| |
| .. _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:`importlib.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 |
| single: module spec |
| |
| 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 :dfn:`module spec`, an encapsulation of the module's import-related |
| information, which the import machinery then uses when loading the module. |
| |
| 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. |
| |
| .. versionchanged:: 3.4 |
| In previous versions of Python, finders returned :term:`loaders <loader>` |
| directly, whereas now they return module specs which *contain* loaders. |
| Loaders are still used during import but have fewer responsibilities. |
| |
| 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_spec |
| |
| 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:`~importlib.abc.MetaPathFinder.find_spec()` which takes three arguments: |
| a name, an import path, and (optionally) a target module. 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 |
| spec 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 spec, then an :exc:`ImportError` is raised. Any other exceptions raised |
| are simply propagated up, aborting the import process. |
| |
| The :meth:`~importlib.abc.MetaPathFinder.find_spec()` method of meta path |
| finders is called with two or three 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 third argument is an |
| existing module object that will be the target of loading later. The |
| import system passes in a target module only during reload. |
| |
| 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_spec("foo", None, 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_spec("foo.bar", foo.__path__, None)``. Once ``foo.bar`` has been |
| imported, the final traversal will call |
| ``mpf.find_spec("foo.bar.baz", foo.bar.__path__, None)``. |
| |
| 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`). |
| |
| .. versionchanged:: 3.4 |
| The :meth:`~importlib.abc.MetaPathFinder.find_spec` method of meta path |
| finders replaced :meth:`~importlib.abc.MetaPathFinder.find_module`, which |
| is now deprecated. While it will continue to work without change, the |
| import machinery will try it only if the finder does not implement |
| ``find_spec()``. |
| |
| |
| Loading |
| ======= |
| |
| If and when a module spec is found, the import machinery will use it (and |
| the loader it contains) when loading the module. Here is an approximation |
| of what happens during the loading portion of import:: |
| |
| module = None |
| if spec.loader is not None and hasattr(spec.loader, 'create_module'): |
| # It is assumed 'exec_module' will also be defined on the loader. |
| module = spec.loader.create_module(spec) |
| if module is None: |
| module = ModuleType(spec.name) |
| # The import-related module attributes get set here: |
| _init_module_attrs(spec, module) |
| |
| if spec.loader is None: |
| if spec.submodule_search_locations is not None: |
| # namespace package |
| sys.modules[spec.name] = module |
| else: |
| # unsupported |
| raise ImportError |
| elif not hasattr(spec.loader, 'exec_module'): |
| module = spec.loader.load_module(spec.name) |
| # Set __loader__ and __package__ if missing. |
| else: |
| sys.modules[spec.name] = module |
| try: |
| spec.loader.exec_module(module) |
| except BaseException: |
| try: |
| del sys.modules[spec.name] |
| except KeyError: |
| pass |
| raise |
| return sys.modules[spec.name] |
| |
| Note the following details: |
| |
| * If there is an existing module object with the given name in |
| :data:`sys.modules`, import will have already returned it. |
| |
| * The module will 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 failing module -- and only the failing module -- |
| gets removed from :data:`sys.modules`. 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. This contrasts with |
| reloading where even the failing module is left in :data:`sys.modules`. |
| |
| * After the module is created but before execution, the import machinery |
| sets the import-related module attributes ("_init_module_attrs" in |
| the pseudo-code example above), as summarized in a |
| :ref:`later section <import-mod-attrs>`. |
| |
| * Module execution is the key moment of loading in which the module's |
| namespace gets populated. Execution is entirely delegated to the |
| loader, which gets to decide what gets populated and how. |
| |
| * The module created during loading and passed to exec_module() may |
| not be the one returned at the end of import [#fnlo]_. |
| |
| .. versionchanged:: 3.4 |
| The import system has taken over the boilerplate responsibilities of |
| loaders. These were previously performed by the |
| :meth:`importlib.abc.Loader.load_module` method. |
| |
| Loaders |
| ------- |
| |
| Module loaders provide the critical function of loading: module execution. |
| The import machinery calls the :meth:`importlib.abc.Loader.exec_module` |
| method with a single argument, the module object to execute. Any value |
| returned from :meth:`~importlib.abc.Loader.exec_module` is ignored. |
| |
| Loaders must satisfy the following requirements: |
| |
| * 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__``). |
| |
