http://hg.python.org/cpython/rev/1cf1df6f62f5 changeset: 78357:1cf1df6f62f5 user: Barry Warsaw <barry@python.org> date: Tue Jul 31 16:03:09 2012 -0400 summary: Finally, a coherent set of terminology for all the lil' beasties involved. files: Doc/glossary.rst | 34 +- Doc/reference/datamodel.rst | 19 +- Doc/reference/import_machinery.rst | 307 ++++++++-------- Doc/reference/index.rst | 2 +- Doc/reference/simple_stmts.rst | 15 +- 5 files changed, 205 insertions(+), 172 deletions(-) diff --git a/Doc/glossary.rst b/Doc/glossary.rst --- a/Doc/glossary.rst +++ b/Doc/glossary.rst @@ -315,6 +315,13 @@ role in places where a constant hash value is needed, for example as a key in a dictionary. + import path + A list of locations (or :term:`path entries <path entry>`) that are + searched by the :term:`path importer` for modules to import. During + import, this list of locations usually comes from :data:`sys.path`, but + for subpackages it may also come from the parent package's ``__path__`` + attribute. + importing The process by which Python code in one module is made available to Python code in another module. @@ -446,8 +453,8 @@ meta path finder A finder returned by a search of :data:`sys.meta_path`. Meta path - finders are related to, but different from :term:`sys path finders <sys - path finder>`. + finders are related to, but different from :term:`path entry finders + <path entry finder>`. metaclass The class of a class. Class definitions create a class name, a class @@ -541,9 +548,23 @@ subpackages. Technically, a package is a Python module with an ``__path__`` attribute. + path entry + A single location on the :term:`import path` which the :term:`path + importer` consults to find modules for importing. + + path entry finder + A :term:`finder` returned by a callable on :data:`sys.path_hooks` + (i.e. a :term:`path entry hook`) which knows how to locate modules given + a :term:`path entry`. + + path entry hook + A callable on the :data:`sys.path_hook` list which returns a :term:`path + entry finder` if it knows how to find modules on a specific :term:`path + entry`. + path importer - A built-in :term:`finder` / :term:`loader` that knows how to find and - load modules from the file system. + One of the default :term:`meta path finders <meta path finder>` which + searches an :term:`import path` for modules. portion A set of files in a single directory (possibly stored in a zip file) @@ -671,11 +692,6 @@ :meth:`~collections.somenamedtuple._asdict`. Examples of struct sequences include :data:`sys.float_info` and the return value of :func:`os.stat`. - sys path finder - A finder returned by a search of :data:`sys.path` by the :term:`path - importer`. Sys path finders are related to, but different from - :term:`meta path finders <meta path finder>`. - triple-quoted string A string which is bound by three instances of either a quotation mark (") or an apostrophe ('). While they don't provide any functionality diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst --- a/Doc/reference/datamodel.rst +++ b/Doc/reference/datamodel.rst @@ -652,15 +652,16 @@ object: module Modules are a basic organizational unit of Python code, and are created by - the :ref:`importmachinery` as invoked either by the :keyword:`import` - statement (see section :ref:`import`) or by calling the built in - :func:`__import__` function. 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). + 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``. diff --git a/Doc/reference/import_machinery.rst b/Doc/reference/import.rst rename from Doc/reference/import_machinery.rst rename to Doc/reference/import.rst --- a/Doc/reference/import_machinery.rst +++ b/Doc/reference/import.rst @@ -1,9 +1,9 @@ -.. _importmachinery: +.. _importsystem: -**************** -Import machinery -**************** +***************** +The import system +***************** .. index:: single: import machinery @@ -33,8 +33,8 @@ cannot be found, an :exc:`ImportError` is raised. Python implements various strategies to search for the named module when the import machinery is invoked. These strategies can be modified and extended by using various hooks -described in the sections below. The entire import machinery itself can be -overridden by replacing built-in :func:`__import__`. +described in the sections below. More coarse-grained overriding of the import +system can be accomplished by replacing built-in :func:`__import__`. :mod:`importlib` @@ -189,25 +189,25 @@ single: finder single: loader -If the named module is not found in :data:`sys.modules` then Python's import -protocol is invoked to find and load the module. As this implies, the import -protocol consists of two conceptual objects, :term:`finders <finder>` and -:term:`loaders <loader>`. A finder's job is to determine whether it can find -the named module using whatever strategy it knows about. For example, there -is a