Python in a Nutshell, 2nd Edition

Garbage Collection

Python’s garbage collection normally proceeds transparently and automatically, but you can choose to exert some direct control. The general principle is that Python collects each object x at some time after x becomes unreachable—that is, when no chain of references can reach x by starting from a local variable of a function instance that is executing, nor from a global variable of a loaded module. Normally, an object x becomes unreachable when there are no references at all to x. In addition, a group of objects can be unreachable when they reference each other but no global nor local variables reference any of them, even indirectly (such a situation is known as a mutual reference loop).

Classic Python keeps with each object x a count, known as a reference count, of how many references to x are outstanding. When x’s reference count drops to 0, CPython immediately collects x. Function getrefcount of module sys accepts any object and returns its reference count (at least 1, since getrefcount itself has a reference to the object it’s examining). Other versions of Python, such as Jython or IronPython, rely on other garbage-collection mechanisms supplied by the platform they run on (e.g., the JVM or the MSCLR). Modules gc and weakref therefore apply only to CPython.

When Python garbage-collects x and there are no references at all to x, Python then finalizes x (i.e., calls x._ _del_ _( )) and makes the memory that x occupied available for other uses. If x held any references ...