| |
| .. _importsystem: |
| |
| ***************** |
| The import system |
| ***************** |
| |
| .. index:: single: import machinery |
| |
| Python code in one :term:`module` gains access to the code in another module |
| by the process of :term:`importing` it. The :keyword:`import` statement is |
| the most common way of invoking the import machinery, but it is not the only |
| way. Functions such as :func:`importlib.import_module` and built-in |
| :func:`__import__` can also be used to invoke the import machinery. |
| |
| The :keyword:`import` statement combines two operations; it searches for the |
| named module, then it binds the results of that search to a name in the local |
| scope. The search operation of the :keyword:`import` statement is defined as |
| a call to the :func:`__import__` function, with the appropriate arguments. |
| The return value of :func:`__import__` is used to perform the name |
| binding operation of the :keyword:`import` statement. See the |
| :keyword:`import` statement for the exact details of that name binding |
| operation. |
| |
| A direct call to :func:`__import__` performs only the module search and, if |
| found, the module creation operation. While certain side-effects may occur, |
| such as the importing of parent packages, and the updating of various caches |
| (including :data:`sys.modules`), only the :keyword:`import` statement performs |
| a name binding operation. |
| |
| When calling :func:`__import__` as part of an import statement, the |
| import system first checks the module global namespace for a function by |
| that name. If it is not found, then the standard builtin :func:`__import__` |
| is called. Other mechanisms for invoking the import system (such as |
| :func:`importlib.import_module`) do not perform this check and will always |
| use the standard import system. |
| |
| When a module is first imported, Python searches for the module and if found, |
| it creates a module object [#fnmo]_, initializing it. If the named module |
| 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. |
| |
| .. versionchanged:: 3.3 |
| The import system has been updated to fully implement the second phase |
| of :pep:`302`. There is no longer any implicit import machinery - the full |
| import system is exposed through :data:`sys.meta_path`. In addition, |
| native namespace package support has been implemented (see :pep:`420`). |
| |
| |
| :mod:`importlib` |
| ================ |
| |
| The :mod:`importlib` module provides a rich API for interacting with the |
| import system. For example :func:`importlib.import_module` provides a |
| recommended, simpler API than built-in :func:`__import__` for invoking the |
| import machinery. Refer to the :mod:`importlib` library documentation for |
| additional detail. |
| |
| |
| |
| Packages |
| ======== |
| |
| .. index:: |
| single: package |
| |
| Python has only one type of module object, and all modules are of this type, |
| regardless of whether the module is implemented in Python, C, or something |
| else. To help organize modules and provide a naming hierarchy, Python has a |
| concept of :term:`packages <package>`. |
| |
| You can think of packages as the directories on a file system and modules as |
| files within directories, but don't take this analogy too literally since |
| packages and modules need not originate from the file system. For the |
| purposes of this documentation, we'll use this convenient analogy of |
| directories and files. Like file system directories, packages are organized |
| hierarchically, and packages may themselves contain subpackages, as well as |
| regular modules. |
| |
| It's important to keep in mind that all packages are modules, but not all |
| modules are packages. Or put another way, packages are just a special kind of |
| module. Specifically, any module that contains a ``__path__`` attribute is |
| considered a package. |
| |
| All modules have a name. Subpackage names are separated from their parent |
| package name by dots, akin to Python's standard attribute access syntax. Thus |
| you might have a module called :mod:`sys` and a package called :mod:`email`, |
| which in turn has a subpackage called :mod:`email.mime` and a module within |
| that subpackage called :mod:`email.mime.text`. |
| |
| |
| Regular packages |
| ---------------- |
| |
| .. index:: |
| pair: package; regular |
| |
| Python defines two types of packages, :term:`regular packages <regular |
| package>` and :term:`namespace packages <namespace package>`. Regular |
| packages are traditional packages as they existed in Python 3.2 and earlier. |
| A regular package is typically implemented as a directory containing an |
| ``__init__.py`` file. When a regular package is imported, this |
| ``__init__.py`` file is implicitly executed, and the objects it defines are |
| bound to names in the package's namespace. The ``__init__.py`` file can |
| contain the same Python code that any other module can contain, and Python |
| will add some additional attributes to the module when it is imported. |
| |
| For example, the following file system layout defines a top level ``parent`` |
| package with three subpackages:: |