| * If the loader cannot execute the module, it should raise an |
| :exc:`ImportError`, although any other exception raised during |
| :meth:`~importlib.abc.Loader.exec_module` will be propagated. |
| |
| In many cases, the finder and loader can be the same object; in such cases the |
| :meth:`~importlib.abc.MetaPathFinder.find_spec` method would just return a |
| spec with the loader set to ``self``. |
| |
| Module loaders may opt in to creating the module object during loading |
| by implementing a :meth:`~importlib.abc.Loader.create_module` method. |
| It takes one argument, the module spec, and returns the new module object |
| to use during loading. ``create_module()`` does not need to set any attributes |
| on the module object. If the method returns ``None``, the |
| import machinery will create the new module itself. |
| |
| .. versionadded:: 3.4 |
| The create_module() method of loaders. |
| |
| .. versionchanged:: 3.4 |
| The :meth:`~importlib.abc.Loader.load_module` method was replaced by |
| :meth:`~importlib.abc.Loader.exec_module` and the import |
| machinery assumed all the boilerplate responsibilities of loading. |
| |
| For compatibility with existing loaders, the import machinery will use |
| the ``load_module()`` method of loaders if it exists and the loader does |
| not also implement ``exec_module()``. However, ``load_module()`` has been |
| deprecated and loaders should implement ``exec_module()`` instead. |
| |
| The ``load_module()`` method must implement all the boilerplate loading |
| functionality described above in addition to executing the module. All |
| the same constraints apply, with some additional clarification: |
| |
| * If there is an existing module object with the given name in |
| :data:`sys.modules`, the loader must use that existing module. |
| (Otherwise, :func:`importlib.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`. |
| |
| * The module *must* exist in :data:`sys.modules` before the loader |
| executes the module code, to prevent unbounded recursion or multiple |
| loading. |
| |
| * If loading fails, the loader must remove any modules it has inserted |
| into :data:`sys.modules`, but it must remove **only** the failing |
| module(s), and only if the loader itself has loaded the module(s) |
| explicitly. |
| |
| .. versionchanged:: 3.5 |
| A :exc:`DeprecationWarning` is raised when ``exec_module()`` is defined but |
| ``create_module()`` is not. Starting in Python 3.6 it will be an error to not |
| define ``create_module()`` on a loader attached to a ModuleSpec. |
| |
| Submodules |
| ---------- |
| |
| When a submodule is loaded using any mechanism (e.g. ``importlib`` APIs, the |
| ``import`` or ``import-from`` statements, or built-in ``__import__()``) a |
| binding is placed in the parent module's namespace to the submodule object. |
| For example, if package ``spam`` has a submodule ``foo``, after importing |
| ``spam.foo``, ``spam`` will have an attribute ``foo`` which is bound to the |
| submodule. Let's say you have the following directory structure:: |
| |
| spam/ |
| __init__.py |
| foo.py |
| bar.py |
| |
| and ``spam/__init__.py`` has the following lines in it:: |
| |
| from .foo import Foo |
| from .bar import Bar |
| |
| then executing the following puts a name binding to ``foo`` and ``bar`` in the |
| ``spam`` module:: |
| |
| >>> import spam |
| >>> spam.foo |
| <module 'spam.foo' from '/tmp/imports/spam/foo.py'> |
| >>> spam.bar |
| <module 'spam.bar' from '/tmp/imports/spam/bar.py'> |
| |
| Given Python's familiar name binding rules this might seem surprising, but |
| it's actually a fundamental feature of the import system. The invariant |
| holding is that if you have ``sys.modules['spam']`` and |
| ``sys.modules['spam.foo']`` (as you would after the above import), the latter |
| must appear as the ``foo`` attribute of the former. |
| |
| Module spec |
| ----------- |
| |
| The import machinery uses a variety of information about each module |
| during import, especially before loading. Most of the information is |
| common to all modules. The purpose of a module's spec is to encapsulate |
| this import-related information on a per-module basis. |
| |
| Using a spec during import allows state to be transferred between import |
| system components, e.g. between the finder that creates the module spec |
| and the loader that executes it. Most importantly, it allows the |
| import machinery to perform the boilerplate operations of loading, |
| whereas without a module spec the loader had that responsibility. |
| |
| See :class:`~importlib.machinery.ModuleSpec` for more specifics on what |
| information a module's spec may hold. |
| |
| .. versionadded:: 3.4 |
| |
| .. _import-mod-attrs: |
| |
| Import-related module attributes |
| -------------------------------- |
| |
| The import machinery fills in these attributes on each module object |
| during loading, based on the module's spec, before the loader executes |
| the module. |
| |
| .. attribute:: __name__ |
| |
| The ``__name__`` attribute must be set to the fully-qualified name of |
| the module. This name is used to uniquely identify the module in |
| the import system. |
| |
| .. attribute:: __loader__ |