file system finder which know how to search the file system for the named -module. Other finders may know how to search a zip file, a web page, or a -database to find the named module. The import machinery is extensible, so new -finders can be added to extend the range and scope of module searching. +If the named module is not found in :data:`sys.modules`, then Python's import +protocol is invoked to find and load the module. This protocol consists of +two conceptual objects, :term:`finders <finder>` and :term:`loaders <loader>`. +A finder's job is to determine whether it can find the named module using +whatever strategy it knows about. + +By default, Python comes with several default finders. One knows how to +locate frozen modules, and another knows how to locate built-in modules. A +third default finder searches an :term:`import path` for modules. The +:term:`import path` is a list of locations that may name file system paths or +zip files. It can also be extended to search for any locatable resource, such +as those identified by URLs. + +The import machinery is extensible, so new finders can be added to extend the +range and scope of module searching. Finders do not actually load modules. If they can find the named module, they -return a loader, which the import machinery later invokes to load the module -and create the corresponding module object. - -There are actually two types of finders, and two different but related APIs -for finders, depending on whether it is a :term:`meta path finder` or a -:term:`sys path finder`. Meta path processing occurs at the beginning of -import processing, while sys path processing happens later, by the :term:`path -importer`. +return a :term:`loader`, which the import machinery then invokes to load the +module and create the corresponding module object. The following sections describe the protocol for finders and loaders in more detail, including how you can create and register new ones to extend the @@ -227,18 +227,18 @@ The import machinery is designed to be extensible; the primary mechanism for this are the *import hooks*. There are two types of import hooks: *meta -hooks* and *path hooks*. +hooks* and *import path hooks*. Meta hooks are called at the start of import processing, before any other -import processing has occurred. This allows meta hooks to override -:data:`sys.path` processing, frozen modules, or even built-in modules. Meta -hooks are registered by adding new finder objects to :data:`sys.meta_path`, as -described below. +import processing has occurred, other than :data:`sys.modules` cache look up. +This allows meta hooks to override :data:`sys.path` processing, frozen +modules, or even built-in modules. Meta hooks are registered by adding new +finder objects to :data:`sys.meta_path`, as described below. -Path hooks are called as part of :data:`sys.path` (or ``package.__path__``) -processing, at the point where their associated path item is encountered. -Path hooks are registered by adding new callables to :data:`sys.path_hooks` as -described below. +Import path hooks are called as part of :data:`sys.path` (or +``package.__path__``) processing, at the point where their associated path +item is encountered. Import path hooks are registered by adding new callables +to :data:`sys.path_hooks` as described below. The meta path @@ -253,9 +253,9 @@ searches :data:`sys.meta_path`, which contains a list of meta path finder objects. These finders are queried in order to see if they know how to handle the named module. Meta path finders must implement a method called -:meth:`find_module()` which takes two arguments, a name and a path. The meta -path finder can use any strategy it wants to determine whether it can handle -the named module or not. +:meth:`find_module()` which takes two arguments, a name and an import path. +The meta path finder can use any strategy it wants to determine whether it can +handle the named module or not. If the meta path finder knows how to handle the named module, it returns a loader object. If it cannot handle the named module, it returns ``None``. If @@ -266,21 +266,21 @@ The :meth:`find_module()` method of meta path finders is called with two arguments. The first is the fully qualified name of the module being imported, for example ``foo.bar.baz``. The second argument is the relative -path for the module search. For top-level modules, the second argument is -``None``, but for submodules or subpackages, the second argument is the value -of the parent package's ``__path__`` attribute, which must exist or an -:exc:`ImportError` is raised. +import path for the module search. For top-level modules, this second +argument will always be ``None``, but for submodules or subpackages, the +second argument is the value of the parent package's ``__path__`` attribute, +which must exist on the parent module or an :exc:`ImportError` is