| |
| parent/ |
| __init__.py |
| one/ |
| __init__.py |
| two/ |
| __init__.py |
| three/ |
| __init__.py |
| |
| Importing ``parent.one`` will implicitly execute ``parent/__init__.py`` and |
| ``parent/one/__init__.py``. Subsequent imports of ``parent.two`` or |
| ``parent.three`` will execute ``parent/two/__init__.py`` and |
| ``parent/three/__init__.py`` respectively. |
| |
| |
| Namespace packages |
| ------------------ |
| |
| .. index:: |
| pair:: package; namespace |
| pair:: package; portion |
| |
| A namespace package is a composite of various :term:`portions <portion>`, |
| where each portion contributes a subpackage to the parent package. Portions |
| may reside in different locations on the file system. Portions may also be |
| found in zip files, on the network, or anywhere else that Python searches |
| during import. Namespace packages may or may not correspond directly to |
| objects on the file system; they may be virtual modules that have no concrete |
| representation. |
| |
| Namespace packages do not use an ordinary list for their ``__path__`` |
| attribute. They instead use a custom iterable type which will automatically |
| perform a new search for package portions on the next import attempt within |
| that package if the path of their parent package (or :data:`sys.path` for a |
| top level package) changes. |
| |
| With namespace packages, there is no ``parent/__init__.py`` file. In fact, |
| there may be multiple ``parent`` directories found during import search, where |
| each one is provided by a different portion. Thus ``parent/one`` may not be |
| physically located next to ``parent/two``. In this case, Python will create a |
| namespace package for the top-level ``parent`` package whenever it or one of |
| its subpackages is imported. |
| |
| See also :pep:`420` for the namespace package specification. |
| |
| |
| Searching |
| ========= |
| |
| To begin the search, Python needs the :term:`fully qualified <qualified name>` |
| name of the module (or package, but for the purposes of this discussion, the |
| difference is immaterial) being imported. This name may come from various |
| arguments to the :keyword:`import` statement, or from the parameters to the |
| :func:`importlib.import_module` or :func:`__import__` functions. |
| |
| This name will be used in various phases of the import search, and it may be |
| the dotted path to a submodule, e.g. ``foo.bar.baz``. In this case, Python |
| first tries to import ``foo``, then ``foo.bar``, and finally ``foo.bar.baz``. |
| If any of the intermediate imports fail, an :exc:`ImportError` is raised. |
| |
| |
| The module cache |
| ---------------- |
| |
| .. index:: |
| single: sys.modules |
| |
| The first place checked during import search is :data:`sys.modules`. This |
| mapping serves as a cache of all modules that have been previously imported, |
| including the intermediate paths. So if ``foo.bar.baz`` was previously |
| imported, :data:`sys.modules` will contain entries for ``foo``, ``foo.bar``, |
| and ``foo.bar.baz``. Each key will have as its value the corresponding module |
| object. |
| |
| During import, the module name is looked up in :data:`sys.modules` and if |
| present, the associated value is the module satisfying the import, and the |
| process completes. However, if the value is ``None``, then an |
| :exc:`ImportError` is raised. If the module name is missing, Python will |
| continue searching for the module. |
| |
| :data:`sys.modules` is writable. Deleting a key may not destroy the |
| associated module (as other modules may hold references to it), |
| but it will invalidate the cache entry for the named module, causing |
| Python to search anew for the named module upon its next |
| import. The key can also be assigned to ``None``, forcing the next import |
| of the module to result in an :exc:`ImportError`. |
| |
| Beware though, as if you keep a reference to the module object, |
| invalidate its cache entry in :data:`sys.modules`, and then re-import the |
| named module, the two module objects will *not* be the same. By contrast, |
| :func:`imp.reload` will reuse the *same* module object, and simply |
| reinitialise the module contents by rerunning the module's code. |
| |
| |
| Finders and loaders |
| ------------------- |
| |
| .. index:: |
| 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. 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. Objects that implement both of these |
| interfaces are referred to as :term:`importers <importer>` - they return |
| themselves when they find that they can load the requested module. |
| |
| Python includes a number of default finders and importers. The first one |
| knows how to locate built-in modules, and the second knows how to locate |
| frozen 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 :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 |
| import machinery. |
| |
| |
| Import hooks |
| ------------ |
| |
| .. index:: |
| single: import hooks |
| single: meta hooks |
| single: path hooks |
| pair: hooks; import |
| pair: hooks; meta |
| pair: hooks; path |
| |
| 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 *import path hooks*. |
| |
