descriptor.po

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#: ../Doc/howto/descriptor.rst:3
msgid "Descriptor HowTo Guide"
msgstr ""

#: ../Doc/howto/descriptor.rst
msgid "Author"
msgstr ""

#: ../Doc/howto/descriptor.rst:5
msgid "Raymond Hettinger"
msgstr ""

#: ../Doc/howto/descriptor.rst
msgid "Contact"
msgstr ""

#: ../Doc/howto/descriptor.rst:6
msgid "<python at rcn dot com>"
msgstr ""

#: ../Doc/howto/descriptor.rst:8
msgid "Contents"
msgstr ""

#: ../Doc/howto/descriptor.rst:11
msgid "Abstract"
msgstr ""

#: ../Doc/howto/descriptor.rst:13
msgid ""
"Defines descriptors, summarizes the protocol, and shows how descriptors are "
"called.  Examines a custom descriptor and several built-in Python "
"descriptors including functions, properties, static methods, and class "
"methods.  Shows how each works by giving a pure Python equivalent and a "
"sample application."
msgstr ""

#: ../Doc/howto/descriptor.rst:18
msgid ""
"Learning about descriptors not only provides access to a larger toolset, it "
"creates a deeper understanding of how Python works and an appreciation for "
"the elegance of its design."
msgstr ""

#: ../Doc/howto/descriptor.rst:24
msgid "Definition and Introduction"
msgstr ""

#: ../Doc/howto/descriptor.rst:26
msgid ""
"In general, a descriptor is an object attribute with \"binding behavior\", "
"one whose attribute access has been overridden by methods in the descriptor "
"protocol.  Those methods are :meth:`__get__`, :meth:`__set__`, and :meth:"
"`__delete__`.  If any of those methods are defined for an object, it is said "
"to be a descriptor."
msgstr ""

#: ../Doc/howto/descriptor.rst:32
msgid ""
"The default behavior for attribute access is to get, set, or delete the "
"attribute from an object's dictionary.  For instance, ``a.x`` has a lookup "
"chain starting with ``a.__dict__['x']``, then ``type(a).__dict__['x']``, and "
"continuing through the base classes of ``type(a)`` excluding metaclasses. If "
"the looked-up value is an object defining one of the descriptor methods, "
"then Python may override the default behavior and invoke the descriptor "
"method instead. Where this occurs in the precedence chain depends on which "
"descriptor methods were defined."
msgstr ""

#: ../Doc/howto/descriptor.rst:41
msgid ""
"Descriptors are a powerful, general purpose protocol.  They are the "
"mechanism behind properties, methods, static methods, class methods, and :"
"func:`super()`. They are used throughout Python itself to implement the new "
"style classes introduced in version 2.2.  Descriptors simplify the "
"underlying C-code and offer a flexible set of new tools for everyday Python "
"programs."
msgstr ""

#: ../Doc/howto/descriptor.rst:49
msgid "Descriptor Protocol"
msgstr ""

#: ../Doc/howto/descriptor.rst:51
msgid "``descr.__get__(self, obj, type=None) -> value``"
msgstr ""

#: ../Doc/howto/descriptor.rst:53
msgid "``descr.__set__(self, obj, value) -> None``"
msgstr ""

#: ../Doc/howto/descriptor.rst:55
msgid "``descr.__delete__(self, obj) -> None``"
msgstr ""

#: ../Doc/howto/descriptor.rst:57
msgid ""
"That is all there is to it.  Define any of these methods and an object is "
"considered a descriptor and can override default behavior upon being looked "
"up as an attribute."
msgstr ""

#: ../Doc/howto/descriptor.rst:61
msgid ""
"If an object defines :meth:`__set__` or :meth:`__delete__`, it is considered "
"a data descriptor.  Descriptors that only define :meth:`__get__` are called "
"non-data descriptors (they are typically used for methods but other uses are "
"possible)."
msgstr ""

