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Updating the short blob about old/new-style classes to reflect (IMHO) more

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precisely the motivation and the differences.

Added links to www.python.org/doc/newstyle.html and a footnote to warn against
the statement "x[i] is equivalent to x.__getitem__(i)", which already caused
many invalid bug reports on SF.
This commit is contained in:
Armin Rigo 2005-12-26 18:06:17 +00:00
parent 07cf5d4bb7
commit ddddd2f7e6
1 changed files with 34 additions and 6 deletions
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40
Doc/ref/ref3.tex
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@ -24,7 +24,8 @@ for example, it \emph{is} now possible in some cases to change an
object's type, under certain controlled conditions. Until this manual
undergoes extensive revision, it must now be taken as authoritative
only regarding ``classic classes'', that are still the default, for
compatibility purposes, in Python 2.2 and 2.3.}
compatibility purposes, in Python 2.2 and 2.3. For more information,
see \url{http://www.python.org/doc/newstyle.html}.}
An object's type determines the operations that the object
supports (e.g., ``does it have a length?'') and also defines the
possible values for objects of that type. The
@ -1036,14 +1037,39 @@ by the built-in \function{classmethod()} constructor.
Classes and instances come in two flavours: old-style or classic, and new-style.
Old-style classes were the only flavour of class available before Python 2.1. While they supported multiple inheritance, the rules for resolving names were chosen for ease of implementation. These rules turn out to make multiple inheritance hard to use in certain situations.
Up to Python 2.1, old-style classes were the only flavour available to the
user. The concept of (old-style) class is unrelated to the concept of type: if
\var{x} is an instance of an old-style class, then \code{x.__class__}
designates the class of \var{x}, but \code{type(x)} is always \code{<type
'instance'>}. This reflects the fact that all old-style instances,
independently of their class, are implemented with a single built-in type,
called \code{instance}.
New-style classes were introduced in Python 2.1, and change the method resolution order to make multiple inheritance more usable.
New-style classes were introduced in Python 2.2 to unify classes and types. A
new-style class neither more nor less than a user-defined type. If \var{x} is
an instance of a new-style class, then \code{type(x)} is the same as
\code{x.__class__}.
The major motivation for introducing new-style classes is to provide a unified
object model with a full meta-model. It also has a number of immediate
benefits, like the ability to subclass most built-in types, or the introduction
of "descriptors", which enable computed properties.
For compatibility reasons, classes are still old-style by default. New-style
classes are created by specifying another new-style class (i.e.\ a type) as a
parent class, or the "top-level type" \class{object} if no other parent is
needed. The behaviour of new-style classes differs from that of old-style
classes in a number of important details in addition to what \function{type}
returns. Some of these changes are fundamental to the new object model, like
the way special methods are invoked. Others are "fixes" that could not be
implemented before for compatibility concerns, like the method resolution order
in case of multiple inheritance.
This manuel is not up-to-date with respect to new-style classes. For now,
please see \url{http://www.python.org/doc/newstyle.html} for more information.
The plan is to eventually drop old-style classes, leaving only the semantics of new-style classes. This change will probably only be feasible in Python 3.0.
%=========================================================================
\section{Special method names\label{specialnames}}
@ -1054,7 +1080,9 @@ This is Python's approach to \dfn{operator overloading}, allowing
classes to define their own behavior with respect to language
operators. For instance, if a class defines
a method named \method{__getitem__()}, and \code{x} is an instance of
this class, then \code{x[i]} is equivalent to
this class, then \code{x[i]} is equivalent\footnote{This, and other
statements, are only roughly true for instances of new-style
classes.} to
\code{x.__getitem__(i)}. Except where mentioned, attempts to execute
an operation raise an exception when no appropriate method is defined.
\withsubitem{(mapping object method)}{\ttindex{__getitem__()}}