5. The import system
********************

Python code in one *module* gains access to the code in another module
by the process of *importing* it.  The "import" statement is the most
common way of invoking the import machinery, but it is not the only
way.  Functions such as "importlib.import_module()" and built-in
"__import__()" can also be used to invoke the import machinery.

The "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 "import" statement is
defined as a call to the "__import__()" function, with the appropriate
arguments. The return value of "__import__()" is used to perform the
name binding operation of the "import" statement.  See the "import"
statement for the exact details of that name binding operation.

A direct call to "__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 "sys.modules"), only the "import"
statement performs a name binding operation.

When calling "__import__()" as part of an import statement, the
standard builtin "__import__()" is called. Other mechanisms for
invoking the import system (such as "importlib.import_module()") may
choose to subvert "__import__()" and use its own solution to implement
import semantics.

When a module is first imported, Python searches for the module and if
found, it creates a module object [1], initializing it.  If the named
module cannot be found, an "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.

Changed in version 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
"sys.meta_path". In addition, native namespace package support has
been implemented (see **PEP 420**).


5.1. "importlib"
================

The "importlib" module provides a rich API for interacting with the
import system.  For example "importlib.import_module()" provides a
recommended, simpler API than built-in "__import__()" for invoking the
import machinery.  Refer to the "importlib" library documentation for
additional detail.


5.2. Packages
=============

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 *packages*.

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 "sys" and a
package called "email", which in turn has a subpackage called
"email.mime" and a module within that subpackage called
"email.mime.text".


5.2.1. Regular packages
-----------------------

Python defines two types of packages, *regular packages* and
*namespace packages*.  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.


5.2.2. Namespace packages
-------------------------

A namespace package is a composite of various *portions*, 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 "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.


5.3. Searching
==============

To begin the search, Python needs the *fully qualified* 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 "import" statement, or from the parameters to
the "importlib.import_module()" or "__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
"ImportError" is raised.


5.3.1. The module cache
-----------------------

The first place checked during import search is "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, "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 "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
"ImportError" is raised.  If the module name is missing, Python will
continue searching for the module.

"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 "ImportError".

Beware though, as if you keep a reference to the module object,
invalidate its cache entry in "sys.modules", and then re-import the
named module, the two module objects will *not* be the same. By
contrast, "importlib.reload()" will reuse the *same* module object,
and simply reinitialise the module contents by rerunning the module’s
code.


5.3.2. Finders and loaders
--------------------------

If the named module is not found in "sys.modules", then Python’s
import protocol is invoked to find and load the module.  This protocol
consists of two conceptual objects, *finders* and *loaders*. 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 *importers* - 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 *import
path* for modules.  The *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 *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.

Changed in version 3.4: In previous versions of Python, finders
returned *loaders* directly, whereas now they return module specs
which *contain* loaders. Loaders are still used during import but have
fewer responsibilities.


5.3.3. Import hooks
-------------------

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 "sys.modules" cache
look up. This allows meta hooks to override "sys.path" processing,
frozen modules, or even built-in modules.  Meta hooks are registered
by adding new finder objects to "sys.meta_path", as described below.

Import path hooks are called as part of "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 "sys.path_hooks" as described below.


5.3.4. The meta path
--------------------

When the named module is not found in "sys.modules", Python next
searches "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 "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 "sys.meta_path" processing reaches the end of its
list without returning a spec, then an "ImportError" is raised.  Any
other exceptions raised are simply propagated up, aborting the import
process.

The "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 "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 "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
*import path* (i.e. the *path based finder*).

Changed in version 3.4: The "find_spec()" method of meta path finders
replaced "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()".


5.4. 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
     "sys.modules", import will have already returned it.

   * The module will exist in "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
     "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 "sys.modules".  Any module already in
     the "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
     "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 later section.

   * 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 [2].

Changed in version 3.4: The import system has taken over the
boilerplate responsibilities of loaders.  These were previously
performed by the "importlib.abc.Loader.load_module()" method.


5.4.1. Loaders
--------------

Module loaders provide the critical function of loading: module
execution. The import machinery calls the
"importlib.abc.Loader.exec_module()" method with a single argument,
the module object to execute.  Any value returned from "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
     "ImportError", although any other exception raised during
     "exec_module()" will be propagated.

In many cases, the finder and loader can be the same object; in such
cases the "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 "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.

New in version 3.4: The "create_module()" method of loaders.

Changed in version 3.4: The "load_module()" method was replaced by
"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
     "sys.modules", the loader must use that existing module.
     (Otherwise, "importlib.reload()" will not work correctly.)  If
     the named module does not exist in "sys.modules", the loader must
     create a new module object and add it to "sys.modules".

   * The module *must* exist in "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 "sys.modules", but it must remove **only** the
     failing module(s), and only if the loader itself has loaded the
     module(s) explicitly.

Changed in version 3.5: A "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.


5.4.2. 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.


5.4.3. 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 "ModuleSpec" for more specifics on what information a module’s
spec may hold.

New in version 3.4.


5.4.4. 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.

__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.

__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.

__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**.

__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 modules initialized during
   interpreter startup.  The one exception is "__main__", where
   "__spec__" is set to None in some cases.

   New in version 3.4.

__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 below.

   Non-package modules should not have a "__path__" attribute.

__file__

__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.


5.4.5. 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 "sys.path", i.e. providing a list of locations to search for
modules during import. However, "__path__" is typically much more
constrained than "sys.path".

