During the discussion about assignment decorators Nick brought up the idea of adding a def-from syntax:<div><br></div><div>def <NAME> from <BUILDER>(<ARGS>):</div><div> ...</div><div><br></div><div>which would effectively call </div>
<div><br></div><div><BUILDER>(<NAME>, <ARGS>, <BODY>)</div><div><br></div><div>This is much like how meta-classes work, though this would be with functions (sort of meta-functions). Ultimately, they would amount to the same thing. The current "def" statement would just have an implicit builder.</div>
<div><br></div><div>During the sprints I explored the idea of this with Nick by making a "build_from" decorator, and an exec_closure builtin to provide the full capability needed to emulate the def-from syntax with a code object. It turned out the exec_closure didn't buy much. However, one thing that became apparent in discussing this with Nick is that just passing the code object of the decorated function (or of the def body) as <BODY> doesn't buy much. To really get much bang out of this you would need to pass the AST of the body. With the AST you could manipulate it as needed before compiling. (Nick's idea) Suddenly class definitions are just a special case of def-from.</div>
<div><br></div><div>If you really wanted to get crazy you could pass the raw string as <BODY> (can't blame this one on Nick). With the raw string you could put just about anything in there, like a DSL or another programming language. Then parse it however you like, and use that result to compile something else or build some data set or call some external library or whatever you care to do with raw data. It would not be restricted to parsable Python In the normal "def" context Python is hardwired to turn it into a function code object, and the implicit builder to generate the function object thereon.</div>
<div><br></div><div>I am reticent to suggest adding undue complexity to the languge or adversely affect readability (potentially) if it doesn't offer plenty of increased expressive power. So, crazy raw string thing aside, how about the def-from syntax, particularly with the AST passed? Nick already indicated to me that we probably should get comfy with metaclass __prepare__ before we get any more metaprogramming, and he's probably right. I was thinking about implementing def-from as an exercise in syntax hacking, regardless. Any thoughts? Are there better syntax hacking exercises (like Raymond's "def x.y(..." or "x.y = ..." or "x.(name)"? Could def-from have a place in the future?</div>
<div><br></div><font color="#888888"><div>-eric</div><div><br></div></font>