```
@staticmethod
def mreturnL(val) :
    return Liste(val)

def bind(self,func):
    def fval(x):
       return func(x).value

    return Liste(concat(map(fval,self.value)))

@staticmethod
def zero():
    return Liste([])
```
def guard(b): if b : return Liste.mreturn(()) else : return Liste.zero()

EXEMPLES

test = eval(do(""" x <- Liste(["1" , "2" ]) y <- Liste(["3" , "4" ]) Liste.mreturn (x + " and " + y) """))

print test.value

Another point. Continuation and may yield be différent on one other point i think. When you want to use delimited continuation, i wonder if it's possible to implement shift/reset with yield. Maybe yes.

Good post anyway :)
Bill Mill
September 25th, 2008 at 5:38 pm

@dolio python 2.5 gained generators that can be sent values; this makes them isomorphic to coroutines.

Are coroutines isomorphic to continuations?
Bill Mill
September 25th, 2008 at 5:43 pm

> Is that called "filter" in Python?

ahha - yes. Guido likes filter when used with builtins, but thinks a list comp is clearer than filter with a lambda.
dolio
September 26th, 2008 at 2:08 am

Bill Mill: I'm relatively sure that continuations are more powerful than coroutines, although I don't have any specific examples.
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October 3rd, 2008 at 10:32 am

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Brock
October 6th, 2008 at 7:02 pm

Coroutines are "one-shot" continuations. That is, with full continuations you can re-activate the same bundle of state multiple times. With coroutines you can stop-and-bundle some state, but you can only resume it once.

Perl's Coro provides coroutines, not continuations.

Interestingly enough, for one interesting use of continuations -- inverting the control of web applications ala Seaside -- it turns out that coroutines are Good Enough (tm).
Sal
February 22nd, 2009 at 8:57 am

I think whitespace is most understood. It is usually seen as a gross feature when you start perusing the language. However once you are have some momentum and actually start using the language, you will come to realize that it is one of its strengths; Code just seems to flow and productivity increases. Long time Perl master Eric Raymond discovered this as well (http://www.linuxjournal.com/article/3882).

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