#: ../Doc/howto/descriptor.rst:66
msgid ""
"Data and non-data descriptors differ in how overrides are calculated with "
"respect to entries in an instance's dictionary.  If an instance's dictionary "
"has an entry with the same name as a data descriptor, the data descriptor "
"takes precedence.  If an instance's dictionary has an entry with the same "
"name as a non-data descriptor, the dictionary entry takes precedence."
msgstr ""

#: ../Doc/howto/descriptor.rst:72
msgid ""
"To make a read-only data descriptor, define both :meth:`__get__` and :meth:"
"`__set__` with the :meth:`__set__` raising an :exc:`AttributeError` when "
"called.  Defining the :meth:`__set__` method with an exception raising "
"placeholder is enough to make it a data descriptor."
msgstr ""

#: ../Doc/howto/descriptor.rst:79
msgid "Invoking Descriptors"
msgstr ""

#: ../Doc/howto/descriptor.rst:81
msgid ""
"A descriptor can be called directly by its method name.  For example, ``d."
"__get__(obj)``."
msgstr ""

#: ../Doc/howto/descriptor.rst:84
msgid ""
"Alternatively, it is more common for a descriptor to be invoked "
"automatically upon attribute access.  For example, ``obj.d`` looks up ``d`` "
"in the dictionary of ``obj``.  If ``d`` defines the method :meth:`__get__`, "
"then ``d.__get__(obj)`` is invoked according to the precedence rules listed "
"below."
msgstr ""

#: ../Doc/howto/descriptor.rst:89
msgid ""
"The details of invocation depend on whether ``obj`` is an object or a class."
msgstr ""

#: ../Doc/howto/descriptor.rst:91
msgid ""
"For objects, the machinery is in :meth:`object.__getattribute__` which "
"transforms ``b.x`` into ``type(b).__dict__['x'].__get__(b, type(b))``.  The "
"implementation works through a precedence chain that gives data descriptors "
"priority over instance variables, instance variables priority over non-data "
"descriptors, and assigns lowest priority to :meth:`__getattr__` if provided. "
"The full C implementation can be found in :c:func:"
"`PyObject_GenericGetAttr()` in :source:`Objects/object.c`."
msgstr ""

#: ../Doc/howto/descriptor.rst:99
msgid ""
"For classes, the machinery is in :meth:`type.__getattribute__` which "
"transforms ``B.x`` into ``B.__dict__['x'].__get__(None, B)``.  In pure "
"Python, it looks like::"
msgstr ""

#: ../Doc/howto/descriptor.rst:110
msgid "The important points to remember are:"
msgstr ""

#: ../Doc/howto/descriptor.rst:112
msgid "descriptors are invoked by the :meth:`__getattribute__` method"
msgstr ""

#: ../Doc/howto/descriptor.rst:113
msgid "overriding :meth:`__getattribute__` prevents automatic descriptor calls"
msgstr ""

#: ../Doc/howto/descriptor.rst:114
msgid ""
":meth:`object.__getattribute__` and :meth:`type.__getattribute__` make "
"different calls to :meth:`__get__`."
msgstr ""

#: ../Doc/howto/descriptor.rst:116
msgid "data descriptors always override instance dictionaries."
msgstr ""

#: ../Doc/howto/descriptor.rst:117
msgid "non-data descriptors may be overridden by instance dictionaries."
msgstr ""

#: ../Doc/howto/descriptor.rst:119
msgid ""
"The object returned by ``super()`` also has a custom :meth:"
"`__getattribute__` method for invoking descriptors.  The attribute lookup "
"``super(B, obj).m`` searches ``obj.__class__.__mro__`` for the base class "
"``A`` immediately following ``B`` and then returns ``A.__dict__['m']."
"__get__(obj, B)``.  If not a descriptor, ``m`` is returned unchanged.  If "
"not in the dictionary, ``m`` reverts to a search using :meth:`object."
"__getattribute__`."
msgstr ""

#: ../Doc/howto/descriptor.rst:126
msgid ""
"The implementation details are in :c:func:`super_getattro()` in :source:"
"`Objects/typeobject.c`.  and a pure Python equivalent can be found in "
"`Guido's Tutorial`_."
msgstr ""

