On Sun, Sep 6, 2009 at 09:41, Rob Cliffe<rob.cliffe@btinternet.com> wrote:
Can I make another plea for the syntax following '@' to be an unrestricted expression? Guido has said he has a 'gut feeling' against this but has not as far as I know rationalised it.
When it comes to Guido's gut, a rationalization isn't needed. Perk of being BDFL. Plus his gut is right so often it tends to not be questioned.
1) It is inconsistent with Python in general (unPythonic) to impose arbitrary restrictions in one particular place, and hard to explain to someone learning the language.
It's not difficult to explain; decorators can only be a dotted name w/ an optional method call and its corresponding arguments. It keeps the syntax simple and clean, IMO. Decorators add a mental overhead of having to think about what they will do to a function when reading the code. If I then also have to figure out what an arbitrary expression evaluates to in order to figure that out that is more mental effort than needed. Yes, you can do whatever with the decorator you are passing in, but hopefully you are not so evil/stupid as to make a decorator that copmlicated. Give people the power of full expressions and that will happen more often.
2) The restriction is in any case more apparent than real, as @ <any-expression> # disallowed, SyntaxError can be implemented, albeit in a more verbose aka less Pythonic was, as:
AnyExpr = <any-expression> @AnyExpr
or as
def Identity(x): return x ... @Identity( <any-expression> ) # smuggle in as func arg
And we almost ditched lambdas in Python 3 because you can implement them in the same way. The only reason they got to stick around was they were already in use and people threw a fit over them.
3) I propose the following as plausible use cases (I know other people will have their own):
3.1) @DecoratorList[index]
3.2) @DecoratorDictionary[key]
3.3) @Decorator1 if <condition> else Decorator2 # Special case of the last one: def Identity(x): return x @Decorator if __debug__ else Identity
Plausible does not equal useful. You need to show that this actually comes up in normal coding for a decent amount of Python code to warrant tweaking the language over.
Xavier Morel has pointed out that 3.1) can be implemented now as @DecoratorList.__getitem__[index] but this doesn't seem a good reason for forbidding the simpler syntax; after all Python allows the simpler syntax in other contexts. Similarly 3.2) can be written as @DecoratorDictionary.get(key)
(As an aside, perhaps a decorator that evaluates to None could be treated at run-time the same as no decorator, i.e. equivalent to the Identity function in the above examples. Currently it naturally raises TypeError: 'NoneType' object is not callable. Just a thought.)
That's not going to happen. =) Complicates the bytecode unnecessarily. Once again, this needs to actually come up in regular usage to warrant even considering the change.
Finally, sorry if I have not sent this e-mail to the right place (I wanted to attach it to the 'allow lambdas as decorators' thread but don't yet know how to do this). Also sorry that this partly duplicates a message I sent to python-dev. I am still finding my way round the Python mailing lists.
No, this is the place to send thought out proposals for changing Python before they get promoted to hitting python-dev. -Brett