[Tutor] Memory and functions

[Peter Otten]

Charles Becker wrote:

> For functions that return values (ie the list method pop): If you don't
> assign the returned value to anything does it still end up residing in
> memory?  Just seems like this could be a potential problem to watch out
> for when memory usage is an issue (probably not most of the time).

Every object in CPython has a counter to keep track how many times it is 
referenced. When this counter drops to zero the object is destroyed and the 
memory is available for reuse. This is simple and works most of the time 
except when there are reference cycles. Consider:

class A: 
    pass
a = A()
b = A()
a.other = b
b.other = a
del a
del b

There is now no way for your program to reach a or b, but the reference 
counter of a is still one because it is referenced by b as b.other, and the 
reference counter of b is still one because it's referenced by a. 
Therefore CPython has another mechanism called garbage collection as a 
backup that periodically searches for reference cycles.

While the specific mechanism described above is only implemented in CPython 
all variants (pypy, jython, ironpython) guarantee that you don't have to 
bother about object lifetime.