Ruby-style Blocks in Python

http://tav.espians.com/ruby-style-blocks-in-python.html

by tav • 17 years ago

46 42 17 years ago

tav.espians.com

inklesspen• 17 years ago

Never gonna happen. As GvR said, 'with' already means something completely different.

Besides, it's not the Python style to have function calls without the () syntax.

I think you could do something interesting with decorators, though.

russell5• 17 years ago

Ditto on that. The two uses of with have completely different meanings. The was an interesting discussion of the use of 'there' as a keyword that ferreted out a lot of issues: http://groups.google.com/group/comp.lang.python/browse_threa...

Sorry about the url. Tinyurl couldnt digest it. :-)

I think 'there' could have good traction, but nobody has made a PEP (see python.org). If a proposal doesnt have a champion, it isn't going anywhere.

thristian• 17 years ago

I wish that thread had progressed to a more complete conclusion - I've been developing software with Twisted Python recently (a notoriously callback-heavy framework) and that syntax would clean up an awful lot of code in our code-base. In particular, I like that it's not a knee-jerk emulation of a particular syntax-feature of another language, but a general solution that works well for Ruby-style maps, defining property getters and setters, and setting up design-by-contract semantics.

I'd happily swap that syntax for some of the other features modern Python has grown, like the "with" statement, or decorators.

aceofspades192• 17 years ago

I don't understand what the use of anonymous functions besides lambdas would be

jrockway3• 17 years ago

The use is the same, it's just that the definition can now be more than one line of code. It's pretty arbitrary to say "you can have one-line anonymous functions, but for two lines, the function must have a name".

No other language does that, because it makes very little sense. This proposal will make Python behave more like every other programming language that supports anonymous functions.

aceofspades192• 17 years ago

I just don't understand where you would need an anonymous function with more then one line

bobbyi• 17 years ago

But how would this new syntax work when you want to pass two or more anonymous functions to another function? Or when the anonymous function is not the last argument?

Having anonymous functions only for the situation where you are passing exactly one of them to another function and it is the last argument is much more arbitrary than the current style of always temporarily binding functions to names.

halo3• 17 years ago

The reason no other language does that is because no other mainstream language uses newline and indentation to indicate blocks, which makes all general-purpose multi-line anonymous function syntaxes inherently ugly and non-Pythonic: where should the new block's indentation begin or end? I also regard it as a relatively minor problem as there's little cognitive overhead from simply naming a multi-line function, and I can see advantages in encouraging naming for complex functions.

Having thought about it for a couple of hours, I don't think there is any good solution for adding multi-line anonymous functions to Python. The suggested syntax feels non-Pythonic, is inherently unexpressive and difficult to read, only useful in a few select use-cases while not solving the holy grail of 'pleasant anonymous functions', and the single problem it's solving isn't common enough or big enough to need dedicated syntax and could already largely be solved with a named function. The fact it enables 'misuse' via Ruby-style iteration, going against Pythonic ideals of 'one way to do things', doesn't work in its favour either.

I don't think analogies with decorators are good either. The advantages of decorators is they transform something otherwise difficult-to-read and unexpressive into something easier-to-read and more expressive, whereas this makes something that's easy-to-read less expressive with the only advantage being terseness. Python, unlike other languages, focuses primarily on clarity over code length.

gaius2• 17 years ago

I don't understand why naming a function is such a problem? You can even do it in the local scope and prefix it _ if you want to.

sah7• 17 years ago

Would you find it problematic to write if and for statements like this?

  def _forbody(x):
    def _ifbody(x):
      print x
    if x % 2: _ifbody(x)
  for x in xrange(10): _forbody(x)

SapphireSun• 17 years ago

I also don't understand why people use lambda like:

  myfunction( lambda a: a.foo > a.bar )

I think the following looks neater:

  aname = lambda a: a.foo > a.bar
  myfunction( aname )

I think that putting unnecessary clutter inside of a function call is just asking for readability problems.

sofal• 17 years ago

If you explored outside of your boundaries and learned different ways of programming (like functional programming), you might realize that there are perfectly valid styles of programming where lambdas are as common as if-statements and for-loops. I think that it is reasonable to assume that creating named local variables for each and every one of them would be silly.

lacker• 17 years ago

Why don't you make a PEP or suggest this to the python mailing list? Might be more effective than blogging for this sort of thing.

tav3• 17 years ago

@lacker I did mail python-dev, but wanted to get general public criticism too...

relme• 17 years ago

Ruby style blocks would have made the with_statement (and a bunch of other stuff) obsolete. In fact most stuff can be implemented from blocks (like decorators and generators) and not the other way around.

jackdied• 17 years ago

"Ruby does it this way and people like it" isn't an argument that will work. You can accomplish the same things in Python with an extra line of overhead.

