PEP 3107 Function Annotations for review and comment

[Tim Smith]

here's a potentially nifty way of adding decorators to input args for python:

  def a(int(arg1), arg2, tuple(arg3)):
    #arg1 is an int (or was converted to an int)
    #arg2's type is not known (ie this decoration is optional)
    #arg3 is a tuple (may have been a list coming in, but is now a tuple)
    pass

this would add optional conversion of input arguments to desired types
(you could drop the parens, making it more like standard type syntax, but I put them there to intone that the int() method will be called on every arg1 coming in, and so on)

this would also add ability to write your own conversion functions to handle type checking as arguments come into a function

should add little to no overhead (as you are likely doing this manually like so if desired:
  def a(arg1, arg2, arg3):
    arg1 = int(arg1)
    arg3 = tuple(arg3)
    pass

addendum:
  any type conversion method should throw ValueError on failure (this would allow python to catch this error and throw a new exception (InputArgError) or something

so this:
  def a(int(arg1), arg2, tuple(arg3)):
    pass

would more or less translate to this:
  def a(arg1, arg2, arg3):
    try:
      arg1 = int(arg1)
      arg3 = tuple(arg3)
    except ValueError:
      raise InputArgError("what went wrong")
    pass

it would likely be desired to create some extra builtin functions like:
  convdict, convlist, convtuple, that if input is already this type, will return the input unmodified, (as opposed to calling a constructor on dict, list, tuple to create a whole new object (copying all the data))

another nice side effect of this is it adds the ability to call by value instead of by reference:
  def a(list(b)):
    pass #call by value

  def a(convlist(b)):
    pass #call by reference (unless input type wasn't list)

 -- Tim



-- On 12/30/06 "John Roth" <JohnRoth1 at jhrothjr.com> wrote:

> BJörn Lindqvist wrote:
> > On 12/29/06, Tony Lownds <tony at pagedna.com> wrote:
> > > Rationale
> > > =========
> > >
> > > Because Python's 2.x series lacks a standard way of annotating a
> > > function's parameters and return values (e.g., with information about
> > > what type a function's return value should be), a variety of tools
> > > and libraries have appeared to fill this gap [#tailexamp]_.  Some
> > > utilise the decorators introduced in "PEP 318", while others parse a
> > > function's docstring, looking for annotations there.
> > >
> > > This PEP aims to provide a single, standard way of specifying this
> > > information, reducing the confusion caused by the wide variation in
> > > mechanism and syntax that has existed until this point.
> >
> > I think this rationale is very lacking and to weak for such a big
> > change to Python. I definitely like to see it expanded.
> >
> > The reference links to two small libraries implementing type checking
> > using decorators and doc strings. None of which to seem to be very
> > popular in the Python community. Surely, those two libraries *alone*
> > can't be enough of a motivation for this? To me, it is far from
> > self-evident what purpose function annotations would serve.
> >
> > I also wonder why a very obtrusive syntax addition is needed when it
> > clearly is possible to annotate functions in today's Python. Why is
> > syntax better than just adding a function annotation decorator to the
> > standard library?
> >
> >     @annotate(a = int, b = dict, c = int)
> >     def foo(a, b, c = 5):
> >         ...
> >
> > Are decorators too ugly?
> >
> > --
> > mvh Björn
> 
> The problem I have with it is that it doesn't solve the problem
> I've got, and I can see some user requests to use it rather than
> the metadata solution I've got now in Python FIT. Neither do
> decorators, by the way.
> 
> So, what are the problems I see?
> 
> First, it only handles functions/methods. Python FIT needs
> metadata on properties and assignable/readable attributes
> of all kinds. So in no sense is it a replacement. Parenthetically,
> neither is the decorator facility, and for exactly the same reason.
> 
> Second, it has the potential to make reading the function
> header difficult. In the languages I'm familiar with, static type
> declarations are a very few, somewhat well chosen words.
> In this proposal, it can be a general expression. In Python
> FIT, that could well turn into a full blown dictionary with
> multiple keys.
> 
> Third, it's half of a proposal. Type checking isn't the only use
> for metadata about functions/methods, classes, properties
> and other objects, and the notion that there are only going to
> be a small number of non-intersecting libraries out there is
> an abdication of responsibility to think this thing through.
> 
> I should note that there are quite a few packages out there
> that use some form of annotation, be they comments
> (like Ned Bachelder's coverage analyzer and the two
> lint packages I'm aware of), docstrings, decorators or
> auxilliary dictionarys (like Python FIT, and a possible
> Python version of Naked Objects). They include a
> fair number of documentation packages.
> 
> On a positive note, what I'd like is something similar to
> Java's Javadoc, but a bit looser. It could be a comment
> convention like Javadoc, but one that the compiler recognizes
> and stashes in the compiled .pyc / .pyo file. Or it could have
> different syntax. What is SHOULDN'T have is a mandatory tie
> to function/method syntax.
> 
> Combined with a convention to identify which annotation
> belongs to who, it could be a quite useful mechanism.
> I, for one, have no difficulty with the notion of using someone
> else's annotations if I can identify them unambiguously.
> 
> John Roth
> Python FIT
> 
> -- 
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