[Python-3000] hash as attribute/property

[Giovanni Bajo]

Guido van Rossum <guido at python.org> wrote:

> The question is where to draw the line. I don't believe you really
> meant "all no-args methods". Perhaps you meant "all side-effect free
> argument-less methods that return the same value each time (or at
> least until some explicit change to the object is made) and that can
> be computed reasonably cheap"?

+1. And "reasonably cheap" also means O(1) (aka "no for loops"), in my own rule
of thumb. The worst thing of VB is when you read a value from a property and
don't realize how much it's going on behind the scenes.

A function call is a function call is a function call. Someone smart said "one
thing is to read code which calls functions that you don't know about, another
thing is to read code where you do not even know where the function calls are".
This is very true, and should be kept in mind when designing code with
properties.

> The __hash__() method qualifies, which is why it's a tough call (I'll
> come back on that; I want to come up with a reason why __hash__()
> should remain a method). The iterator-getting example above doesn't
> qualify.

__hash__() isn't always O(1), so in my world it wouldn't be a property.

Giovanni Bajo