#: ../Doc/howto/descriptor.rst:132
msgid ""
"The details above show that the mechanism for descriptors is embedded in "
"the :meth:`__getattribute__()` methods for :class:`object`, :class:`type`, "
"and :func:`super`.  Classes inherit this machinery when they derive from :"
"class:`object` or if they have a meta-class providing similar functionality. "
"Likewise, classes can turn-off descriptor invocation by overriding :meth:"
"`__getattribute__()`."
msgstr ""

#: ../Doc/howto/descriptor.rst:141
msgid "Descriptor Example"
msgstr ""

#: ../Doc/howto/descriptor.rst:143
msgid ""
"The following code creates a class whose objects are data descriptors which "
"print a message for each get or set.  Overriding :meth:`__getattribute__` is "
"alternate approach that could do this for every attribute.  However, this "
"descriptor is useful for monitoring just a few chosen attributes::"
msgstr ""

#: ../Doc/howto/descriptor.rst:181
msgid ""
"The protocol is simple and offers exciting possibilities.  Several use cases "
"are so common that they have been packaged into individual function calls. "
"Properties, bound methods, static methods, and class methods are all based "
"on the descriptor protocol."
msgstr ""

#: ../Doc/howto/descriptor.rst:188
msgid "Properties"
msgstr ""

#: ../Doc/howto/descriptor.rst:190
msgid ""
"Calling :func:`property` is a succinct way of building a data descriptor "
"that triggers function calls upon access to an attribute.  Its signature is::"
msgstr ""

#: ../Doc/howto/descriptor.rst:195
msgid ""
"The documentation shows a typical use to define a managed attribute ``x``::"
msgstr ""

#: ../Doc/howto/descriptor.rst:203
msgid ""
"To see how :func:`property` is implemented in terms of the descriptor "
"protocol, here is a pure Python equivalent::"
msgstr ""

#: ../Doc/howto/descriptor.rst:243
msgid ""
"The :func:`property` builtin helps whenever a user interface has granted "
"attribute access and then subsequent changes require the intervention of a "
"method."
msgstr ""

#: ../Doc/howto/descriptor.rst:247
msgid ""
"For instance, a spreadsheet class may grant access to a cell value through "
"``Cell('b10').value``. Subsequent improvements to the program require the "
"cell to be recalculated on every access; however, the programmer does not "
"want to affect existing client code accessing the attribute directly.  The "
"solution is to wrap access to the value attribute in a property data "
"descriptor::"
msgstr ""

#: ../Doc/howto/descriptor.rst:263
msgid "Functions and Methods"
msgstr ""

#: ../Doc/howto/descriptor.rst:265
msgid ""
"Python's object oriented features are built upon a function based "
"environment. Using non-data descriptors, the two are merged seamlessly."
msgstr ""

#: ../Doc/howto/descriptor.rst:268
msgid ""
"Class dictionaries store methods as functions.  In a class definition, "
"methods are written using :keyword:`def` or :keyword:`lambda`, the usual "
"tools for creating functions.  Methods only differ from regular functions in "
"that the first argument is reserved for the object instance.  By Python "
"convention, the instance reference is called *self* but may be called *this* "
"or any other variable name."
msgstr ""

#: ../Doc/howto/descriptor.rst:275
msgid ""
"To support method calls, functions include the :meth:`__get__` method for "
"binding methods during attribute access.  This means that all functions are "
"non-data descriptors which return bound methods when they are invoked from "
"an object.  In pure Python, it works like this::"
msgstr ""

#: ../Doc/howto/descriptor.rst:288
msgid ""
"Running the interpreter shows how the function descriptor works in practice::"
msgstr ""

#: ../Doc/howto/descriptor.rst:326
msgid "Static Methods and Class Methods"
msgstr ""

#: ../Doc/howto/descriptor.rst:328
msgid ""
"Non-data descriptors provide a simple mechanism for variations on the usual "
"patterns of binding functions into methods."
msgstr ""

