getattr is a python built-in function taking (at least) two arguments: the object from which you're getting the attribute, and the string name of the attribute.
If the string name is a constant, say 'foo', getattr(obj, 'foo') is exactly the same thing as obj.foo.
So, the main use case for the built-in function getattr is when you don't have the attribute name as a constant, but rather as a variable. A second important use case is when you pass it three arguments, rather than just two: in that case, if the attribute is absent from the object, getattrreturns the third, "default", argument, rather than raising an exception.
__getattr__ is a special method, defined in a class, that gets invoked when some attribute of an instance of that class is requested, and other normal ways to supply that attribute (via the instance's __dict__, slots, properties, and so on) all failed. You can define it, for example, when you want to delegate otherwise-undefined attribute lookups to other objects.
So your second example is wrong because the builtin getattr can never be called with a single argument.
If the string name is a constant, say 'foo', getattr(obj, 'foo') is exactly the same thing as obj.foo.
So, the main use case for the built-in function getattr is when you don't have the attribute name as a constant, but rather as a variable. A second important use case is when you pass it three arguments, rather than just two: in that case, if the attribute is absent from the object, getattrreturns the third, "default", argument, rather than raising an exception.
__getattr__ is a special method, defined in a class, that gets invoked when some attribute of an instance of that class is requested, and other normal ways to supply that attribute (via the instance's __dict__, slots, properties, and so on) all failed. You can define it, for example, when you want to delegate otherwise-undefined attribute lookups to other objects.
So your second example is wrong because the builtin getattr can never be called with a single argument.
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class A(dict): def __init__(self, **arg): self.a = "I am value of a.a variable attribute accessed via getattr(a, 'a')" def __getattr__(self, name): return "You asked for %s, but I'm giving you default" % name a = A(x=1) print getattr(a, 'a') >> I am value of a.a variable attribute accessed via getattr(a, 'a') print getattr(a, 'b') >> You asked for b, but I'm giving you default |
Using the above model, we can get the attribute by supplying the attribute_name. This works when we give a name as a string and you need to look up the instance attribute referenced by that name.
Use of __getattr__
You can also tell a class how to deal with attributes which it doesn't explicitly manage and do that via __getattr__ method.
Python will call this method whenever you request an attribute that hasn't already been defined. So you can define what to do with it.
Caveats and use of __getattribute__
If you need to catch every attribute regardless whether it exists or not, use __getattribute__ instead.
Difference__getattr__ only gets called for attributes that don't actually exist. If you set the attribute directly, referencing that attribute will retrieve it without calling __getattr__.
__getattribute__ is called all the times.
A key difference between __getattr__ and __getattribute__ is that __getattr__ is only invoked if the attribute wasn't found the usual ways. It's good for implementing a fallback for missing attributes, and is probably the one of two you want.
Use of __getattr__
You can also tell a class how to deal with attributes which it doesn't explicitly manage and do that via __getattr__ method.
Python will call this method whenever you request an attribute that hasn't already been defined. So you can define what to do with it.
Caveats and use of __getattribute__
If you need to catch every attribute regardless whether it exists or not, use __getattribute__ instead.
Difference__getattr__ only gets called for attributes that don't actually exist. If you set the attribute directly, referencing that attribute will retrieve it without calling __getattr__.
__getattribute__ is called all the times.
A key difference between __getattr__ and __getattribute__ is that __getattr__ is only invoked if the attribute wasn't found the usual ways. It's good for implementing a fallback for missing attributes, and is probably the one of two you want.
more bits and pieces : http://www.devx.com/opensource/Article/31482/0/page/4
