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Weak references are hashable if the object is hashable. Recently registered callback to the oldest registered callback.Įxceptions raised by the callback will be noted on the standard error output,īut cannot be propagated they are handled in exactly the same way as exceptions It is allowable for many weak references to be constructed for the same object.Ĭallbacks registered for each weak reference will be called from the most Parameter to the callback the referent will no longer be available. Weakref object is still alive, the callback will be called when the object isĪbout to be finalized the weak reference object will be passed as the only If callback is provided and not None, and the returned No longer alive, calling the reference object will cause None to be The original object can be retrieved byĬalling the reference object if the referent is still alive if the referent is
Python library reference 2.7 code#
Regular expression pattern objects, and code objects.ĬPython implementation detail: Other built-in types such as tuple and long do not supportĮxtension types can easily be made to support weak references see Unbound), sets, frozensets, file objects, generators, type objects,ĭBcursor objects from the bsddb module, sockets, arrays, deques, Instances, functions written in Python (but not in C), methods (both bound and Not all objects can be weakly referenced those objects which can include class The low-level machinery used by the weakĭictionary implementations is exposed by the weakref module for the Most programs should find that using one of these weakĭictionary types is all they need – it’s not usually necessary to create your That notify the weak dictionaries when a key or value has been reclaimed by In their implementation, setting up callback functions on the weak references WeakKeyDictionary and WeakValueDictionary use weak references Image object are the weak references held by weak mappings, garbage collectionĬan reclaim the object, and its corresponding entries in weak mappings are
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WeakValueDictionary, then when the last remaining references to that If, for example, an image object is a value in a
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Mappings that don’t keep objects alive solely because they appear in the mapping The weakref module are an alternative, using weak references to construct WeakKeyDictionary and WeakValueDictionary classes supplied by They appeared as values or keys in the dictionaries. Images, or images to names, the image objects would remain alive just because If you used a Python dictionary to map names to Kept alive solely because it appears in a cache or mapping.įor example, if you have a number of large binary image objects, you may wish toĪssociate a name with each. Mappings holding large objects, where it’s desired that a large object not be A primary use for weak references is to implement caches or
Python library reference 2.7 free#
Garbage collection is free to destroy the referent and reuse its memoryįor something else. Only remaining references to a referent are weak references, In the following, the term referent means the object which is referred toĪ weak reference to an object is not enough to keep the object alive: when the The weakref module allows the Python programmer to create weak