[Python-Dev] Re: metaclass and author are already decorators (original) (raw)
"Martin v. Löwis" martin at v.loewis.de
Sun Aug 22 17:10:48 CEST 2004
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Paul Morrow wrote:
That's a terrible way to 'think' about this kind of programming --- programming thru imperative mutation.
This is Pythonic. In Python, we don't have any declarations (well, perhaps except for global). Instead, everything in a module is executed, including class and def statements. This is an important property, as it allows:
conditional function definition: if some_condition: def foo():some_code else: def foo():other_code
computed inheritance: if some_condition: base = B1 else: base = B2
class D(base): body
and more
Decorators extend this pattern.
So regardless of what is going on under the hood, it's better to think of this technique as a function annotation --- something additionally stated in its description --- than something that we do to the function.
No. It is best to simultaneously think of this as descriptive and imperative. I.e. for most purposes, it is sufficient to consider it descriptive, but one always needs to be aware when it is more correct to consider it imperative. For example, you have to understand that
@foo def bar():self
might cause bar not to be a function. This can only be understood if you don't think of decorators as kind of attribute.
When a feature gets added/applied to a class/function shouldn't matter to the programmer. We've screwed up if we make them think that hard.
Depends on the situation. In some cases, it does matter, and then the programmer needs to think hard. However, she can only succeed in that case if she is even aware that the declarative view on these things is a simplification.
They should simply be able to state which features they want a class/function to have (as they can now with metaclass, lt, etc.), and the system makes sure they are there when the class/function is called. It's magical, but its easy to think about.
Yes, it works most of the time. If this simplification is wrong, it gets so terribly wrong that the programmer needs somebody else who really understands it solves the problem. To avoid that, we should not try to hide the semantics, but make it obvious - and simultaneously explain that in most cases, the simplification will serve fine.
That should be irrelevant to the programmer. It's magic.
No, no, no. In a well-designed programming language, there is no place for magic, in the sense that what really happens is deliberately hidden for nobody to find. Instead, people should know the inner workings. Many people know the inner workings of the current language, and they can easily extend their mental model to decorators.
Regards, Martin
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