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An Introduction to Python
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 124 pages
ISBN 0954161769
RRP £12.95 ($19.95)

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11.6 Weak References

Python does automatic memory management (reference counting for most objects and garbage collection to eliminate cycles). The memory is freed shortly after the last reference to it has been eliminated.

This approach works fine for most applications but occasionally there is a need to track objects only as long as they are being used by something else. Unfortunately, just tracking them creates a reference that makes them permanent. The ‘weakref’ module provides tools for tracking objects without creating a reference. When the object is no longer needed, it is automatically removed from a weakref table and a callback is triggered for weakref objects. Typical applications include caching objects that are expensive to create:

    >>> import weakref, gc
    >>> class A:
    ...     def __init__(self, value):
    ...             self.value = value
    ...     def __repr__(self):
    ...             return str(self.value)
    >>> a = A(10)    # create a reference
    >>> d = weakref.WeakValueDictionary()
    >>> d['primary'] = a # does not create a reference
    >>> d['primary'] # fetch the object if it is still alive
    >>> del a        # remove the one reference
    >>> gc.collect() # run garbage collection right away
    >>> d['primary'] # entry was automatically removed
    Traceback (most recent call last):
      File "<pyshell#108>", line 1, in -toplevel-
        d['primary'] # entry was automatically removed
      File "C:/PY24/lib/weakref.py", line 46, in __getitem__
        o = self.data[key]()
    KeyError: 'primary'
ISBN 0954161769An Introduction to PythonSee the print edition