| Meta hooks are called at the start of import processing, before any other |
| 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. |
| |
| 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 |
| ------------- |
| |
| .. index:: |
| single: sys.meta_path |
| pair: finder; find_module |
| pair: finder; find_loader |
| |
| When the named module is not found in :data:`sys.modules`, Python next |
| 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 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 |
| :data:`sys.meta_path` processing reaches the end of its list without returning |
| a loader, then an :exc:`ImportError` is raised. Any other exceptions raised |
| are simply propagated up, aborting the import process. |
| |
| 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 path |
| entries to use 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. If |
| the appropriate ``__path__`` attribute cannot be accessed, an |
| :exc:`ImportError` is raised. |
| |
| The meta path may be traversed multiple times for a single import request. |
| For example, assuming none of the modules involved has already been cached, |
| importing ``foo.bar.baz`` will first perform a top level import, calling |
| ``mpf.find_module("foo", None)`` on each meta path finder (``mpf``). After |
| ``foo`` has been imported, ``foo.bar`` will be imported by traversing the |
| meta path a second time, calling |
| ``mpf.find_module("foo.bar", foo.__path__)``. Once ``foo.bar`` has been |
| imported, the final traversal will call |
| ``mpf.find_module("foo.bar.baz", foo.bar.__path__)``. |
| |
| Some meta path finders only support top level imports. These importers will |
| always return ``None`` when anything other than ``None`` is passed as the |
| second argument. |
| |
| 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 an :term:`import path` |
| (i.e. the :term:`path based finder`). |
| |
| |
| Loaders |
| ======= |
| |
| 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 |
| loaded [#fnlo]_. If it cannot load the module, it should raise an |
| :exc:`ImportError`, although any other exception raised during |
| :meth:`load_module()` will be propagated. |
| |
| 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: |
| |
| * If there is an existing module object with the given name in |
| :data:`sys.modules`, the loader must use that existing module. (Otherwise, |
| :func:`imp.reload` will not work correctly.) If the named module does |
| not exist in :data:`sys.modules`, the loader must create a new module |
| object and add it to :data:`sys.modules`. |
| |
| Note that the module *must* exist in :data:`sys.modules` before the loader |
| executes the module code. This is crucial because the module code may |
| (directly or indirectly) import itself; adding it to :data:`sys.modules` |
| beforehand prevents unbounded recursion in the worst case and multiple |
| loading in the best. |
| |
| 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 |
| ``__file__`` unset if it has no semantic meaning (e.g. a module loaded from |
| a database). |
| |
| * The loader may set the ``__name__`` attribute of the module. While not |
| required, setting this attribute is highly recommended so that the |
| :meth:`repr()` of the module is more informative. |
| |
| * If the module is a package (either regular or namespace), the loader must |
| set the module object's ``__path__`` attribute. The value must be |
| iterable, but may be empty if ``__path__`` has no further significance |
| to the loader. If ``__path__`` is not empty, it must produce strings |
| when iterated over. More details on the semantics of ``__path__`` are |
| given :ref:`below <package-path-rules>`. |
| |
| * The ``__loader__`` attribute must be set to the loader object that loaded |
| the module. This is mostly for introspection and reloading, but can be |
| used for additional loader-specific functionality, for example getting |
| data associated with a loader. |
| |
| * The module's ``__package__`` attribute should be set. Its value must be a |
| 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`. |
| |
| * If the module is a Python module (as opposed to a built-in module or a |
| dynamically loaded extension), the loader should execute the module's code |
| in the module's global name space (``module.__dict__``). |
| |
| |
| Module reprs |
| ------------ |
| |
| By default, all modules have a usable repr, however depending on the |
| attributes set above, and hooks in the loader, you can more explicitly control |
| the repr of module objects. |
| |
| Loaders may implement a :meth:`module_repr()` method which takes a single |
| argument, the module object. When ``repr(module)`` is called for a module |
| with a loader supporting this protocol, whatever is returned from |
| ``module.__loader__.module_repr(module)`` is returned as the module's repr |
| without further processing. This return value must be a string. |
| |
| If the module has no ``__loader__`` attribute, or the loader has no |
| :meth:`module_repr()` method, then the module object implementation itself |
| will craft a default repr using whatever information is available. It will |
