diff options
Diffstat (limited to 'Doc/reference/datamodel.rst')
| -rw-r--r-- | Doc/reference/datamodel.rst | 253 |
1 files changed, 177 insertions, 76 deletions
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst index 4bad2d5..7c51068 100644 --- a/Doc/reference/datamodel.rst +++ b/Doc/reference/datamodel.rst @@ -35,12 +35,19 @@ represented by objects.) Every object has an identity, a type and a value. An object's *identity* never changes once it has been created; you may think of it as the object's address in memory. The ':keyword:`is`' operator compares the identity of two objects; the -:func:`id` function returns an integer representing its identity (currently -implemented as its address). An object's :dfn:`type` is also unchangeable. [#]_ +:func:`id` function returns an integer representing its identity. + +.. impl-detail:: + + For CPython, ``id(x)`` is the memory address where ``x`` is stored. + 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 :func:`type` function returns an object's type (which is an object -itself). The *value* of some objects can change. Objects whose value can +itself). Like its identity, an object's :dfn:`type` is also unchangeable. +[#]_ + +The *value* of some objects can change. Objects whose value can change are said to be *mutable*; objects whose value is unchangeable once they are created are called *immutable*. (The value of an immutable container object that contains a reference to a mutable object can change when the latter's value @@ -267,25 +274,27 @@ Sequences The following types are immutable sequences: + .. index:: + single: string; immutable sequences + Strings .. index:: builtin: chr builtin: ord - builtin: str single: character single: integer single: Unicode - The items of a string object are Unicode code units. A Unicode code - unit is represented by a string object of one item and can hold either - a 16-bit or 32-bit value representing a Unicode ordinal (the maximum - value for the ordinal is given in ``sys.maxunicode``, and depends on - how Python is configured at compile time). Surrogate pairs may be - present in the Unicode object, and will be reported as two separate - items. The built-in functions :func:`chr` and :func:`ord` convert - between code units and nonnegative integers representing the Unicode - ordinals as defined in the Unicode Standard 3.0. Conversion from and to - other encodings are possible through the string method :meth:`encode`. + A string is a sequence of values that represent Unicode codepoints. + All the codepoints in range ``U+0000 - U+10FFFF`` can be represented + in a string. Python doesn't have a :c:type:`chr` type, and + every character in the string is represented as a string object + with length ``1``. The built-in function :func:`ord` converts a + character to its codepoint (as an integer); :func:`chr` converts + an integer in range ``0 - 10FFFF`` to the corresponding character. + :meth:`str.encode` can be used to convert a :class:`str` to + :class:`bytes` using the given encoding, and :meth:`bytes.decode` can + be used to achieve the opposite. Tuples .. index:: @@ -448,6 +457,11 @@ Callable types +-------------------------+-------------------------------+-----------+ | :attr:`__name__` | The function's name | Writable | +-------------------------+-------------------------------+-----------+ + | :attr:`__qualname__` | The function's | Writable | + | | :term:`qualified name` | | + | | | | + | | .. versionadded:: 3.3 | | + +-------------------------+-------------------------------+-----------+ | :attr:`__module__` | The name of the module the | Writable | | | function was defined in, or | | | | ``None`` if unavailable. | | @@ -639,17 +653,20 @@ Modules statement: import object: module - Modules are imported by the :keyword:`import` statement (see section - :ref:`import`). A module object has a - namespace implemented by a dictionary object (this is the dictionary referenced - by the __globals__ attribute of functions defined in the module). Attribute - references are translated to lookups in this dictionary, e.g., ``m.x`` is - equivalent to ``m.__dict__["x"]``. A module object does not contain the code - object used to initialize the module (since it isn't needed once the - initialization is done). - - Attribute assignment updates the module's namespace dictionary, e.g., ``m.x = - 1`` is equivalent to ``m.__dict__["x"] = 1``. + Modules are a basic organizational unit of Python code, and are created by + the :ref:`import system <importsystem>` as invoked either by the + :keyword:`import` statement (see :keyword:`import`), or by calling + functions such as :func:`importlib.import_module` and built-in + :func:`__import__`. A module object has a namespace implemented by a + dictionary object (this is the dictionary referenced by the ``__globals__`` + attribute of functions defined in the module). Attribute references are + translated to lookups in this dictionary, e.g., ``m.x`` is equivalent to + ``m.__dict__["x"]``. A module object does not contain the code object used + to initialize the module (since it isn't needed once the initialization is + done). + + Attribute assignment updates the module's namespace dictionary, e.g., + ``m.x = 1`` is equivalent to ``m.