mirror of https://github.com/python/cpython
Address substantially all of Eric Snow's comments in issue #15295, except for
those which now have additional XXX's here. I'll get to those later. :)
This commit is contained in:
Barry Warsaw
parent
38732ea832
commit
c1e721b979
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@ -475,7 +475,7 @@ Glossary
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module
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An object that serves as an organizational unit of Python code. Modules
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have a namespace contain arbitrary Python objects. Modules are loaded
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have a namespace containing arbitrary Python objects. Modules are loaded
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into Python by the process of :term:`importing`.
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MRO
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@ -8,30 +8,44 @@ Import machinery
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.. index:: single: import machinery
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Python code in one :term:`module` gains access to the code in another module
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by the process of :term:`importing` it. Most commonly, the :keyword:`import`
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statement is used to invoke the import machinery, but it can also be invoked
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by calling the built-in :func:`__import__` function.
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by the process of :term:`importing` it. The :keyword:`import` statement is
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the most common way of invoking the import machinery, but it is not the only
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way. Functions such as :func:`importlib.import_module` and built-in
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:func:`__import__` can also be used to invoke the import machinery.
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The :keyword:`import` statement combines two operations; it searches for the
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named module, then it binds the results of that search to a name in the local
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scope. The search operation of the :keyword:`import` statement is defined as
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a call to the :func:`__import__` function, with the appropriate arguments.
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The return value of :func:`__import__` is used to perform the name binding
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operation of the :keyword:`import` statement. See the :keyword:`import`
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statement for the exact details of that name binding operation.
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a call to the built-in :func:`__import__` function, with the appropriate
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arguments. The return value of :func:`__import__` is used to perform the name
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binding operation of the :keyword:`import` statement. See the
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:keyword:`import` statement for the exact details of that name binding
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operation.
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A direct call to :func:`__import__` performs only the search for the module.
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The function's return value is used like any other function call in Python;
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there is no special side-effects (e.g. name binding) associated with
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:func:`__import__`.
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A direct call to :func:`__import__` performs only the module search and, if
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found, the module creation operation. While certain side-effects may occur,
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such as the importing of parent packages, and the updating of various caches
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(including :data:`sys.modules`), only the :keyword:`import` statement performs
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a name binding operation.
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When a module is first imported, Python searches for the module and if found,
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it creates a module object, initializing it. If the named module cannot be
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found, an :exc:`ImportError` is raised. Python implements various strategies
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to search for the named module when the import machinery is invoked. These
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strategies can be modified and extended by using various hooks described in
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the sections below. The entire import machinery itself can be overridden by
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replacing built-in :func:`__import__`.
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it creates a module object [#fnmo]_, initializing it. If the named module
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cannot be found, an :exc:`ImportError` is raised. Python implements various
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strategies to search for the named module when the import machinery is
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invoked. These strategies can be modified and extended by using various hooks
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described in the sections below. The entire import machinery itself can be
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overridden by replacing built-in :func:`__import__`.
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:mod:`importlib`
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================
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The :mod:`importlib` module provides a rich API for interacting with the
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import system. For example :func:`importlib.import_module` provides a
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recommended, simpler API than built-in :func:`__import__` for invoking the
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import machinery. Refer to the :mod:`importlib` library documentation for
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additional detail.
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Packages
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@ -43,25 +57,26 @@ Packages
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Python has only one type of module object, and all modules are of this type,
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regardless of whether the module is implemented in Python, C, or something
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else. To help organize modules and provide a naming hierarchy, Python has a
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concept of :term:`packages <package>`. It's important to keep in mind that
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all packages are modules, but not all modules are packages. Or put another
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way, packages are just a special kind of module. Although usually
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unnecessary, introspection of various module object attributes can determine
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whether a module is a package or not.
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concept of :term:`packages <package>`.
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Packages can contain other packages and modules, while modules generally do
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not contain other modules or packages. You can think of packages as the
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directories on a file system and modules as files within directories, but
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don't take this analogy too literally since packages and modules need not
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originate from the file system. For the purposes of this documentation, we'll
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use this convenient analogy of directories and files.
