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| -rw-r--r-- | Doc/ref/ref3.tex | 62 |
1 files changed, 54 insertions, 8 deletions
diff --git a/Doc/ref/ref3.tex b/Doc/ref/ref3.tex index 064794e..874fe10 100644 --- a/Doc/ref/ref3.tex +++ b/Doc/ref/ref3.tex @@ -1155,9 +1155,6 @@ to Unicode using the system default encoding. The following methods can be defined to customize the meaning of attribute access (use of, assignment to, or deletion of \code{x.name}) for class instances. -For performance reasons, these methods are cached in the class object -at class definition time; therefore, they cannot be changed after the -class definition is executed. \begin{methoddesc}[object]{__getattr__}{self, name} Called when an attribute lookup has not found the attribute in the @@ -1171,10 +1168,11 @@ Note that if the attribute is found through the normal mechanism, asymmetry between \method{__getattr__()} and \method{__setattr__()}.) This is done both for efficiency reasons and because otherwise \method{__setattr__()} would have no way to access other attributes of -the instance. -Note that at least for instance variables, you can fake -total control by not inserting any values in the instance -attribute dictionary (but instead inserting them in another object). +the instance. Note that at least for instance variables, you can fake +total control by not inserting any values in the instance attribute +dictionary (but instead inserting them in another object). See the +\method{__getattribute__()} method below for a way to actually get +total control in new-style classes. \withsubitem{(object method)}{\ttindex{__setattr__()}} \end{methoddesc} @@ -1188,7 +1186,10 @@ If \method{__setattr__()} wants to assign to an instance attribute, it should not simply execute \samp{self.\var{name} = value} --- this would cause a recursive call to itself. Instead, it should insert the value in the dictionary of instance attributes, e.g., -\samp{self.__dict__[\var{name}] = value}. +\samp{self.__dict__[\var{name}] = value}. For new-style classes, +rather than accessing the instance dictionary, it should call the base +class method with the same name, for example, +\samp{object.__setattr__(self, name, value)}. \withsubitem{(instance attribute)}{\ttindex{__dict__}} \end{methoddesc} @@ -1198,6 +1199,51 @@ assignment. This should only be implemented if \samp{del obj.\var{name}} is meaningful for the object. \end{methoddesc} +\subsubsection{More attribute access for new-style classes \lable{new-style-attribute-access}} + +The following methods only apply to new-style classes. + +\begin{methoddesc}[object]{__getattribute__}{self, name} +Called unconditionally to implement attribute accesses for instances +of the class. If the class also defines \method{__getattr__}, it will +never be called (unless called explicitly). +This method should return the (computed) attribute +value or raise an \exception{AttributeError} exception. +In order to avoid infinite recursion in this method, its +implementation should always call the base class method with the same +name to access any attributes it needs to access, for example, +\samp{object.__getattribute__(self, name)}. +\end{methoddesc} + +\subsubsubsection{Implementing Descriptors \label{descriptors}} + +The following methods only apply when an instance of the class +containing the method (a so-called \emph{descriptor} class) is in +the class dictionary of another new-style class, known as the +\emph{owner} class. In the examples below, ``the attribute'' refers to +the attribute whose name is the key of the property in the accessed +class' \code{__dict__}. + +\begin{methoddesc}[object]{__get__}{self, instance, owner} +Called to get the attribute of the owner class (class attribute acess) +or of an instance of that class (instance attribute acces). +\var{owner} is always the owner class, while \var{instance} is the +instance that the attribute was accessed through, or \code{None} when +the attribute is accessed through the \var{owner}. This method should +return the (computed) attribute value or raise an +\exception{AttributeError} exception. +\end{methoddesc} + +\begin{methoddesc}[object]{__set__}{self, instance, value} +Called to set the attribute on an instance \{instance} of the owner +class to a new value, \var{value}. +\end{methoddesc} + +\begin{methoddesc}[object]{__delete__}{self, instance} +Called to delete the attribute on an instance \{instance} of the owner +class. +\end{methoddesc} + \subsection{Emulating callable objects\label{callable-types}} |