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They aren't looked up by index into an array, they are looked up by name, even if stored in an array. Roughly, an object with slots is a mutable counterpart

They aren't looked up by index into an array, they are looked up by name, even if stored in an array. Roughly, an object with __slots__ is a mutable counterpart of namedtuple. Both need to map a name into a field/slot index. Of course, there're ways to optimize that lookup (which primitive Python implementations, like CPython, may not do, but other implementations definitely do).

pdonis 6y ago
> They aren't looked up by index into an array, they are looked up by name...[which is mapped] into a field/slot index Yes, this is a fair point. Still, accessing the slot does not require a dictionary lookup, as it would for an ordinary instance attribute, which was the main point I was trying to make. > there're ways to optimize that lookup The way CPython does this, if you can call it an "optimization", is to implement the lookup as a data descriptor, which directly accesses the slot array location by index. (The namedtuple implementation correspondingly implements accessing the attribute as a read-only, non-data descriptor that directly accesses the appropriate tuple location by index.) Quite possibly the fact that the descriptor lookup comes before anything else in the attribute access code is considered "optimization" enough for this case.
- pfalcon 6y ago
  > Still, accessing the slot does not require a dictionary lookup, as it would for an ordinary instance attribute, which was the main point I was trying to make. I'm sorry but mapping a string (slot name) to an index [in an overdynamic language like Python] does require a dictionary lookup. It's just this dictionary is located in the class, not in each instance. > The way CPython does this, if you can call it an "optimization" The usual way to optimize lookups-by-name in dynamic languages is using (inline) caches. AFAIK, CPython now does that too.
  - pdonis 6y ago
    > mapping a string (slot name) to an index [in an overdynamic language like Python] does require a dictionary lookup Yes, you're right, I wasn't clear enough. What I meant to say was that accessing the value of the slot attribute (to either get or set it) on the instance does not require a dictionary lookup, just an array access. But of course finding out that the string (attribute name) is the name of a slot and getting the slot index does require a dictionary lookup (on the class, as you say).

They aren't looked up by index into an array, they are looked up by name, even if stored in an array. Roughly, an object with __slots__ is a mutable counterpart

pdonis 6y ago

> They aren't looked up by index into an array, they are looked up by name...[which is mapped] into a field/slot index Yes, this is a fair point. Still, accessing the slot does not require a dictionary lookup, as it would for an ordinary instance attribute, which was the main point I was trying to make. > there're ways to optimize that lookup The way CPython does this, if you can call it an "optimization", is to implement the lookup as a data descriptor, which directly accesses the slot array location by index. (The namedtuple implementation correspondingly implements accessing the attribute as a read-only, non-data descriptor that directly accesses the appropriate tuple location by index.) Quite possibly the fact that the descriptor lookup comes before anything else in the attribute access code is considered "optimization" enough for this case.

pfalcon 6y ago
> Still, accessing the slot does not require a dictionary lookup, as it would for an ordinary instance attribute, which was the main point I was trying to make. I'm sorry but mapping a string (slot name) to an index [in an overdynamic language like Python] does require a dictionary lookup. It's just this dictionary is located in the class, not in each instance. > The way CPython does this, if you can call it an "optimization" The usual way to optimize lookups-by-name in dynamic languages is using (inline) caches. AFAIK, CPython now does that too.
- pdonis 6y ago
  > mapping a string (slot name) to an index [in an overdynamic language like Python] does require a dictionary lookup Yes, you're right, I wasn't clear enough. What I meant to say was that accessing the value of the slot attribute (to either get or set it) on the instance does not require a dictionary lookup, just an array access. But of course finding out that the string (attribute name) is the name of a slot and getting the slot index does require a dictionary lookup (on the class, as you say).