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You can think of packages as the directories on a file system and modules as
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files within directories, but don't take this analogy too literally since
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packages and modules need not originate from the file system. For the
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purposes of this documentation, we'll use this convenient analogy of
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directories and files. Like file system directories, packages are organized
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hierarchically, and packages may themselves contain subpackages, as well as
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regular modules.
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All modules have a name. Packages also have names, and subpackages can be
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nested arbitrarily deeply. Subpackage names are separated from their parent
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package by dots, akin to Python's standard attribute access syntax. Thus you
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might have a module called :mod:`sys` and a package called :mod:`email`, which
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in turn has a subpackage called :mod:`email.mime` and a module within that
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subpackage called :mod:`email.mime.text`.
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It's important to keep in mind that all packages are modules, but not all
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modules are packages. Or put another way, packages are just a special kind of
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module. Specifically, any module that contains an ``__path__`` attribute is
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considered a package.
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All modules have a name. Subpackage names are separated from their parent
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package name by dots, akin to Python's standard attribute access syntax. Thus
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you might have a module called :mod:`sys` and a package called :mod:`email`,
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which in turn has a subpackage called :mod:`email.mime` and a module within
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that subpackage called :mod:`email.mime.text`.
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Regular packages
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@ -80,22 +95,6 @@ bound to names in the package's namespace. The ``__init__.py`` file can
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contain the same Python code that any other module can contain, and Python
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will add some additional attributes to the module when it is imported.
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Namespace packages
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------------------
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.. index::
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pair:: package; namespace
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pair:: package; portion
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A namespace package is a composite of various :term:`portions <portion>`,
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where each portion contributes a subpackage to the parent package. Portions
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may reside in different locations on the file system. Portions may also be
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found in zip files, on the network, or anywhere else that Python searches
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during import. Namespace packages may or may not correspond directly to
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objects on the file system; they may be virtual modules that have no concrete
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representation.
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For example, the following file system layout defines a top level ``parent``
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package with three subpackages::
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@ -113,14 +112,31 @@ Importing ``parent.one`` will implicitly import ``parent/__init__.py`` and
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``parent.three`` will import ``parent/two/__init__.py`` and
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``parent/three/__init__.py`` respectively.
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Namespace packages
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------------------
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.. index::
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pair:: package; namespace
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pair:: package; portion
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A namespace package is a composite of various :term:`portions <portion>`,
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where each portion contributes a subpackage to the parent package. Portions
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may reside in different locations on the file system. Portions may also be
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found in zip files, on the network, or anywhere else that Python searches
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during import. Namespace packages may or may not correspond directly to
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objects on the file system; they may be virtual modules that have no concrete
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representation.
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With namespace packages, there is no ``parent/__init__.py`` file. In fact,
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there may be multiple ``parent`` directories found during import search, where
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each one is provided by a separate vendor installed container, and none of
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them contain an ``__init__.py`` file. Thus ``parent/one`` may not be
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each one is provided by a different portion. Thus ``parent/one`` may not be
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physically located next to ``parent/two``. In this case, Python will create a
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namespace package for the top-level ``parent`` package whenever it or one of
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its subpackages is imported.
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See also :pep:`420` for the namespace package specification.
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Searching
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=========
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@ -129,7 +145,7 @@ To begin the search, Python needs the :term:`fully qualified <qualified name>`
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name of the module (or package, but for the purposes of this discussion, the
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difference is immaterial) being imported. This name may come from various
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arguments to the :keyword:`import` statement, or from the parameters to the
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:func:`__import__` function.
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:func:`importlib.import_module` or :func:`__import__` functions.
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This name will be used in various phases of the import search, and it may be
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the dotted path to a submodule, e.g. ``foo.bar.baz``. In this case, Python
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@ -156,8 +172,8 @@ process completes. However, if the value is ``None``, then an
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:exc:`ImportError` is raised. If the module name is missing, Python will
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continue searching for the module.
