Description
Bug report
Bug description:
On Python 3.13, instances that have both __slots__ and __dict__ defined (e.g. through inheritance)
can have a size that is more than four times of that in previous Python versions (e.g. 3.12).
import tracemalloc
import gc
class _Point2D:
__slots__ = ("x", "y")
class Point3D_OnlySlots(_Point2D):
__slots__ = ("z",)
def __init__(self, x, y, z):
self.x, self.y, self.z = x, y, z
class Point3D_DictAndSlots(_Point2D):
def __init__(self, x, y, z):
self.x, self.y, self.z = x, y, z
class Point3D_OnlyDict:
def __init__(self, x, y, z):
self.x, self.y, self.z = x, y, z
gc.collect() # clear freelists
tracemalloc.start()
_ = [Point3D_OnlySlots(1, 2, 3) for _ in range(1_000_000)]
print(
f"1M Point3D (only __slots__) instances: {tracemalloc.get_traced_memory()[0]:_} bytes"
)
gc.collect() # clear freelists
tracemalloc.start()
_ = [Point3D_OnlyDict(1, 2, 3) for _ in range(1_000_000)]
print(
f"1M Point3D (only __dict__) instances: {tracemalloc.get_traced_memory()[0]:_} bytes"
)
gc.collect() # clear freelists
tracemalloc.start()
_ = [Point3D_DictAndSlots(1, 2, 3) for _ in range(1_000_000)]
print(
f"1M Point3D (__dict__ and __slots__) instances: {tracemalloc.get_traced_memory()[0]:_} bytes"
)On python 3.13.3, this prints:
1M Point3D (only __slots__) instances: 64_448_792 bytes
1M Point3D (only __dict__) instances: 104_451_928 bytes
1M Point3D (__dict__ and __slots__) instances: 416_448_848 bytes
On python 3.12.11, this prints:
1M Point3D (only __slots__) instances: 64_448_792 bytes
1M Point3D (only __dict__) instances: 96_451_808 bytes
1M Point3D (__dict__ and __slots__) instances: 96_452_232 bytes
The apparent cause seems to be the object layout changes (see here)
which introduced inline values. It appears that these don't mesh well with __slots__.
Could this be because the inline values need to have a fixed offset?
What appears to cause the dramatic increase above is that having (nonempty) __slots__ will
trigger __dict__ materialization (size 30) as soon as a non-slot attribute is set. In contrast to the optimized "no slots" case, this dict doesn't shrink as more instances are created.
Of course, mixing __slots__ and __dict__ is not recommended, but it's a trap that can be easily fallen into.
For example, if forgetting to define __slots__ anywhere in a complex class hierarchy.
I'm unsure if I'm understanding exactly what's going on though. @markshannon perhaps you can shed some light on this?
CPython versions tested on:
3.13
Operating systems tested on:
macOS