Hi, list! I have a doubt about the design of dynamic attribute creation by assignments in Python.
As we know, in Python, we are able to create a new attribute of a class dynamically by an assignment: >>> class test: pass ... >>> test.a = "hello" >>> test.a 'hello' >>> However, I still don't get the points why Python designs it like this. My points are: 1) Modifying a class attribute is metaprogramming, and this is modifying a class, i.e. adding a new attribute to it, thus this should belong to metaprogramming. (I know, strictly speaking, maybe my definition of "metaprogramming" here is incorrect, I _do_ welcome someone could correct me if I am really wrong, but that is not the main point here, please don't go off-topic.) 2) Metaprogramming should be distinguished with non-meta programming, like templates in C++, it is obvious to see if you are using template metaprogramming in C++. 3) Thus, allowing dynamic attribute creation by assignment _by default_ is not a good design for me. It is not obvious at all to see if I am doing metaprogramming at a first glance. 4) Also, this will _somewhat_ violate the OOP princples, in OOP, this is and should be implemented by inherence. 5) This could hide errors silently, for example, when I do: >>>test.typooooo = "blah" I am expecting Python will compllain that "Hey! You have a typo in the attribute name!". Also, this could make code worse to read, if I add a new attribute in one place, and add another one in the another place, and so on, what attributes the hell do I have finally?! I know someone will say you can change this by overriding the __set_attr__ function, like Recipe 6.3 in Python Cookbook. However, this is not default. In my points of view, a better design should be: 1) Disallow dynamic attribute creations by assignments _by default_, thus I expect an error when I do: >>> foo.new_attr = "blah" AttributeError: by default. 2) For people who want to add a new attribute at runtime, but not to override __set_attr__, he/she should switch to: >>> setattr(foo, "new_attr", "blah") This will be more like doing metaprogramming rather than non-meta programming, at least more obvious than using an assignment. 3) Allow users who don't like this to change by __set_attr__, of course. Someone argued with me that Python is a dynamic language, allowing this is natural. True, I do understand that attributes in Python are stored in an internal dictionary (__dict__), allowing assignments to an non-existing key is natural. However, this will be a little different when we talk about classes attributes, simple assignments could have side-effects, besides the traditional assignments effect, like in C, that is, creating a new attribute silently. So even from a view of beauty, this is not a good design. I hope someone could teach me more about why Python design it like it is. Any reply is more than welcome. Thanks for your time! -- Live like a child, think like the god. -- http://mail.python.org/mailman/listinfo/python-list
