[Python-Dev] Python memory model (low level)

Tim Peters tim.peters at gmail.com
Fri Jun 30 21:14:24 CEST 2006

[Nick Maclaren]
> I Have been thinking about software floating point, and there are
> some aspects of Python and decimal that puzzle me.  Basically, they
> are things that are wanted for this sort of thing and seem to be
> done in very contorted ways, so I may have missed something.
> Firstly, can Python C code assume no COMPACTING garbage collector,
> or should it allow for things shifting under its feet?

CPython never relocates objects.  The address of a Python object is
fixed from birth to death.

> Secondly, is there any documentation on the constraints and necessary
> ritual when allocating chunks of raw data and/or types of variable
> size?  Decimal avoids the latter.

Note that because CPython never relocates objects, it's impossible to
allocate "in one gulp" an object of a mutable type with a dynamically
size-varying member.  So, e.g., list objects have a fixed-size object
header (which never moves), while the list "guts" are separately
allocated; OTOH, tuples are immutable, so can (& do) allocate all the
space they need in one gulp.

See the Python/C API Reference Manual, esp. "Allocating Objects on the
Heap", "Supporting Cyclic Garbage Collection", and "Memory

> Thirdly, I can't find an efficient way for object-mangling code to
> access class data and/or have some raw data attached to a class (as
> distinct from an instance).

Don't know what "raw data" might mean here.  Any Python object can be
bound to any attribute of a class.  In Python, e.g.,

class MyClass:

     mydata = ['xyz', 12]

     def method(self):
         # or, more inheritance-friendly

This is substantially more long-winded in C.

> Fourthly, can I assume that no instance of a class will remain active
> AFTER the class disappears?  This would mean that it could use a
> pointer to class-level raw data.

It would be an error to free the memory for a class if the class is
reachable.  So long as an instance I of a class C is reachable, C is
also reachable (at least via I.__class__) so C _must_ not disappear.
C disappears only if it becomes unreachable, and one of the garbage
collection mechanisms then gets around to freeing its memory.  All gc
mechanisms in CPython rely on accurate reference counts.

> I can explain why all of those are the 'right' way to approach the
> problem, at an abstract level, but it is quite possible that Python
> does not support the abstract model of class implementation that I
> am thinking of.

Since we don't know what you're thinking of, you're on your own there ;-)

More information about the Python-Dev mailing list