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:data:`sys.modules` is writable. Deleting a key will generally not destroy
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the associated module, but it will invalidate the cache entry for the named
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:data:`sys.modules` is writable. Deleting a key will not destroy the
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associated module, but it will invalidate the cache entry for the named
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module, causing Python to search anew for the named module upon its next
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import. Beware though, because if you keep a reference to the module object,
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invalidate its cache entry in :data:`sys.modules`, and then re-import the
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@ -265,11 +281,12 @@ Meta path loaders
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-----------------
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Once a loader is found via a meta path finder, the loader's
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:meth:`load_module()` method is called, with a single argument, the fully
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qualified name of the module being imported. This method has several
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responsibilities, and should return the module object it has loaded [#fn1]_.
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If it cannot load the module, it should raise an :exc:`ImportError`, although
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any other exception raised during :meth:`load_module()` will be propagated.
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:meth:`~importlib.abc.Loader.load_module` method is called, with a single
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argument, the fully qualified name of the module being imported. This method
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has several responsibilities, and should return the module object it has
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loaded [#fnlo]_. If it cannot load the module, it should raise an
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:exc:`ImportError`, although any other exception raised during
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:meth:`load_module()` will be propagated.
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In many cases, the meta path finder and loader can be the same object,
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e.g. :meth:`finder.find_module()` would just return ``self``.
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@ -278,8 +295,8 @@ Loaders must satisfy the following requirements:
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* If there is an existing module object with the given name in
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:data:`sys.modules`, the loader must use that existing module. (Otherwise,
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the :func:`reload()` builtin will not work correctly.) If the named module
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does not exist in :data:`sys.modules`, the loader must create a new module
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the :func:`imp.reload` will not work correctly.) If the named module does
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not exist in :data:`sys.modules`, the loader must create a new module
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object and add it to :data:`sys.modules`.
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Note that the module *must* exist in :data:`sys.modules` before the loader
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@ -314,28 +331,29 @@ Loaders must satisfy the following requirements:
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* The module's ``__package__`` attribute should be set. Its value must be a
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string, but it can be the same value as its ``__name__``. This is the
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recommendation when the module is a package. When the module is not a
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package, ``__package__`` should be set to the parent package's name.
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package, ``__package__`` should be set to the parent package's
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name [#fnpk]_.
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This attribute is used instead of ``__name__`` to calculate explicit
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relative imports for main modules, as defined in :pep:`366`.
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* If the module is a Python module (as opposed to a built-in module or a
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dynamically loaded extension), it should execute the module's code in the
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module's global name space (``module.__dict__``).
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dynamically loaded extension), the loader should execute the module's code
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in the module's global name space (``module.__dict__``).
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Module reprs
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------------
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By default, all modules have a usable repr, however depending on the
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attributes set above, and hooks in the loader, you can more tightly control
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attributes set above, and hooks in the loader, you can more explicitly control
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the repr of module objects.
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Loaders may implement a :meth:`module_repr()` method which takes a single
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argument, the module object. When ``repr(module)`` is called for a module
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with a loader supporting this protocol, whatever is returned from
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``loader.module_repr(module)`` is returned as the module's repr without
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further processing. This return value must be a string.
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``module.__loader__.module_repr(module)`` is returned as the module's repr
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without further processing. This return value must be a string.
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If the module has no ``__loader__`` attribute, or the loader has no
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:meth:`module_repr()` method, then the module object implementation itself
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@ -385,7 +403,7 @@ However, ``__path__`` is typically much more constrained than
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``__path__`` must be a list, but it may be empty. The same rules used for
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:data:`sys.path` also apply to a package's ``__path__``, and
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:data:`sys.path_hooks` (described below) are consulted when traversing a
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:data:`sys.path_hooks` (described below) is consulted when traversing a
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package's ``__path__``.
