Python and the power of 'first class' everything

http://www.itworld.com/development/76397/python-and-power-first-class-everything

by abennett • 17 years ago

33 24 17 years ago

masklinn6• 17 years ago

Of course, not everything is actually first-class in Python.

There's a whole bunch of statements — simple (assert, pass, del, break, continue, import, raise, …) and compound (if, while, for, with, try) — which definitely don't qualify

jerf2• 17 years ago

The list is shorter than that, though.

How we'd override "pass" boggles my mind. Same for break and continue, and mostly raise (though you have control over the exception hierarchy).

del can be modified to some extent by changing the "del" double-underscore method of the target. import can be modified in several ways, which is how the zipimport is implemented (not an atomic capability). if, while, and try are effectively atomic, but "for" loops can be redefined by defining an iterator, and with is specified entirely in terms of an interface that allows you to affect what it does too.

Personally, I don't think the right way to phrase it is as "everything is first-class in Python", though. I see it as "when the language designer wants to put a capability into the language, like, say, importing from zip files, instead of hard coding it into the interpreter, an interface is coded into the interpreter that anybody can use", which unfortunately I don't have a snappy word on hand for. Looking over the history of Python you can even see this evolving, with the increasing openness of "import" and metaclasses being the most obvious examples.

"Everything is first class" would lead you to something like Lisp macros, which Python eschews. (For better and for worse.) However, I for one would wish more languages would learn the Python lesson above; it's so frustrating to me when I see a language implement something like "import from a different source" but still give you no way to use this new capability yourself. If it's convenient and powerful to you as a language designer, it'll be even more convenient and powerful in the hands of all your users.

richcollins2• 17 years ago

Io has no keywords; everything is a message. This includes return, break and continue.

masklinn6• 17 years ago

> How we'd override "pass" boggles my mind.

An expression that does nothing. Putting `''` in a function is equivalent to `pass` in that it lets the parser do its stuff. `pass` could be a builtin defined as `None` and the behavior would be the same as today, without a keyword.

> Same for break and continue

I'm not saying I know how to replace everything (though I'd probably just remove break and continue, they're nice from time to time but not exactly essential), just that there's a lot of stuff which isn't first-class in Python.

> if, while, and try are effectively atomic, but "for" loops can be redefined by defining an iterator, and with is specified entirely in terms of an interface that allows you to affect what it does too.

I don't get what you're trying to say here.

didroe• 17 years ago

>Same for break and continue, and mostly raise

These are all escape continuations.

>which unfortunately I don't have a snappy word on hand for

Abstractions?

dkarl• 17 years ago

Missing: first-class syntax.

arohner2• 17 years ago

and first-class anonymous functions

masklinn6• 17 years ago

unrestricted first-class anonymous functions.

Python already has first-class anonymous functions (`lambda`), but highly restricted ones.

moe• 17 years ago

What do you mean?

silentbicycle3• 17 years ago

Lisp macros.

Whether Python would actually be improved by macros is debatable, though.

tetha2• 17 years ago

Mh, I have to say, this is far less impressive if you think in interfaces everywhere (or haskells type classes everywhere)... This is just something like:

interface TypeConstructor<T,V> { T buildType(V value); }

List<TypeConstructor<T,V>>;

or something like this.

natrius2• 17 years ago

Is this a joke? That's nowhere near as simple.

silentbicycle3• 17 years ago

Haskell is making explicit (and inferring - you don't need to type it!) what Python implicitly assumes will probably work, but checking whether it's actually correct.

The terminology is math-y to the point of exclusiveness because of Haskell's close association with academia* , but there's a point after which it's simpler to make the compiler check than to rewrite the same kinds of tests over and over and over. Type systems are for expressing your requirements in a way that the compiler can do the tests once, at compile time. Computers are good at automating tedious and repetitive work, y'know?

It's too bad most of the people discussing how these things work seem hell-bent on using Greek letters and other insular terminology for everything. Static typing is kind of a wash for smaller projects (I'd just use Lua or Python for something that will only be a few pages of code), but once you start worrying about maintenance or testing changing interfaces between modules, the immediate feedback from a good static type system really comes in handy.

OCaml is a pretty great language, but has some major usability warts and not that much written about it in English. (And Haskell's "avoid success at all costs" motto is...telling.)

hboon• 17 years ago

People should look at Smalltalk if they want "first class" everything.

masklinn6• 17 years ago

first-class almost everything. I don't think "^" is a first-class element in Smalltalk (though I might be wrong)

Python and the power of 'first class' everything

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