__dict__["x"] = 1``. .. index:: single: __dict__ (module attribute) @@ -671,11 +688,12 @@ Modules Predefined (writable) attributes: :attr:`__name__` is the module's name; :attr:`__doc__` is the module's documentation string, or ``None`` if - unavailable; :attr:`__file__` is the pathname of the file from which the module - was loaded, if it was loaded from a file. The :attr:`__file__` attribute is not - present for C modules that are statically linked into the interpreter; for - extension modules loaded dynamically from a shared library, it is the pathname - of the shared library file. + unavailable; :attr:`__file__` is the pathname of the file from which the + module was loaded, if it was loaded from a file. The :attr:`__file__` + attribute may be missing for certain types of modules, such as C modules + that are statically linked into the interpreter; for extension modules + loaded dynamically from a shared library, it is the pathname of the shared + library file. Custom classes Custom class types are typically created by class definitions (see section @@ -1153,7 +1171,7 @@ Basic customization Called by :func:`str(object) <str>` and the built-in functions :func:`format` and :func:`print` to compute the "informal" or nicely printable string representation of an object. The return value must be a - :ref:`string <typesseq>` object. + :ref:`string <textseq>` object. This method differs from :meth:`object.__repr__` in that there is no expectation that :meth:`__str__` return a valid Python expression: a more @@ -1172,14 +1190,14 @@ Basic customization Called by :func:`bytes` to compute a byte-string representation of an object. This should return a ``bytes`` object. - -.. method:: object.__format__(self, format_spec) - .. index:: + single: string; __format__() (object method) pair: string; conversion - builtin: str builtin: print + +.. method:: object.__format__(self, format_spec) + Called by the :func:`format` built-in function (and by extension, the :meth:`str.format` method of class :class:`str`) to produce a "formatted" string representation of an object. The ``format_spec`` argument is @@ -1258,10 +1276,10 @@ Basic customization immutable (if the object's hash value changes, it will be in the wrong hash bucket). - User-defined classes have :meth:`__eq__` and :meth:`__hash__` methods by default; with them, all objects compare unequal (except with themselves) - and ``x.__hash__()`` returns ``id(x)``. + and ``x.__hash__()`` returns an appropriate value such that ``x == y`` + implies both that ``x is y`` and ``hash(x) == hash(y)``. A class that overrides :meth:`__eq__` and does not define :meth:`__hash__` will have its :meth:`__hash__` implicitly set to ``None``. When the @@ -1280,7 +1298,27 @@ Basic customization a :exc:`TypeError` would be incorrectly identified as hashable by an ``isinstance(obj, collections.Hashable)`` call. - See also the :option:`-R` command-line option. + + .. note:: + + By default, the :meth:`__hash__` values of str, bytes and datetime + objects are "salted" with an unpredictable random value. Although they + remain constant within an individual Python process, they are not + predictable between repeated invocations of Python. + + This is intended to provide protection against a denial-of-service caused + by carefully-chosen inputs that exploit the worst case performance of a + dict insertion, O(n^2) complexity. See + http://www.ocert.org/advisories/ocert-2011-003.html for details. + + Changing hash values affects the iteration order of dicts, sets and + other mappings. Python has never made guarantees about this ordering + (and it typically varies between 32-bit and 64-bit builds). + + See also :envvar:`PYTHONHASHSEED`. + + .. versionchanged:: 3.3 + Hash randomization is enabled by default. .. method:: object.__bool__(self) @@ -1361,7 +1399,8 @@ access (use of, assignment to, or deletion of ``x.name``) for class instances. .. method:: object.__dir__(self) - Called when :func:`dir` is called on the object. A list must be returned. + Called when :func:`dir` is called on the object. A sequence must be + returned. :func:`dir` converts the returned sequence to a list and sorts it. .. _descriptors: @@ -1532,53 +1571,115 @@ Notes on using *__slots__* Customizing class creation -------------------------- -By default, classes are constructed using :func:`type`. A class definition is -read into a separate namespace and the value of class name is bound to the -result of ``type(name, bases, dict)``. +By default, classes are constructed using :func:`type`. The class body is +executed in a new namespace and the class name is bound locally to the +result of ``type(name, bases, namespace)``. -When the class definition is read, if a callable ``metaclass`` keyword argument -is passed after the bases in the class definition, the callable given will be -called instead of :func:`type`. If other keyword arguments are passed, they -will also be passed to the metaclass. This allows classes or functions to be -written which monitor or alter the class creation process: +The class creation process can be customised by passing the ``metaclass`` +keyword argument in the class definition line, or by inheriting from an +existing class that included such an argument. In the following example, +both ``MyClass`` and ``MySubclass`` are instances of ``Meta``:: -* Modifying the class dictionary prior to the class being created. + class Meta(type): + pass -* Returning an instance of another class -- essentially performing the role of a - factory function. + class MyClass(metaclass=Meta): + pass -These steps will have to be performed in the metaclass's :meth:`__new__` method --- :meth:`type.__new__` can then be called from this method to create a class -with different properties. This example adds a new element to the class -dictionary before creating the class:: + class MySubclass(MyClass): + pass - class metacls(type): - def __new__(mcs, name, bases, dict): - dict['foo'] = 'metacls was here' - return type.