raised. Python's default :data:`sys.meta_path` has three meta path finders, one that knows how to import built-in modules, one that knows how to import frozen -modules, and one that knows how to import modules from the file system +modules, and one that knows how to import modules from an :term:`import path` (i.e. the :term:`path importer`). -Meta path loaders ------------------ +Loaders +======= -Once a loader is found via a meta path finder, the loader's +If and when a module loader is found its :meth:`~importlib.abc.Loader.load_module` method is called, with a single argument, the fully qualified name of the module being imported. This method has several responsibilities, and should return the module object it has @@ -288,8 +288,8 @@ :exc:`ImportError`, although any other exception raised during :meth:`load_module()` will be propagated. -In many cases, the meta path finder and loader can be the same object, -e.g. :meth:`finder.find_module()` would just return ``self``. +In many cases, the finder and loader can be the same object; in such cases the +:meth:`finder.find_module()` would just return ``self``. Loaders must satisfy the following requirements: @@ -305,9 +305,11 @@ beforehand prevents unbounded recursion in the worst case and multiple loading in the best. - If the load fails, the loader needs to remove any modules it may have - inserted into ``sys.modules``. If the module was already in - ``sys.modules`` then the loader should leave it alone. + If loading fails, the loader must remove any modules it has inserted into + :data:`sys.modules`, but it must remove **only** the failing module, and + only if the loader itself has loaded it explicitly. Any module already in + the :data:`sys.modules` cache, and any module that was successfully loaded + as a side-effect, must remain in the cache. * The loader may set the ``__file__`` attribute of the module. If set, this attribute's value must be a string. The loader may opt to leave @@ -329,10 +331,13 @@ data associated with an importer. * The module's ``__package__`` attribute should be set. Its value must be a - string, but it can be the same value as its ``__name__``. This is the - recommendation when the module is a package. When the module is not a - package, ``__package__`` should be set to the parent package's - name [#fnpk]_. + string, but it can be the same value as its ``__name__``. If the attribute + is set to ``None`` or is missing, the import system will fill it in with a + more appropriate value. When the module is a package, its ``__package__`` + value should be set to its ``__name__``. When the module is not a package, + ``__package__`` should be set to the empty string for top-level modules, or + for submodules, to the parent package's name. See :pep:`366` for further + details. This attribute is used instead of ``__name__`` to calculate explicit relative imports for main modules, as defined in :pep:`366`. @@ -421,32 +426,44 @@ single: path importer As mentioned previously, Python comes with several default meta path finders. -One of these, called the :term:`path importer`, knows how to provide -traditional file system imports. It implements all the semantics for finding -modules on the file system, handling special file types such as Python source -code (``.py`` files), Python byte code (``.pyc`` and ``.pyo`` files) and -shared libraries (e.g. ``.so`` files). +One of these, called the :term:`path importer`, searches an :term:`import +path`, which contains a list of :term:`path entries <path entry>`. Each path +entry names a location to search for modules. -In addition to being able to find such modules, there is built-in support for -loading these modules. To accomplish these two related tasks, additional -hooks and protocols are provided so that you can extend and customize the path -importer semantics. +Path entries may name file system locations, and by default the :term:`path +importer` knows how to provide traditional file system imports. It implements +all the semantics for finding modules on the file system, handling special +file types such as Python source code (``.py`` files), Python byte code +(``.pyc`` and ``.pyo`` files) and shared libraries (e.g. ``.so`` files). + +Path entries need not be limited to file system locations. They can refer to +the contents of zip files, URLs, database queries, or any other location that +can be specified as a string. + +The :term:`path importer` provides additional hooks and protocols so that you +can extend and customize the types of searchable path entries. For example, +if you wanted to support path entries as network URLs, you could write a hook +that implements HTTP semantics to find modules on the web. This hook (a +callable) would return a :term:`path entry finder` supporting the protocol +described below, which was then used to get a loader for the module from the +web. A word of