On Mon, 04 Jun 2007 22:19:35 +0000, Lenard Lindstrom wrote: > What is "magic" about __init__ and __repr__? They are identifiers just > like "foo" or "JustAnotherClass". They have no special meaning to the > Python compiler. The leading and trailing double underscores represent > no special incantation. It is just a naming convention.
That's not quite true, as you point out: > So a number of method names like __init__ and __repr__ have a > pre-defined usage. That makes them magic, in the best possible way. > In every other respect they are just normal methods. That is *almost* true. Or, to put it another way, that is wrong. Some of the double-underscore magic methods are automatically called on the class instead of the instance, bypassing normal inheritance. >>> import new >>> class Test(object): ... def foo(self): ... return "foo called from the class" ... def __str__(self): ... return "__str__ called from the class" ... >>> obj = Test() >>> obj.foo() 'foo called from the class' >>> str(obj) # calls obj.__str__ '__str__ called from the class' Now see what happens when we add methods to the instance. >>> obj.foo = new.instancemethod( ... lambda self: "foo called from the instance", obj, Test) >>> obj.foo() 'foo called from the instance' >>> obj.__class__.foo(obj) # Check the method in the class is still there. 'foo called from the class' So calling a random method like foo() goes through the usual procedure: check the instance, then check the class. Now let's try it with a magic method. >>> obj.__str__ = new.instancemethod( ... lambda self: "__str__ called from the instance", obj, Test) >>> obj.__str__() '__str__ called from the instance' >>> str(obj) # calls obj.__str__() maybe? '__str__ called from the class' [snip] > Back to "magic names". Python is a consistent language. What may be > special cased - "magic" - in another language is handled normally with > Python's general purpose dynamic type system. In fact I can think of > only one name I would call "magic": __future__. Certainly I think __future__ is *more* magic than other double-underscore names. -- Steven. -- http://mail.python.org/mailman/listinfo/python-list
