Some of my assumptions about Python object allocation
In a reddit comment thread
I was accurately dinged for being a bit casual in how I talked about
object allocation at the end of my previous entry. As the comment notes, calling '
)' creates and throws away a new object each time the key already
exists, just as '
.setdefaults(k, SomeClass())' would.
Well, sort of. One of the things I assume about Python is that
allocating and freeing new empty instances of built-in types (such
') is highly optimized and in practice is very cheap, while
allocating and freeing instances of Python-level classes is relatively
expensive both in memory and in time. I am thus much more casual about
churning simple primitive instances than I am about churning user
(Things like zero-length strings and empty tuples can be optimized even further; since they are unchangeable once created, you can have a single null instance of each and just hand out references to it.)
I think of instances of Python-level classes as expensive to create and
destroy for two reasons. First, even a very minimal class instance has
a not insignificant amount of overhead; unless it uses
it has at least a dict instance for its
__dict__ and a C-level
class instance structure of some sort. Second, if the class has an
__init__ method, it must be called, with the overhead in time
and object creation and churn that that implies. (And of coure the
__init__ can wind up attaching more data on the new instance, making
it more expensive.)