warning: this section and the previous both use the term *finder*, distinguishing between them by using the terms :term:`meta path finder` and -:term:`sys path finder`. Meta path finders and sys path finders are very -similar, support similar protocols, and function in similar ways during the -import process, but it's important to keep in mind that they are subtly -different. In particular, meta path finders operate at the beginning of the -import process, as keyed off the :data:`sys.meta_path` traversal. +:term:`path entry finder`. These two types of finders are very similar, +support similar protocols, and function in similar ways during the import +process, but it's important to keep in mind that they are subtly different. +In particular, meta path finders operate at the beginning of the import +process, as keyed off the :data:`sys.meta_path` traversal. -On the other hand, sys path finders are in a sense an implementation detail of -the path importer, and in fact, if the path importer were to be removed from -:data:`sys.meta_path`, none of the sys path finder semantics would be invoked. +On the other hand, path entry finders are in a sense an implementation detail +of the :term:`path importer`, and in fact, if the path importer were to be +removed from :data:`sys.meta_path`, none of the path entry finder semantics +would be invoked. -sys path finders ----------------- +Path entry finders +------------------ .. index:: single: sys.path @@ -454,112 +471,107 @@ single: sys.path_importer_cache single: PYTHONPATH -The path importer is responsible for finding and loading Python modules and -packages from the file system. As a meta path finder, it implements the +The :term:`path importer` is responsible for finding and loading Python +modules and packages whose location is specified with a string :term:`path +entry`. Most path entries name locations in the file system, but they need +not be limited to this. + +As a meta path finder, the :term:`path importer` implements the :meth:`find_module()` protocol previously described, however it exposes additional hooks that can be used to customize how modules are found and -loaded from the file system. +loaded from the :term:`import path`. -Three variables are used during file system import, :data:`sys.path`, -:data:`sys.path_hooks` and :data:`sys.path_importer_cache`. These provide -additional ways that the import machinery can be customized, in this case -specifically during file system path import. +Three variables are used by the :term:`path importer`, :data:`sys.path`, +:data:`sys.path_hooks` and :data:`sys.path_importer_cache`. The ``__path__`` +attribute on package objects is also used. These provide additional ways that +the import machinery can be customized. :data:`sys.path` contains a list of strings providing search locations for modules and packages. It is initialized from the :data:`PYTHONPATH` environment variable and various other installation- and implementation-specific defaults. Entries in :data:`sys.path` can name directories on the file system, zip files, and potentially other "locations" -(see the :mod:`site` module) that should be searched for modules. +(see the :mod:`site` module) that should be searched for modules, such as +URLs, or database queries. -The path importer is a meta path finder, so the import machinery begins file -system search by calling the path importer's :meth:`find_module()` method as -described previously. When the ``path`` argument to :meth:`find_module()` is -given, it will be a list of string paths to traverse. If not, +The :term:`path importer` is a :term:`meta path finder`, so the import +machinery begins :term:`import path` search by calling the path importer's +:meth:`find_module()` method as described previously. When the ``path`` +argument to :meth:`find_module()` is given, it will be a list of string paths +to traverse. If the ``path`` argument is not given or is ``None``, :data:`sys.path` is used. -The path importer iterates over every entry in the search path, and for each -of these, searches for an appropriate sys path finder for the path entry. -Because this can be an expensive operation (e.g. there are `stat()` call -overheads for this search), the path importer maintains a cache mapping path -entries to sys path finders. This cache is maintained in -:data:`sys.path_importer_cache`. In this way, the expensive search for a -particular path location's sys path finder need only be done once. User code -is free to remove cache entries from :data:`sys.path_importer_cache` forcing -the path importer to perform the path search again [#fnpic]_. +The :term:`path importer` iterates over every entry in the search path, and +for each of these, looks for an appropriate :term:`path entry finder` for the +path entry. Because this can be an expensive operation (e.g. there may be +`stat()` call overheads for this