Tuesday, January 27, 2009

every python thingy is an object with a type

I like this particular insight in [[py ref]] (i.e. "python essential reference") though I can't easily verify it in code -- Every thingy in a python program is an object with a type. (There might possibly be some exceptions but this is a rather good starting point.)

* An object is defined, same way in C++, as a storage_location, with an (obviously) immutable address.
** content could be mutable.
** Even an int is an object, unlike java

* Every storage_location (Say ObjectA) has a specific data_type. Required by python interpreter, java compiler or c++ compiler... That specific data_type is technically a Type OBJECT.
* [[peref]] made it clear that Type tells us the features supported by this storage_location. Features include methods and fields...

The thing about the python language is that Everything seems to be an object with a type. Here are some special objects and their types
$ a free function. This OBJECT has a __name__ attribute. Try qq/dir(len)/
$ a bound instance method. Such an OBJECT has an im_self attribute.
$ a bound class method. Such an OBJECT has a __name__ attribute
$ (There's no such thing as a bound static method -- nothing to bind.)
$ a regular instance method. Such an OBJECT has a __self__ attribute pointing to the host OBJECT
$ a regular class method. Such an OBJECT has a __name__ attribute
$ a regular static method. Such an OBJECT has a __name__ attribute

Warning -- these special objects each have a distinct data_type but not by type()! It's worthwhile to step back and recognize these special OBJECTS as separate data_types with their own features. The "feature set" difference between any 2 data_types is reflected in various ways (such as the attributes...)

Footnotes
* the built-in type() function is an almost-perfect way to see the data_type of a storage_location, but I don't think it reveals all the details I need in a foolproof way.
* The "type" OBJECT is comparable to type_info objects and java "class" objects.
* What's the type of the "type" OBJECTS? We don't need to know.

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