"__path__" must be an iterable of strings, but it may be empty. The
same rules used for "sys.path" also apply to a package’s "__path__",
and "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.


5.4.6. 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.

Changed in version 3.4: Use of "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 "module_repr()"
method, if defined, before trying either approach described above.
However, the method is deprecated.


5.5. The Path Based Finder
==========================

As mentioned previously, Python comes with several default meta path
finders. One of these, called the *path based finder* ("PathFinder"),
searches an *import path*, which contains a list of *path entries*.
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
"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 *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 *meta path
finder* and *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 "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 "sys.meta_path", none of the path entry finder
semantics would be invoked.


5.5.1. Path entry finders
-------------------------

The *path based finder* is responsible for finding and loading Python
modules and packages whose location is specified with a string *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 *path based finder* implements the
"find_spec()" protocol previously described, however it exposes
additional hooks that can be used to customize how modules are found
and loaded from the *import path*.

Three variables are used by the *path based finder*, "sys.path",
"sys.path_hooks" and "sys.path_importer_cache".  The "__path__"
attributes on package objects are also used.  These provide additional
ways that the import machinery can be customized.

"sys.path" contains a list of strings providing search locations for
modules and packages.  It is initialized from the "PYTHONPATH"
environment variable and various other installation- and
implementation-specific defaults.  Entries in "sys.path" can name
directories on the file system, zip files, and potentially other
“locations” (see the "site" module) that should be searched for
modules, such as URLs, or database queries.  Only strings and bytes
should be present on "sys.path"; all other data types are ignored.
The encoding of bytes entries is determined by the individual *path
entry finders*.

The *path based finder* is a *meta path finder*, so the import
machinery begins the *import path* search by calling the path based
finder’s "find_spec()" method as described previously.  When the
"path" argument to "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 "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 *path entry finder*
("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
"sys.path_importer_cache" (despite the name, this cache actually
stores finder objects rather than being limited to *importer*
objects). In this way, the expensive search for a particular *path
entry* location’s *path entry finder* need only be done once.  User
code is free to remove cache entries from "sys.path_importer_cache"
forcing the path based finder to perform the path entry search again
[3].

If the path entry is not present in the cache, the path based finder
iterates over every callable in "sys.path_hooks".  Each of the *path
entry hooks* in this list is called with a single argument, the path
entry to be searched.  This callable may either return a *path entry
finder* that can handle the path entry, or it may raise "ImportError".
An "ImportError" is used by the path based finder to signal that the
hook cannot find a *path entry finder* for that *path entry*.  The
exception is ignored and *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 "ImportError".

If "sys.path_hooks" iteration ends with no *path entry finder* being
returned, then the path based finder’s "find_spec()" method will store
"None" in "sys.path_importer_cache" (to indicate that there is no
finder for this path entry) and return "None", indicating that this
*meta path finder* could not find the module.

If a *path entry finder* *is* returned by one of the *path entry hook*
callables on "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 "sys.path". First,
if the current working directory is found to not exist, no value is
stored in "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 "sys.path_importer_cache" and returned by
"importlib.machinery.PathFinder.find_spec()" will be the actual
current working directory and not the empty string.


5.5.2. 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 "find_spec()" method.

"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 *portion*. the path entry finder sets “loader” on the spec
to "None" and “submodule_search_locations” to a list containing the
portion.

Changed in version 3.4: "find_spec()" replaced "find_loader()" and
"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.
However, if "find_spec()" is implemented on the path entry finder, the
legacy methods are ignored."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 *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()".


5.6. Replacing the standard import system
=========================================

The most reliable mechanism for replacing the entire import system is
to delete the default contents of "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 "__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 "ImportError" directly from "find_spec()"
instead of returning "None". The latter indicates that the meta path
search should continue, while raising an exception terminates it
immediately.


5.7. Special considerations for __main__
========================================

The "__main__" module is a special case relative to Python’s import
system.  As noted elsewhere, the "__main__" module is directly
initialized at interpreter startup, much like "sys" and "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.


5.7.1. __main__.__spec__
------------------------

Depending on how "__main__" is initialized, "__main__.__spec__" gets
set appropriately or to "None".

When Python is started with the "-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 "sys.path" entry.

In the remaining cases "__main__.__spec__" is set to "None", as the
code used to populate the "__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 "-m" switch if valid module metadata is desired in
"__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.


5.8. 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**).


5.9. References
===============

The import machinery has evolved considerably since Python’s early
days.  The original specification for packages is still available to
read, although some details have changed since the writing of that
document.

The original specification for "sys.meta_path" was **PEP 302**, with
subsequent extension in **PEP 420**.

**PEP 420** introduced *namespace packages* for Python 3.3.  **PEP
420** also introduced the "find_loader()" protocol as an alternative
to "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.

-[ Footnotes ]-

[1] See "types.ModuleType".

[2] The importlib implementation avoids using the return value
    directly. Instead, it gets the module object by looking the module
    name up in "sys.modules".  The indirect effect of this is that an
    imported module may replace itself in "sys.modules".  This is
    implementation-specific behavior that is not guaranteed to work in
    other Python implementations.

[3] In legacy code, it is possible to find instances of
    "imp.NullImporter" in the "sys.path_importer_cache".  It is
    recommended that code be changed to use "None" instead.  See
    Porting Python code for more details.