| try to use the ``module.__name__``, ``module.__file__``, and |
| ``module.__loader__`` as input into the repr, with defaults for whatever |
| information is missing. |
| |
| Here are the exact rules used: |
| |
| * If the module has a ``__loader__`` and that loader has a |
| :meth:`module_repr()` method, call it with a single argument, which is the |
| module object. The value returned is used as the module's repr. |
| |
| * If an exception occurs in :meth:`module_repr()`, the exception is caught |
| and discarded, and the calculation of the module's repr continues as if |
| :meth:`module_repr()` did not exist. |
| |
| * If the module has a ``__file__`` attribute, this is used as part of the |
| module's repr. |
| |
| * If the module has no ``__file__`` but does have a ``__loader__``, then the |
| loader's repr is used as part of the module's repr. |
| |
| * Otherwise, just use the module's ``__name__`` in the repr. |
| |
| This example, from :pep:`420` shows how a loader can craft its own module |
| repr:: |
| |
| class NamespaceLoader: |
| @classmethod |
| def module_repr(cls, module): |
| return "<module '{}' (namespace)>".format(module.__name__) |
| |
| |
| .. _package-path-rules: |
| |
| module.__path__ |
| --------------- |
| |
| By definition, if a module has an ``__path__`` attribute, it is a package, |
| regardless of its value. |
| |
| A package's ``__path__`` attribute is used during imports of its subpackages. |
| Within the import machinery, it functions much the same as :data:`sys.path`, |
| i.e. providing a list of locations to search for modules during import. |
| However, ``__path__`` is typically much more constrained than |
| :data:`sys.path`. |
| |
| ``__path__`` must be an iterable of strings, but it may be empty. |
| The same rules used for :data:`sys.path` also apply to a package's |
| ``__path__``, and :data:`sys.path_hooks` (described below) are |
| consulted when traversing a package's ``__path__``. |
| |
| A package's ``__init__.py`` file may set or alter the package's ``__path__`` |
| attribute, and this was typically the way namespace packages were implemented |
| prior to :pep:`420`. With the adoption of :pep:`420`, namespace packages no |
| longer need to supply ``__init__.py`` files containing only ``__path__`` |
| manipulation code; the namespace loader automatically sets ``__path__`` |
| correctly for the namespace package. |
| |
| |
| The Path Based Finder |
| ===================== |
| |
| .. index:: |
| single: path based finder |
| |
| As mentioned previously, Python comes with several default meta path finders. |
| One of these, called the :term:`path based finder`, 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. |
| |
| The path based finder itself doesn't know how to import anything. Instead, it |
| traverses the individual path entries, associating each of them with a |
| path entry finder that knows how to handle that particular kind of path. |
| |
| The default set of path entry finders implement 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). When supported by the :mod:`zipimport` |
| module in the standard library, the default path entry finders also handle |
| loading all of these file types (other than shared libraries) from zipfiles. |
| |
| Path entries need not be limited to file system locations. They can refer to |
| URLs, database queries, or any other location that can be specified as a |
| string. |
| |
| The path based finder 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:`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. |
| |
| By contrast, path entry finders are in a sense an implementation detail |
| of the path based finder, and in fact, if the path based finder were to be |
| removed from :data:`sys.meta_path`, none of the path entry finder semantics |
| would be invoked. |
| |
| |
| Path entry finders |
| ------------------ |
| |
| .. index:: |
| single: sys.path |
| single: sys.path_hooks |
| single: sys.path_importer_cache |
| single: PYTHONPATH |
| |
| The :term:`path based finder` 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 based finder` 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 :term:`import path`. |
| |
| Three variables are used by the :term:`path based finder`, :data:`sys.path`, |
| :data:`sys.path_hooks` and :data:`sys.path_importer_cache`. The ``__path__`` |
| attributes on package objects are 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, such as |
| URLs, or database queries. Only strings and bytes should be present on |
| :data:`sys.path`; all other data types are ignored. The encoding of bytes |
| entries is determined by the individual :term:`path entry finders <path entry |
| finder>`. |
| |
| The :term:`path based finder` is a :term:`meta path finder`, so the import |
| machinery begins the :term:`import path` search by calling the path |
| based finder'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 - typically a package's ``__path__`` |
| attribute for an import within that package. If the ``path`` argument |
| is ``None``, this indicates a top level import and :data:`sys.path` is used. |
| |
| The path based finder 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 path based finder maintains |
