magic-methods

What methods need to be overridden/implemented when making user-defined classes sortable and/or hashable in python? What are the gotchas to watch out for? I type dir({}) into my interpreter to get a list of methods on built-in dicts. Of those, I assume I need to some implement some subset of ['__cmp__', '__eq__', '__ge__', '__gt__', '__hash__', '__le__', '__lt__', '__ne__'] Is there a difference in which methods must be implemented for Python3 as opposed to Python2? I almost posted this as a comment to the other answers but it's really an answer in and of itself. To make your items sortable,

I've been playing around with Python recently, and one thing I'm finding a bit odd is the extensive use of 'magic methods', e.g. to make its length available, an object implements a method, def __len__(self) , and then it is called when you write len(obj) . I was just wondering why objects don't simply define a len(self) method and have it called directly as a member of the object, e.g. obj.len() ? I'm sure there must be good reasons for Python doing it the way it does, but as a newbie I haven't worked out what they are yet. AFAIK, len is special in this respect and has historical roots. Here

I know this question has been asked several times, but none of them have a real answer for a workaround. Maybe there's one for my specific case. I'm building a mapper class which uses the magic method __get() to lazy load other objects. It looks something like this: public function __get ( $index ) { if ( isset ($this->vars[$index]) ) { return $this->vars[$index]; } // $index = 'role'; $obj = $this->createNewObject ( $index ); return $obj; } In my code I do: $user = createObject('user'); $user->role->rolename; This works so far. The User object doesn't have a property called 'role', so it uses

I know that __call__ method in a class is triggered when the instance of a class is called. However, I have no idea when I can use this special method, because one can simply create a new method and perform the same operation done in __call__ method and instead of calling the instance, you can call the method. I would really appreciate it if someone gives me a practical usage of this special method. Django forms module uses __call__ method nicely to implement a consistent API for form validation. You can write your own validator for a form in Django as a function. def custom_validator(value):

In Scala, if I define a method called apply in a class or a top-level object, that method will be called whenever I append a pair a parentheses to an instance of that class, and put the appropriate arguments for apply() in between them. For example: class Foo(x: Int) { def apply(y: Int) = { x*x + y*y } } val f = new Foo(3) f(4) // returns 25 So basically, object(args) is just syntactic sugar for object.apply(args) . How does Scala do this conversion? Is there a globally defined implicit conversion going on here, similar to the implicit type conversions in the Predef object (but different in

>>> class A(object): pass ... >>> A.__dict__ <dictproxy object at 0x173ef30> >>> A.__dict__.__dict__ Traceback (most recent call last): File "<string>", line 1, in <fragment> AttributeError: 'dictproxy' object has no attribute '__dict__' >>> A.__dict__.copy() {'__dict__': <attribute '__dict__' of 'A' objects> ... } >>> A.__dict__['__dict__'] <attribute '__dict__' of 'A' objects> # What is this object? If I do A.something = 10 , this goes into A.__dict__ . What is this <attribute '__dict__' of 'A' objects> found in A.__dict__.__dict__ , and when does it contain something? Rosh Oxymoron First of