search), the :term:`path importer` maintains +a cache mapping path entries to path entry finders. This cache is maintained +in :data:`sys.path_importer_cache`. In this way, the expensive search for a +particular :term:`path entry` location's :term:`path entry finder` need only +be done once. User code is free to remove cache entries from +:data:`sys.path_importer_cache` forcing the :term:`path importer` to perform +the path entry search again [#fnpic]_. If the path entry is not present in the cache, the path importer iterates over -every callable in :data:`sys.path_hooks`. Each entry in this list is called -with a single argument, the path entry being searched. This callable may -either return a sys path finder that can handle the path entry, or it may -raise :exc:`ImportError`. An :exc:`ImportError` is used by the path importer -to signal that the hook cannot find a sys path finder for that path entry. -The exception is ignored and :data:`sys.path_hooks` iteration continues. +every callable in :data:`sys.path_hooks`. Each of the :term:`path entry hooks +<path entry hook>` in this list is called with a single argument, the path +entry being searched. This callable may either return a :term:`path entry +finder` that can handle the path entry, or it may raise :exc:`ImportError`. +An :exc:`ImportError` is used by the path importer to signal that the hook +cannot find a :term:`path entry finder` for that :term:`path entry`. The +exception is ignored and :term:`import path` iteration continues. -If :data:`sys.path_hooks` iteration ends with no sys path finder being -returned then the path importer's :meth:`find_module()` method will return -``None`` and an :exc:`ImportError` will be raised. +If :data:`sys.path_hooks` iteration ends with no :term:`path entry finder` +being returned, then the path importer's :meth:`find_module()` method will +return ``None``, indicating that this :term:`meta path finder` could not find +the module. -If a sys path finder *is* returned by one of the callables on -:data:`sys.path_hooks`, then the following protocol is used to ask the sys -path finder for a module loader, which is then used to load the module as -previously described (i.e. its :meth:`load_module()` method is called). +If a :term:`path entry finder` *is* returned by one of the :term:`path entry +hook` callables on :data:`sys.path_hooks`, then the following protocol is used +to ask the finder for a module loader, which is then used to load the module. -sys path finder protocol ------------------------- +Path entry finder protocol +-------------------------- -sys path finders support the same, traditional :meth:`find_module()` method -that meta path finders support, however sys path finder :meth:`find_module()` +Path entry finders support the same :meth:`find_module()` method that meta +path finders support, however path entry finder's :meth:`find_module()` methods are never called with a ``path`` argument. -The :meth:`find_module()` method on sys path finders is deprecated though, and -instead sys path finders should implement the :meth:`find_loader()` method. -If it exists on the sys path finder, :meth:`find_loader()` will always be -called instead of :meth:`find_module()`. +The :meth:`find_module()` method on path entry finders is deprecated though, +and instead path entry finders should implement the :meth:`find_loader()` +method. If it exists on the path entry finder, :meth:`find_loader()` will +always be called instead of :meth:`find_module()`. :meth:`find_loader()` takes one argument, the fully qualified name of the module being imported. :meth:`find_loader()` returns a 2-tuple where the first item is the loader and the second item is a namespace :term:`portion`. When the first item (i.e. the loader) is ``None``, this means that while the -sys path finder does not have a loader for the named module, it knows that the -path entry contributes to a namespace portion for the named module. This will -almost always be the case where Python is asked to import a namespace package -that has no physical presence on the file system. When a sys path finder -returns ``None`` for the loader, the second item of the 2-tuple return value -must be a sequence, although it can be empty. +path entry finder does not have a loader for the named module, it knows that +the :term:`path entry` contributes to a namespace portion for the named +module. This will almost always be the case where Python is asked to import a +:term:`namespace package` that has no physical presence on the file system. +When a path entry finder returns ``None`` for the loader, the second item of +the 2-tuple return value must be a sequence, although it can be empty. If :meth:`find_loader()` returns a non-``None`` loader value, the portion is -ignored and the loader is returned from the path importer, terminating the -:data:`sys.path` search. +ignored and the