[Python-Dev] Defining properties - a use case for class decorators? (original) (raw)

Josiah Carlson jcarlson at uci.edu
Wed Oct 19 22:10:30 CEST 2005

Jim Jewett <jimjjewett at gmail.com> wrote:

(In http://mail.python.org/pipermail/python-dev/2005-October/057409.html,) Nick Coghlan suggested allowing attribute references as binding targets.

> x = property("Property x (must be less than 5)") > def x.get(instance): ... Josiah shivered and said it was hard to tell what was even intended, and (in http://mail.python.org/pipermail/python-dev/2005-October/057437.html) Nick agreed that it was worse than > x.get = f given: > def f(): ... Could someone explain to me why it is worse?

def x.get(...): ...

Seems to imply that one is defining a method on x. This is not the case. It is also confused by the x.get(instance) terminology that I doubt has ever seen light of day in production code. Instance of what? Instance of x? The class? ...

I'm personally averse to the 'given:' syntax, if only because under certain situations, it can be reasonably emulated.

I understand not wanting to modify object x outside of its definition.

I understand that there is some trickiness about instancemethods and bound variables. But these objections seem equally strong for both forms, as well as for the current "equivalent" of def f(): ... x.get = f The first form (def x.get) at least avoids repeating (or even creating) the temporary function name. The justification for decorators was to solve this very problem within a module or class. How is this different? Is it just that attributes shouldn't be functions, and this might encourage the practice?

Many will agree that there is a problem with how properties are defined. There are many proposed solutions, some of which use decorators, custom subclasses, metaclasses, etc.

I have a problem with it because from the description, you could use...

def x.y.z.a.b.c.foobarbaz(...):
    ...

...and it woud be unclear to the reader or writer what the hell x.y.z.a.b.c is (class, instance, module), which can come up if the definition/import of x is far enough away from the definition of x. . Again, ick.

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