__new__(mcs, name, bases, dict) +Any other keyword arguments that are specified in the class definition are +passed through to all metaclass operations described below. -You can of course also override other class methods (or add new methods); for -example defining a custom :meth:`__call__` method in the metaclass allows custom -behavior when the class is called, e.g. not always creating a new instance. +When a class definition is executed, the following steps occur: -If the metaclass has a :meth:`__prepare__` attribute (usually implemented as a -class or static method), it is called before the class body is evaluated with -the name of the class and a tuple of its bases for arguments. It should return -an object that supports the mapping interface that will be used to store the -namespace of the class. The default is a plain dictionary. This could be used, -for example, to keep track of the order that class attributes are declared in by -returning an ordered dictionary. +* the appropriate metaclass is determined +* the class namespace is prepared +* the class body is executed +* the class object is created -The appropriate metaclass is determined by the following precedence rules: +Determining the appropriate metaclass +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The appropriate metaclass for a class definition is determined as follows: + +* if no bases and no explicit metaclass are given, then :func:`type` is used +* if an explicit metaclass is given and it is *not* an instance of + :func:`type`, then it is used directly as the metaclass +* if an instance of :func:`type` is given as the explicit metaclass, or + bases are defined, then the most derived metaclass is used + +The most derived metaclass is selected from the explicitly specified +metaclass (if any) and the metaclasses (i.e. ``type(cls)``) of all specified +base classes. The most derived metaclass is one which is a subtype of *all* +of these candidate metaclasses. If none of the candidate metaclasses meets +that criterion, then the class definition will fail with ``TypeError``. + + +Preparing the class namespace +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Once the appropriate metaclass has been identified, then the class namespace +is prepared. If the metaclass has a ``__prepare__`` attribute, it is called +as ``namespace = metaclass.__prepare__(name, bases, **kwds)`` (where the +additional keyword arguments, if any, come from the class definition). + +If the metaclass has no ``__prepare__`` attribute, then the class namespace +is initialised as an empty :func:`dict` instance. + +.. seealso:: + + :pep:`3115` - Metaclasses in Python 3000 + Introduced the ``__prepare__`` namespace hook + + +Executing the class body +^^^^^^^^^^^^^^^^^^^^^^^^ + +The class body is executed (approximately) as +``exec(body, globals(), namespace)``. The key difference from a normal +call to :func:`exec` is that lexical scoping allows the class body (including +any methods) to reference names from the current and outer scopes when the +class definition occurs inside a function. + +However, even when the class definition occurs inside the function, methods +defined inside the class still cannot see names defined at the class scope. +Class variables must be accessed through the first parameter of instance or +class methods, and cannot be accessed at all from static methods. + + +Creating the class object +^^^^^^^^^^^^^^^^^^^^^^^^^ + +Once the class namespace has been populated by executing the class body, +the class object is created by calling +``metaclass(name, bases, namespace, **kwds)`` (the additional keywords +passed here are the same as those passed to ``__prepare__``). + +This class object is the one that will be referenced by the zero-argument +form of :func:`super`. ``__class__`` is an implicit closure reference +created by the compiler if any methods in a class body refer to either +``__class__`` or ``super``. This allows the zero argument form of +:func:`super` to correctly identify the class being defined based on +lexical scoping, while the class or instance that was used to make the +current call is identified based on the first argument passed to the method. + +After the class object is created, it is passed to the class decorators +included in the class definition (if any) and the resulting object is bound +in the local namespace as the defined class. + +.. seealso:: -* If the ``metaclass`` keyword argument is passed with the bases, it is used. + :pep:`3135` - New super + Describes the implicit ``__class__`` closure reference -* Otherwise, if there is at least one base class, its metaclass is used. -* Otherwise, the default metaclass (:class:`type`) is used. +Metaclass example +^^^^^^^^^^^^^^^^^ The potential uses for metaclasses are boundless. Some ideas that have been -explored including logging, interface checking, automatic delegation, automatic +explored include logging, interface checking, automatic delegation, automatic property creation, proxies, frameworks, and automatic resource locking/synchronization. @@ -1591,9 +1692,9 @@ to remember the order that class members were defined:: def __prepare__(metacls, name, bases, **kwds): return collections.OrderedDict() - def __new__(cls, name, bases, classdict): - result = type.__new__(cls, name, bases, dict(classdict)) - result.members = tuple(classdict) + def __new__(cls, name, bases, namespace, **kwds): + result = type.__new__(cls, name, bases, dict(namespace)) + result.members = tuple(namespace) return result class A(metaclass=OrderedClass): |