loader is returned from the :term:`path importer`, terminating +the :term:`import path` search. Open issues =========== -XXX Find a better term than "path importer" for class PathFinder and update -the glossary. - -XXX In the glossary, "though I'd change ":term:`finder` / :term:`loader`" to -"metapath importer". - -XXX Find a better term than "sys path finder". - XXX It would be really nice to have a diagram. XXX * (import_machinery.rst) how about a section devoted just to the attributes of modules and packages, perhaps expanding upon or supplanting the related entries in the data model reference page? -XXX * (import_machinery.rst) Meta path loaders, end of paragraph 2: "The -finder could also be a classmethod that returns an instance of the class." +XXX Module reprs: how does module.__qualname__ fit in? -XXX * (import_machinery.rst) Meta path loaders: "If the load fails, the loader -needs to remove any modules..." is a pretty exceptional case, since the -modules is not in charge of its parent or children, nor of import statements -executed for it. Is this a new requirement? - -XXX Module reprs: how does module.__qualname__ fit in? +XXX runpy, pkgutil, et al in the library manual should all get "See Also" +links at the top pointing to the new import system section. References @@ -571,13 +583,21 @@ although some details have changed since the writing of that document. The original specification for :data:`sys.meta_path` was :pep:`302`, with -subsequent extension in :pep:`420`, which also introduced namespace packages -without ``__init__.py`` files in Python 3.3. :pep:`420` also introduced the -:meth:`find_loader` protocol as an alternative to :meth:`find_module`. +subsequent extension in :pep:`420`. + +:pep:`420` introduced :term:`namespace packages <namespace package>` for +Python 3.3. :pep:`420` also introduced the :meth:`find_loader` protocol as an +alternative to :meth:`find_module`. :pep:`366` describes the addition of the ``__package__`` attribute for explicit relative imports in main modules. +:pep:`328` introduced absolute and relative imports and initially proposed +``__name__`` for semantics :pep:`366` would eventually specify for +``__package__``. + +:pep:`338` defines executing modules as scripts. + Footnotes ========= @@ -591,13 +611,6 @@ implementation-specific behavior that is not guaranteed to work in other Python implementations. -.. [#fnpk] In practice, within CPython there is little consistency in the - values of ``__package__`` for top-level modules. In some, such as in the - :mod:`email` package, both the ``__name__`` and ``__package__`` are set to - "email". In other top-level modules (non-packages), ``__package__`` may be - set to ``None`` or the empty string. The recommendation for top-level - non-package modules is to set ``__package__`` to the empty string. - .. [#fnpic] In legacy code, it is possible to find instances of :class:`imp.NullImporter` in the :data:`sys.path_importer_cache`. It recommended that code be changed to use ``None`` instead. See diff --git a/Doc/reference/index.rst b/Doc/reference/index.rst --- a/Doc/reference/index.rst +++ b/Doc/reference/index.rst @@ -24,7 +24,7 @@ lexical_analysis.rst datamodel.rst executionmodel.rst - import_machinery.rst + import.rst expressions.rst simple_stmts.rst compound_stmts.rst diff --git a/Doc/reference/simple_stmts.rst b/Doc/reference/simple_stmts.rst --- a/Doc/reference/simple_stmts.rst +++ b/Doc/reference/simple_stmts.rst @@ -662,14 +662,17 @@ Import statements are executed in two steps: (1) find a module, loading and initializing it if necessary; (2) define a name or names in the local -namespace (of the scope where the :keyword:`import` statement occurs). The -statement comes in two forms differing on whether it uses the :keyword:`from` -keyword. The first form (without :keyword:`from`) repeats these steps for each -identifier in the list. The form with :keyword:`from` performs step (1) once, -and then performs step (2) repeatedly. +namespace (of the scope where the :keyword:`import` statement occurs). +Step (1) may be performed recursively if the named module is a submodule or +subpackage of a parent package. + +The :keyword:`import` statement comes in two forms differing on whether it +uses the :keyword:`from` keyword. The first form (without :keyword:`from`) +repeats these steps for each identifier in the list. The form with +:keyword:`from` performs step (1), and then performs step (2) repeatedly. The details of step (1), finding and loading modules is described in greater -detail in the section on the :ref:`import machinery <importmachinery>`, which +detail in the section on the :ref:`import system <importsystem>`, which also describes the various types of packages and modules that can be imported, as well as all the hooks that can be used to customize Python's import. -- Repository URL: http://hg.python.org/cpython