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A package's ``__init__.py`` file may set or alter the package's ``__path__``
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@ -452,7 +470,7 @@ modules and packages. It is initialized from the :data:`PYTHONPATH`
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environment variable and various other installation- and
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implementation-specific defaults. Entries in :data:`sys.path` can name
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directories on the file system, zip files, and potentially other "locations"
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that should be searched for modules.
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(see the :mod:`site` module) that should be searched for modules.
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The path importer is a meta path finder, so the import machinery begins file
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system search by calling the path importer's :meth:`find_module()` method as
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@ -468,7 +486,7 @@ entries to sys path finders. This cache is maintained in
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:data:`sys.path_importer_cache`. In this way, the expensive search for a
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particular path location's sys path finder need only be done once. User code
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is free to remove cache entries from :data:`sys.path_importer_cache` forcing
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the path importer to perform the path search again.
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the path importer to perform the path search again [#fnpic]_.
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If the path entry is not present in the cache, the path importer iterates over
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every callable in :data:`sys.path_hooks`. Each entry in this list is called
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@ -484,9 +502,8 @@ returned then the path importer's :meth:`find_module()` method will return
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If a sys path finder *is* returned by one of the callables on
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:data:`sys.path_hooks`, then the following protocol is used to ask the sys
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path finder for a module loader. If a loader results from this step, it is
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used to load the module as previously described (i.e. its
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:meth:`load_module()` method is called).
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path finder for a module loader, which is then used to load the module as
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previously described (i.e. its :meth:`load_module()` method is called).
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sys path finder protocol
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@ -520,14 +537,24 @@ ignored and the loader is returned from the path importer, terminating the
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Open issues
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===========
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XXX What to say about `imp.NullImporter` when it's found in
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:data:`sys.path_importer_cache`?
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XXX Find a better term than "path importer" for class PathFinder and update
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the glossary.
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XXX It would be really nice to have a diagram.
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.. [#fn1] The importlib implementation appears not to use the return value
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directly. Instead, it gets the module object by looking the module name up
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in ``sys.modules``.)
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XXX * (import_machinery.rst) how about a section devoted just to the
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attributes of modules and packages, perhaps expanding upon or supplanting the
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related entries in the data model reference page?
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XXX * (import_machinery.rst) Meta path loaders, end of paragraph 2: "The
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finder could also be a classmethod that returns an instance of the class."
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XXX * (import_machinery.rst) Meta path loaders: "If the load fails, the loader
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needs to remove any modules..." is a pretty exceptional case, since the
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modules is not in charge of its parent or children, nor of import statements
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executed for it. Is this a new requirement?
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XXX Module reprs: how does module.__qualname__ fit in?
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References
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@ -545,3 +572,28 @@ without ``__init__.py`` files in Python 3.3. :pep:`420` also introduced the
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:pep:`366` describes the addition of the ``__package__`` attribute for
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explicit relative imports in main modules.
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Footnotes
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=========
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.. [#fnmo] See :class:`types.ModuleType`.
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.. [#fnlo] The importlib implementation appears not to use the return value
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directly. Instead, it gets the module object by looking the module name up
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in :data:`sys.modules`.) The indirect effect of this is that an imported
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module may replace itself in :data:`sys.modules`. This is
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implementation-specific behavior that is not guaranteed to work in other
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Python implementations.
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.. [#fnpk] In practice, within CPython there is little consistency in the
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values of ``__package__`` for top-level modules. In some, such as in the
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:mod:`email` package, both the ``__name__`` and ``__package__`` are set to
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"email". In other top-level modules (non-packages), ``__package__`` may be
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set to ``None`` or the empty string. The recommendation for top-level
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non-package modules is to set ``__package__`` to the empty string.
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.. [#fnpic] In legacy code, it is possible to find instances of
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:class:`imp.NullImporter` in the :data:`sys.path_importer_cache`. It
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recommended that code be changed to use ``None`` instead. See
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:ref:`portingpythoncode` for more details.
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@ -1677,6 +1677,8 @@ Porting to Python 3.3
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This section lists previously described changes and other bugfixes
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that may require changes to your code.
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.. _portingpythoncode:
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Porting Python code
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-------------------
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