[Python-Dev] Are property descriptors intended to be immutable? (original) (raw)

Guido van Rossum guido at python.org
Sat Feb 7 19:23:20 CET 2009

On Sat, Feb 7, 2009 at 9:41 AM, Curt Hagenlocher <curt at hagenlocher.org> wrote:

On Fri, Feb 6, 2009 at 4:04 PM, Guido van Rossum <guido at python.org> wrote:

On Fri, Feb 6, 2009 at 2:44 PM, Curt Hagenlocher <curt at hagenlocher.org> wrote:

...because they're not quite :). Should I file this as a bug report?

No, this is just how it works. I hope they aren't documented as immuable? Not that I know of :). But the individual properties of the descriptor are all read-only and that the implementations of setter, getter and deleter return new objects instead of mutating and returning the old descriptor. So it seemed a little odd that there was just one way remaining in which the object could be mutated.

You're basically using an "API" that wasn't meant to be invoked directly. I don't particularly care that properties can be mutated, but I don't really care that they should be 100% immutable either. IMO it's enough that the APIs that exist to be used explicitly don't modify the property in place; getter and friends return a new object so that you can override or extend a property in a subclass, and there obviously you don't want to mutate the base class.

In the balance I'd say that the "backdoor" you found is probably harmless, and perhaps someone can find a good use for it, so I'm disinclined to "fix" this.

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

More information about the Python-Dev mailing list