Python: how to implement __getattr__()?

Question

My class has a dict, for example:

class MyClass(object):     def __init__(self):         self.data = {'a': 'v1', 'b': 'v2'} 

Then I want to use the dict's key with MyClass instance to access the dict, for example:

ob = MyClass() v = ob.a   # Here I expect ob.a returns 'v1' 

I know this should be implemented by __getattr__, but I'm new to Python, I don't exactly know how to implement it.

Answer 1

class MyClass(object):      def __init__(self):         self.data = {'a': 'v1', 'b': 'v2'}      def __getattr__(self, attr):         return self.data[attr] 

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>>> ob = MyClass() >>> v = ob.a >>> v 'v1' 

Be careful when implementing __setattr__ though, you will need to make a few modifications:

class MyClass(object):      def __init__(self):         # prevents infinite recursion from self.data = {'a': 'v1', 'b': 'v2'}         # as now we have __setattr__, which will call __getattr__ when the line         # self.data[k] tries to access self.data, won't find it in the instance          # dictionary and return self.data[k] will in turn call __getattr__         # for the same reason and so on.... so we manually set data initially         super(MyClass, self).__setattr__('data', {'a': 'v1', 'b': 'v2'})      def __setattr__(self, k, v):         self.data[k] = v      def __getattr__(self, k):         # we don't need a special call to super here because getattr is only          # called when an attribute is NOT found in the instance's dictionary         try:             return self.data[k]         except KeyError:             raise AttributeError 

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>>> ob = MyClass() >>> ob.c = 1 >>> ob.c 1 

If you don't need to set attributes just use a namedtuple eg.

>>> from collections import namedtuple >>> MyClass = namedtuple("MyClass", ["a", "b"]) >>> ob = MyClass(a=1, b=2) >>> ob.a 1 

If you want the default arguments you can just write a wrapper class around it:

class MyClass(namedtuple("MyClass", ["a", "b"])):      def __new__(cls, a="v1", b="v2"):         return super(MyClass, cls).__new__(cls, a, b) 

or maybe it looks nicer as a function:

def MyClass(a="v1", b="v2", cls=namedtuple("MyClass", ["a", "b"])):     return cls(a, b) 

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>>> ob = MyClass() >>> ob.a 'v1'