Why do class methods always need 'self' as the first parameter?
steve+comp.lang.python at pearwood.info
Wed Aug 31 12:06:50 EDT 2011
John Gordon wrote:
> In <0dc26f12-2541-4d41-8678-4fa53f347acf at g9g2000yqb.googlegroups.com> "T.
> Goodchild" <tgoodchild at gmail.com> writes:
>> So why is 'self' necessary on class methods? It seems to me that the
>> most common practice is that class methods *almost always* operate on
>> the instance that called them. It would make more sense to me if this
>> was assumed by default, and for "static" methods (methods that are
>> part of a class, but never associated with a specific instance) to be
>> labelled instead.
>> Just curious about the rationale behind this part of the language.
> How would a method access instance variables without 'self'?
If Python had compile time declarations, the compiler could know whether x=1
was referring to a local variable x or an attribute x.
The reader might not, but the compiler would :)
By the way, although the Python docs are a little inconsistent, the usual
term here is "attribute" rather than "instance variable".
Attributes need not live on the instance: they can also live on the class, a
superclass, or be computed at run-time via at least three different
mechanisms I can think of (__getattribute__, __getattr__, properties).
Local variables are treated a bit differently from attributes, but broadly
speaking, if you need a dot to access something, it's an attribute, if you
don't, it's a name binding (or variable).
Python even has two different sorts of errors for "variable" lookup
failures: NameError (or UnboundLocalError) for un-dotted names, and
AttributeError for dotted names.
> They probably could have made 'self' a magical attribute that just
> appears out of thin air instead of being passed as an argument, like
> 'this' in C++. But would that really provide any benefit?
Well obviously the C++ people thought so :)
The effort to type "self, " in method signatures is pretty low. I don't
think it is a problem. But other languages are free to choose differently.
Cobra, for example, is explicitly derived from Python in many ways, but it
drops the "self" (as well as other changes).
More information about the Python-list