| |
| The ``__loader__`` attribute must be set to the loader object that |
| the import machinery used when loading the module. This is mostly |
| for introspection, but can be used for additional loader-specific |
| functionality, for example getting data associated with a loader. |
| |
| .. attribute:: __package__ |
| |
| The module's ``__package__`` attribute must be set. Its value must |
| be a string, but it can be the same value as its ``__name__``. 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`. |
| |
| .. attribute:: __spec__ |
| |
| The ``__spec__`` attribute must be set to the module spec that was |
| used when importing the module. This is used primarily for |
| introspection and during reloading. Setting ``__spec__`` |
| appropriately applies equally to :ref:`modules initialized during |
| interpreter startup <programs>`. The one exception is ``__main__``, |
| where ``__spec__`` is :ref:`set to None in some cases <main_spec>`. |
| |
| .. versionadded:: 3.4 |
| |
| .. attribute:: __path__ |
| |
| If the module is a package (either regular or namespace), the module |
| object's ``__path__`` attribute must be set. The value must be |
| iterable, but may be empty if ``__path__`` has no further significance. |
| 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>`. |
| |
| Non-package modules should not have a ``__path__`` attribute. |
| |
| .. attribute:: __file__ |
| .. attribute:: __cached__ |
| |
| ``__file__`` is optional. If set, this attribute's value must be a |
| string. The import system may opt to leave ``__file__`` unset if it |
| has no semantic meaning (e.g. a module loaded from a database). |
| |
| If ``__file__`` is set, it may also be appropriate to set the |
| ``__cached__`` attribute which is the path to any compiled version of |
| the code (e.g. byte-compiled file). The file does not need to exist |
| to set this attribute; the path can simply point to where the |
| compiled file would exist (see :pep:`3147`). |
| |
| It is also appropriate to set ``__cached__`` when ``__file__`` is not |
| set. However, that scenario is quite atypical. Ultimately, the |
| loader is what makes use of ``__file__`` and/or ``__cached__``. So |
| if a loader can load from a cached module but otherwise does not load |
| from a file, that atypical scenario may be appropriate. |
| |
| .. _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 import machinery automatically sets ``__path__`` |
| correctly for the namespace package. |
| |
| Module reprs |
| ------------ |
| |
| By default, all modules have a usable repr, however depending on the |
| attributes set above, and in the module's spec, you can more explicitly |
| control the repr of module objects. |
| |
| If the module has a spec (``__spec__``), the import machinery will try |
| to generate a repr from it. If that fails or there is no spec, the import |
| system will craft a default repr using whatever information is available |
| on the module. 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 ``__spec__`` attribute, the information in the spec |
| is used to generate the repr. The "name", "loader", "origin", and |
| "has_location" attributes are consulted. |
| |
| * 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__`` that is not |
| ``None``, then the loader's repr is used as part of the module's repr. |
| |
| * Otherwise, just use the module's ``__name__`` in the repr. |
| |
| .. versionchanged:: 3.4 |
| Use of :meth:`loader.module_repr() <importlib.abc.Loader.module_repr>` |
| has been deprecated and the module spec is now used by the import |
| machinery to generate a module repr. |
| |
| For backward compatibility with Python 3.3, the module repr will be |
| generated by calling the loader's |
| :meth:`~importlib.abc.Loader.module_repr` method, if defined, before |
| trying either approach described above. However, the method is deprecated. |
| |
| |
| 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` |
| (:class:`~importlib.machinery.PathFinder`), 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`` 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:`~importlib.abc.MetaPathFinder.find_spec` 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:`~importlib.machinery.PathFinder.find_spec` method as |
| described previously. When the ``path`` argument to |
| :meth:`~importlib.machinery.PathFinder.find_spec` 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` |
| (:class:`~importlib.abc.PathEntryFinder`) 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:`~importlib.machinery.PathFinder.find_spec` 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 spec, which is then used when loading the |
| module. |
| |
| The current working directory -- denoted by an empty string -- is handled |
| slightly differently from other entries on :data:`sys.path`. First, if the |
| current working directory is found to not exist, no value is stored in |
| :data:`sys.path_importer_cache`. Second, the value for the current working |
| directory is looked up fresh for each module lookup. Third, the path used for |
| :data:`sys.path_importer_cache` and returned by |
| :meth:`importlib.machinery.PathFinder.find_spec` will be the actual current |