#: ../Doc/howto/descriptor.rst:331
msgid ""
"To recap, functions have a :meth:`__get__` method so that they can be "
"converted to a method when accessed as attributes.  The non-data descriptor "
"transforms an ``obj.f(*args)`` call into ``f(obj, *args)``.  Calling ``klass."
"f(*args)`` becomes ``f(*args)``."
msgstr ""

#: ../Doc/howto/descriptor.rst:336
msgid "This chart summarizes the binding and its two most useful variants:"
msgstr ""

#: ../Doc/howto/descriptor.rst:339
msgid "Transformation"
msgstr ""

#: ../Doc/howto/descriptor.rst:339
msgid "Called from an Object"
msgstr ""

#: ../Doc/howto/descriptor.rst:339
msgid "Called from a Class"
msgstr ""

#: ../Doc/howto/descriptor.rst:342
msgid "function"
msgstr ""

#: ../Doc/howto/descriptor.rst:342
msgid "f(obj, \\*args)"
msgstr ""

#: ../Doc/howto/descriptor.rst:342 ../Doc/howto/descriptor.rst:344
msgid "f(\\*args)"
msgstr ""

#: ../Doc/howto/descriptor.rst:344
msgid "staticmethod"
msgstr ""

#: ../Doc/howto/descriptor.rst:346
msgid "classmethod"
msgstr ""

#: ../Doc/howto/descriptor.rst:346
msgid "f(type(obj), \\*args)"
msgstr ""

#: ../Doc/howto/descriptor.rst:346
msgid "f(klass, \\*args)"
msgstr ""

#: ../Doc/howto/descriptor.rst:349
msgid ""
"Static methods return the underlying function without changes.  Calling "
"either ``c.f`` or ``C.f`` is the equivalent of a direct lookup into ``object."
"__getattribute__(c, \"f\")`` or ``object.__getattribute__(C, \"f\")``. As a "
"result, the function becomes identically accessible from either an object or "
"a class."
msgstr ""

#: ../Doc/howto/descriptor.rst:355
msgid ""
"Good candidates for static methods are methods that do not reference the "
"``self`` variable."
msgstr ""

#: ../Doc/howto/descriptor.rst:358
msgid ""
"For instance, a statistics package may include a container class for "
"experimental data.  The class provides normal methods for computing the "
"average, mean, median, and other descriptive statistics that depend on the "
"data. However, there may be useful functions which are conceptually related "
"but do not depend on the data.  For instance, ``erf(x)`` is handy conversion "
"routine that comes up in statistical work but does not directly depend on a "
"particular dataset. It can be called either from an object or the class:  "
"``s.erf(1.5) --> .9332`` or ``Sample.erf(1.5) --> .9332``."
msgstr ""

#: ../Doc/howto/descriptor.rst:367
msgid ""
"Since staticmethods return the underlying function with no changes, the "
"example calls are unexciting::"
msgstr ""

#: ../Doc/howto/descriptor.rst:380
msgid ""
"Using the non-data descriptor protocol, a pure Python version of :func:"
"`staticmethod` would look like this::"
msgstr ""

#: ../Doc/howto/descriptor.rst:392
msgid ""
"Unlike static methods, class methods prepend the class reference to the "
"argument list before calling the function.  This format is the same for "
"whether the caller is an object or a class::"
msgstr ""

#: ../Doc/howto/descriptor.rst:407
msgid ""
"This behavior is useful whenever the function only needs to have a class "
"reference and does not care about any underlying data.  One use for "
"classmethods is to create alternate class constructors.  In Python 2.3, the "
"classmethod :func:`dict.fromkeys` creates a new dictionary from a list of "
"keys.  The pure Python equivalent is::"
msgstr ""

#: ../Doc/howto/descriptor.rst:423
msgid "Now a new dictionary of unique keys can be constructed like this::"
msgstr ""

#: ../Doc/howto/descriptor.rst:428
msgid ""
"Using the non-data descriptor protocol, a pure Python version of :func:"
"`classmethod` would look like this::"
msgstr ""