If you want to get this past python-dev you'll need to explain the benefits, here's a start:

1) Lower cognitive load: Users can understand blocks but the extra naming/reference is one too many things for their brain stack to handle.

2) More descriptive: with blocks the first line says "I'm about to define a function for use with X" instead of the existing way which says "I'm defining a function. Now I'm using that function with X."

3) Show me: take a chunk of an existing project (stdlib is best) and rewrite it with the proposed block syntax. You get to cherry pick the example so if the new version doesn't read much cleaner than the old version maybe the idea isn't so hot.

I'm -1 on the idea but if you want to change my mind you have to make an argument that doesn't start with "In Ruby..."

tav3• 17 years ago

@jackdied that's brilliant advice!! thank you -- will bear in mind for the future.

sah7• 17 years ago

A similar feature was already proposed and rejected: http://www.python.org/dev/peps/pep-0340/

Anyway, I'm kind of sad that people seem to want a straight copy of Ruby's blocks. I'd prefer syntax that allowed passing more than one block to a function, like the C-like syntax I proposed here: http://www.hackerdashery.com/2006/10/code-blocks-and-c-like-...

tav3• 17 years ago

@sah Nice! Sadly it wouldn't ever fly in the Python context, but I like it nevertheless!

russell5• 17 years ago

Actually 340 was withdrawn in favor of the 'with' statement, so blocks haven't been ruled out, as far as I can see. However, it will have to be Pythonic (no braces of block terminators) and well thought out.

Jebdm• 17 years ago

I was wondering if the one-block limit bothered anyone else. Maybe something like Haskell's where clause:

  map(foo, pairs) where:
    pairs = [(0,1), (20, 10), (30, 10)]
    def foo(pair):
      a = pair[1] * 2
      b = pair[0] * a
      return (b, a)

I'm not terribly familiar with Python's grammar, so it might be difficult to introduce a closing where-clause without ambiguity. In that case, it might be useful to use a do clause as well:

  do:
    <statements>
  where:
    <statements>

Another alternative would be to simply create a real function literal syntax (i.e., lambda that doesn't suck).

jamongkad2• 17 years ago

The only time I see where Python will adopt a lambda that doesn't suck, is the day when the community itself embraces functional programming. At the rate that is going and from what I've read from the commenters of the article. I don't think that's happening anytime soon.

tdavis• 17 years ago

If Python were made to look like Ruby, I'd have to find a new language. I don't want to do that. Luckily, this will never actually happen.

sah7• 17 years ago

Obviously no reasonable person wants Python to be identical to Ruby, since those who like Ruby better can go use it. But Python has over its lifetime incorporated new ideas from other languages. Giving substantive reasons why you don't like this particular idea would be a lot more interesting than just registering your distaste for it.

anamax• 17 years ago

Python's lambdas aren't really functions. They're callable expressions.

Once you understand that, a lot of the "issues" go away.

critic• 17 years ago

I've had a little bit of wine, so you'll have to pardon my honesty: I think Python is a crap language with a great syntax. I'd rather use C++ when performance is very important, or Haskell, when it's not.

globalrev• 17 years ago

This has been up so many times. It's not happening, live with it. Python "lambdas" can do most things you need anyway.Normally I don't really find the need for supercomplicated lambdas and you can do if-else in lambda s in Python with the ternary operator, see below:

####### def throwaway_function(emp): if emp.salary > developer.salary: return fireEmployee(emp) else: return extendContract(emp)

employees.select(throwaway_function) ######

Python lambda: filter(lambda emp: fireEmployee(emp) if emp.salary > developer.salary else extendContract(emp), employees)

Don't know what select does but it sounds like filter. So if that works as intended(is the ruby version both returning value and side-effecting?) or not I'm not sure but you probably could make it.

Ruby-style Blocks in Python

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