# Create a weakref in .NET framework

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# Create a weakref
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def _ _del_ _(self): acc = selfaccountref() # Get account if acc: # Unregister if still exists accunregister(self) def update(self): print("Balance is %02f" % selfaccountref()balance) def close(self): print("Account no longer in use") # Example setup a = Account('Dave',100000) a_ob = AccountObserver(a)
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In this example, a weak reference accountref is createdTo access the underlying Account, you call it like a functionThis either returns the Account or None if it s no longer aroundWith this modification, there is no longer a reference cycle If the Account object is destroyed, its _ _del_ _ method runs and observers receive notificationThe gc module also works properly More information about the weakref module can be found in 13, Python Runtime Services
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Internally, instances are implemented using a dictionary that s accessible as the instance s __dict__ attributeThis dictionary contains the data that s unique to each instance Here s an example:
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>>> a = Account('Guido', 11000) >>> a_ _dict_ _ {'balance': 11000, 'name': 'Guido'}
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New attributes can be added to an instance at any time, like this:
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anumber = 123456 # Add attribute 'number' to a_ _dict_ _
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Modifications to an instance are always reflected in the local _ _dict_ _ attribute Likewise, if you make modifications to _ _dict_ _ directly, those modifications are reflected in the attributes Instances are linked back to their class by a special attribute _ _class_ _The class itself is also just a thin layer over a dictionary which can be found in its own _ _dict_ _ attributeThe class dictionary is where you find the methods For example:
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>>> a_ _class_ _ <class '_ _main_ _Account'> >>> Account_ _dict_ _keys() ['_ _dict_ _', '_ _module_ _', 'inquiry', 'deposit', 'withdraw', '_ _del_ _', 'num_accounts', '_ _weakref_ _', '_ _doc_ _', '_ _init_ _'] >>>
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Finally, classes are linked to their base classes in a special attribute _ _bases_ _, which is a tuple of the base classesThis underlying structure is the basis for all of the operations that get, set, and delete the attributes of objects Whenever an attribute is set using objname = value, the special method obj_ _setattr_ _("name", value) is invoked If an attribute is deleted using del objname, the special method obj_ _delattr_ _("name") is invokedThe default behavior of these methods is to modify or remove values from the local _ _dict_ _ of obj unless the requested attribute happens to correspond to a property or descriptor In
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that case, the set and delete operation will be carried out by the set and delete functions associated with the property For attribute lookup such as objname, the special method obj_ _getattrribute_ _("name") is invokedThis method carries out the search process for finding the attribute, which normally includes checking for properties, looking in the local _ _dict_ _ attribute, checking the class dictionary, and searching the base classes If this search process fails, a final attempt to find the attribute is made by trying to invoke the _ _getattr_ _() method of the class (if defined) If this fails, an AttributeError exception is raised User-defined classes can implement their own versions of the attribute access functions, if desired For example:
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class Circle(object): def _ _init_ _(self,radius): selfradius = radius def _ _getattr_ _(self,name): if name == 'area': return mathpi*selfradius**2 elif name == 'perimeter': return 2*mathpi*selfradius else: return object_ _getattr_ _(self,name) def _ _setattr_ _(self,name,value): if name in ['area','perimeter']: raise TypeError("%s is readonly" % name) object_ _setattr_ _(self,name,value)
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A class that reimplements these methods should probably rely upon the default implementation in object to carry out the actual workThis is because the default implementation takes care of the more advanced features of classes such as descriptors and properties As a general rule, it is relatively uncommon for classes to redefine the attribute access operators However, one application where they are often used is in writing generalpurpose wrappers and proxies to existing objects By redefining _ _getattr_ _(), _ _setattr_ _(), and _ _delattr_ _(), a proxy can capture attribute access and transparently forward those operations on to another object
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_ _slots_ _
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A class can restrict the set of legal instance attribute names by defining a special variable called _ _slots_ _ Here s an example:
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class Account(object): _ _slots_ _ = ('name','balance')
When _ _slots_ _ is defined, the attribute names that can be assigned on instances are restricted to the names specified Otherwise, an AttributeError exception is raised This restriction prevents someone from adding new attributes to existing instances and solves the problem that arises if someone assigns a value to an attribute that they can t spell correctly In reality, _ _slots_ _ was never implemented to be a safety feature Instead, it is actually a performance optimization for both memory and execution speed Instances of a class that uses _ _slots_ _ no longer use a dictionary for storing instance data Instead, a much more compact data structure based on an array is used In programs that