| working directory and not the empty string. |
| |
| 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:`~importlib.abc.PathEntryFinder.find_spec` method. |
| |
| :meth:`~importlib.abc.PathEntryFinder.find_spec` takes two argument, the |
| fully qualified name of the module being imported, and the (optional) target |
| module. ``find_spec()`` returns a fully populated spec for the module. |
| This spec will always have "loader" set (with one exception). |
| |
| To indicate to the import machinery that the spec represents a namespace |
| :term:`portion`. the path entry finder sets "loader" on the spec to |
| ``None`` and "submodule_search_locations" to a list containing the |
| portion. |
| |
| .. versionchanged:: 3.4 |
| :meth:`~importlib.abc.PathEntryFinder.find_spec` replaced |
| :meth:`~importlib.abc.PathEntryFinder.find_loader` and |
| :meth:`~importlib.abc.PathEntryFinder.find_module`, both of which |
| are now deprecated, but will be used if ``find_spec()`` is not defined. |
| |
| Older path entry finders may implement one of these two deprecated methods |
| instead of ``find_spec()``. The methods are still respected for the |
| sake of backward compatibility. Howevever, if ``find_spec()`` is |
| implemented on the path entry finder, the legacy methods are ignored. |
| |
| :meth:`~importlib.abc.PathEntryFinder.find_loader` takes one argument, the |
| fully qualified name of the module being imported. ``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 ``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 ``find_module()`` method that meta path finders support. |
| However path entry finder ``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 ``find_module()`` method on path entry finders is deprecated, |
| as it does not allow the path entry finder to contribute portions to |
| namespace packages. If both ``find_loader()`` and ``find_module()`` |
| exist on a path entry finder, the import system will always call |
| ``find_loader()`` in preference to ``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:`~importlib.abc.MetaPathFinder.find_spec` instead of returning |
| ``None``. The latter indicates that the meta path search should continue, |
| while raising an exception terminates it immediately. |
| |
| |
| Special considerations for __main__ |
| =================================== |
| |
| The :mod:`__main__` module is a special case relative to Python's import |
| system. As noted :ref:`elsewhere <programs>`, the ``__main__`` module |
| is directly initialized at interpreter startup, much like :mod:`sys` and |
| :mod:`builtins`. However, unlike those two, it doesn't strictly |
| qualify as a built-in module. This is because the manner in which |
| ``__main__`` is initialized depends on the flags and other options with |
| which the interpreter is invoked. |
| |
| .. _main_spec: |
| |
| __main__.__spec__ |
| ----------------- |
| |
| Depending on how :mod:`__main__` is initialized, ``__main__.__spec__`` |
| gets set appropriately or to ``None``. |
| |
| When Python is started with the :option:`-m` option, ``__spec__`` is set |
| to the module spec of the corresponding module or package. ``__spec__`` is |
| also populated when the ``__main__`` module is loaded as part of executing a |
| directory, zipfile or other :data:`sys.path` entry. |
| |
| In :ref:`the remaining cases <using-on-interface-options>` |
| ``__main__.__spec__`` is set to ``None``, as the code used to populate the |
| :mod:`__main__` does not correspond directly with an importable module: |
| |
| - interactive prompt |
| - -c switch |
| - running from stdin |
| - running directly from a source or bytecode file |
| |
| Note that ``__main__.__spec__`` is always ``None`` in the last case, |
| *even if* the file could technically be imported directly as a module |
| instead. Use the :option:`-m` switch if valid module metadata is desired |
| in :mod:`__main__`. |
| |
| Note also that even when ``__main__`` corresponds with an importable module |
| and ``__main__.__spec__`` is set accordingly, they're still considered |
| *distinct* modules. This is due to the fact that blocks guarded by |
| ``if __name__ == "__main__":`` checks only execute when the module is used |
| to populate the ``__main__`` namespace, and not during normal import. |
| |
| |
| 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. |
| |
| XXX Add more explanation regarding the different ways in which |
| ``__main__`` is initialized? |
| |
| XXX Add more info on ``__main__`` quirks/pitfalls (i.e. copy from |
| :pep:`395`). |
| |
| |
| References |
| ========== |
| |
| The import machinery has evolved considerably since Python's early days. The |
| original `specification for packages |
| <http://legacy.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. |
| |
| :pep:`451` adds the encapsulation of per-module import state in spec |
| objects. It also off-loads most of the boilerplate responsibilities of |
| loaders back onto the import machinery. These changes allow the |
| deprecation of several APIs in the import system and also addition of new |
| methods to finders and loaders. |
| |
| .. rubric:: 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. |