
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
import system first checks the module global namespace for a function
by that name. If it is not found, then the standard builtin
``__import__()`` is called. Other mechanisms for invoking the import
system (such as ``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 [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**).


``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.


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``.


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.


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.


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.


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, ``imp.reload()`` will reuse the *same* module object, and
simply reinitialise the module contents by rerunning the module's
code.


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 *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
------------

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.


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_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 ``sys.meta_path`` processing reaches the end of
its list without returning a loader, then an ``ImportError`` is
raised.  Any other exceptions raised are simply propagated up,
aborting the import process.

The ``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 ``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 ``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*).


Loaders
=======

If and when a module loader is found its ``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 [2].  If it cannot load the
module, it should raise an ``ImportError``, although any other
exception raised during ``load_module()`` will be propagated.

In many cases, the finder and loader can be the same object; in such
cases the ``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
     ``sys.modules``, the loader must use that existing module.
     (Otherwise, ``imp.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``.

     Note that the module *must* 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 loader must remove any modules it has
     inserted into ``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 ``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 ``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 *below*.

   * 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 ``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
``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
     ``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 ``module_repr()``, the exception is
     caught and discarded, and the calculation of the module's repr
     continues as if ``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__)


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 namespace loader automatically sets ``__path__`` correctly for the
namespace package.


The Path Based Finder
=====================

As mentioned previously, Python comes with several default meta path
finders. One of these, called the *path based finder*, 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`` and
``.pyo`` 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.


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_module()`` 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_module()`` method as described previously.  When the
``path`` argument to ``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 ``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*
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_module()`` 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 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 ``find_loader()`` method.

``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. Instead path entry finders should implement the
``find_loader()`` method as described above. If it exists on the 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 ``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_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 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.


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.
