[Python-Dev] PEP 318: Decorators last before colon (original) (raw)

Phillip J. Eby pje at telecommunity.com
Tue Mar 30 17:19:44 EST 2004

At 01:21 PM 3/30/04 -0800, Guido van Rossum wrote:

> Another possibility that has been suggested is > > [decorator] > def func(arg, arg):

And one that I currently favor. I'm out of bandwidth to participate on a msg-by-msg basis, but perhaps folks can see if they can come to terms with this solution? --Guido van Rossum (home page: http://www.python.org/~guido/)

I showed this to a Python programmer at my office. He immediately asked if this:

if foo: [decorator1] else: [decorator2] def func(arg,arg): ...

was valid. He then further commented that it was strange to have a Python construct split across two lines, and inquired whether these variants:

[decorator] def func(arg,arg): ...

[decorator1, decorator2 ] def func(arg,arg):

[decorator]
def func(arg,arg): ...

would be considered legal as well.

I also showed him the implemented syntax of 'def func(args) [decorators]:', and he thought it seemed natural and Pythonic, and he suggested 'as' as another possible alternative. We also discussed the issue of evaluation order, and wondered if perhaps the decorator order in your latest syntax should be in reverse of the order used for decorators-at-the-end. His last comment was that he thought it seemed very Perlish to begin a Python statement with special symbols, that then modify the behavior of code on a subsequent line.

Some of his questions also got me to wondering how the grammar would even handle the initial proposal, since it seems it would have to be phrased as "lists can have an optional 'NEWLINE function-definition' clause after them".

Anyway, I think I could manage to live with this syntax, although I really don't like it either, for many of the same reasons. I don't think that putting decorators at the end of the definition impedes visibility unless there are a really large number of arguments or a large amount of decorator data. In both cases, you're going to need to be reading more carefully anyway. For the common cases, it's all going to be on one line anyway.

Oh... one more thing... the new proposal brings back up (to a limited extent) the "what to look up" question, in a different form. More precisely, someone who has learned basic Python syntax may wonder why somebody is creating a list with 'classmethod' in it, but unless they already know about the syntax rule, they have no way to even guess that it has something to do with the function definition that follows. By contrast, the decorators-last syntax is visibly part of the 'def' statement and gives much more clue as to its purpose.

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