| a cache mapping path entries to path entry finders. This cache is maintained |
| in :data:`sys.path_importer_cache` (despite the name, this cache actually |
| stores finder objects rather than being limited to :term:`importer` objects). |
| 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 path based finder to perform the path entry search again [#fnpic]_. |
| |
| If the path entry is not present in the cache, the path based finder iterates |
| over 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 to be 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 based finder 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. The hook should expect either a string or bytes object; |
| the encoding of bytes objects is up to the hook (e.g. it may be a file system |
| encoding, UTF-8, or something else), and if the hook cannot decode the |
| argument, it should raise :exc:`ImportError`. |
| |
| If :data:`sys.path_hooks` iteration ends with no :term:`path entry finder` |
| being returned, then the path based finder's :meth:`find_module()` method |
| will store ``None`` in :data:`sys.path_importer_cache` (to indicate that |
| there is no finder for this path entry) and return ``None``, indicating that |
| this :term:`meta path finder` could not find the module. |
| |
| 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. |
| |
| |
| Path entry finder protocol |
| -------------------------- |
| |
| In order to support imports of modules and initialized packages and also to |
| contribute portions to namespace packages, path entry finders must implement |
| the :meth:`find_loader()` method. |
| |
| :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 |
| path entry 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 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 based finder, terminating |
| the search through the path entries. |
| |
| For backwards compatibility with other implementations of the import |
| protocol, many path entry finders also support the same, |
| traditional :meth:`find_module()` method that meta path finders support. |
| However path entry finder :meth:`find_module()` methods are never called |
| with a ``path`` argument (they are expected to record the appropriate |
| path information from the initial call to the path hook). |
| |
| The :meth:`find_module()` method on path entry finders is deprecated, |
| as it does not allow the path entry finder to contribute portions to |
| namespace packages. Instead path entry finders should implement the |
| :meth:`find_loader()` method as described above. If it exists on the path |
| entry finder, the import system will always call :meth:`find_loader()` |
| in preference to :meth:`find_module()`. |
| |
| |
| Replacing the standard import system |
| ==================================== |
| |
| The most reliable mechanism for replacing the entire import system is to |
| delete the default contents of :data:`sys.meta_path`, replacing them |
| entirely with a custom meta path hook. |
| |
| If it is acceptable to only alter the behaviour of import statements |
| without affecting other APIs that access the import system, then replacing |
| the builtin :func:`__import__` function may be sufficient. This technique |
| may also be employed at the module level to only alter the behaviour of |
| import statements within that module. |
| |
| To selectively prevent import of some modules from a hook early on the |
| meta path (rather than disabling the standard import system entirely), |
| it is sufficient to raise :exc:`ImportError` directly from |
| :meth:`find_module` instead of returning ``None``. The latter indicates |
| that the meta path search should continue. while raising an exception |
| terminates it immediately. |
| |
| |
| Open issues |
| =========== |
| |
| 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 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 |
| ========== |
| |
| The import machinery has evolved considerably since Python's early days. The |
| original `specification for packages |
| <http://www.python.org/doc/essays/packages.html>`_ is still available to read, |
| 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`. |
| |
| :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 explicit relative imports and initially |
| proposed ``__name__`` for semantics :pep:`366` would eventually specify for |
| ``__package__``. |
| |
| :pep:`338` defines executing modules as scripts. |
| |
| |
| Footnotes |
| ========= |
| |
| .. [#fnmo] See :class:`types.ModuleType`. |
| |
| .. [#fnlo] The importlib implementation avoids using the return value |
| directly. Instead, it gets the module object by looking the module name up |
| in :data:`sys.modules`. The indirect effect of this is that an imported |
| module may replace itself in :data:`sys.modules`. This is |
| implementation-specific behavior that is not guaranteed to work in other |
| Python implementations. |
| |
| .. [#fnpic] In legacy code, it is possible to find instances of |
| :class:`imp.NullImporter` in the :data:`sys.path_importer_cache`. It |
| is recommended that code be changed to use ``None`` instead. See |
| :ref:`portingpythoncode` for more details. |