[Python-ideas] wild idea : target decorators

Guido van Rossum guido at python.org
Fri Sep 25 03:19:28 CEST 2009

On Thu, Sep 24, 2009 at 5:38 PM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
> I was thinking about something like this myself recently.
> I have some custom property classes that I use extensively
> in a couple of projects, declared like this:
>  foo = fancy_property('foo', 'The fancy foo property')
> It would be nice to be able to write this as something
> like
>  @fancy_property
>  foo = 'The fancy foo property'

How did one occurrence of 'foo' suddenly turn into foo (without
quotes)? That's not how decorators behave elsewhere.

> or perhaps
>  @fancy_property('The fancy foo property')
>  foo = default_foo_value

I think the interpretation for decorators on things other than classes
and functions should be derived by carefully reinterpreting what a
decorator does for a function or class.

We already have the rule that

  def foo.........  # or class foo..........

is equivalent to

  def foo..........  # or class foo..........
  foo = fancy(foo)

Now in addition we know that

  def foo........  # or class foo...........


  foo = <create something>

so we have

  def foo........  # or class foo.........

as a shorthand for

  foo = fancy(<create something>)

>From this we can conclude that

  foo = <expression>

just means

  foo = fancy(<expression>)

which is (IMO) an utterly unattractive violation of TOOWTDI.

If you were going to object "but def foo and class foo also pass the
string 'foo' into <create something>", my counter-objection is that
those semantics are implied by the def/class keywords and not by the
@decorator syntax.

Ergo, -1.

--Guido van Rossum (home page: http://www.python.org/~guido/)

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