python-peps/peps/pep-0302.rst

PEP: 302
Title: New Import Hooks
Version: $Revision$
Last-Modified: $Date$
Author: Just van Rossum <just@letterror.com>,
        Paul Moore <p.f.moore@gmail.com>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 19-Dec-2002
Python-Version: 2.3
Post-History: 19-Dec-2002

.. warning::
   The language reference for import [10]_ and importlib documentation
   [11]_ now supersede this PEP. This document is no longer updated
   and provided for historical purposes only.


Abstract
========

This PEP proposes to add a new set of import hooks that offer better
customization of the Python import mechanism.  Contrary to the current
``__import__`` hook, a new-style hook can be injected into the existing
scheme, allowing for a finer grained control of how modules are found and how
they are loaded.


Motivation
==========

The only way to customize the import mechanism is currently to override the
built-in ``__import__`` function.  However, overriding ``__import__`` has many
problems.  To begin with:

* An ``__import__`` replacement needs to *fully* reimplement the entire
  import mechanism, or call the original ``__import__`` before or after the
  custom code.

* It has very complex semantics and responsibilities.

* ``__import__`` gets called even for modules that are already in
  ``sys.modules``, which is almost never what you want, unless you're writing
  some sort of monitoring tool.

The situation gets worse when you need to extend the import mechanism from C:
it's currently impossible, apart from hacking Python's ``import.c`` or
reimplementing much of ``import.c`` from scratch.

There is a fairly long history of tools written in Python that allow extending
the import mechanism in various way, based on the ``__import__`` hook.  The
Standard Library includes two such tools: ``ihooks.py`` (by GvR) and
``imputil.py`` [1]_ (Greg Stein), but perhaps the most famous is ``iu.py`` by
Gordon McMillan, available as part of his Installer package.  Their usefulness
is somewhat limited because they are written in Python; bootstrapping issues
need to worked around as you can't load the module containing the hook with
the hook itself.  So if you want the entire Standard Library to be loadable
from an import hook, the hook must be written in C.


Use cases
=========

This section lists several existing applications that depend on import hooks.
Among these, a lot of duplicate work was done that could have been saved if
there had been a more flexible import hook at the time.  This PEP should make
life a lot easier for similar projects in the future.

Extending the import mechanism is needed when you want to load modules that
are stored in a non-standard way.  Examples include modules that are bundled
together in an archive; byte code that is not stored in a ``pyc`` formatted
file; modules that are loaded from a database over a network.

The work on this PEP was partly triggered by the implementation of :pep:`273`,
which adds imports from Zip archives as a built-in feature to Python.  While
the PEP itself was widely accepted as a must-have feature, the implementation
left a few things to desire.  For one thing it went through great lengths to
integrate itself with ``import.c``, adding lots of code that was either
specific for Zip file imports or *not* specific to Zip imports, yet was not
generally useful (or even desirable) either.  Yet the :pep:`273` implementation
can hardly be blamed for this: it is simply extremely hard to do, given the
current state of ``import.c``.

Packaging applications for end users is a typical use case for import hooks,
if not *the* typical use case.  Distributing lots of source or ``pyc`` files
around is not always appropriate (let alone a separate Python installation),
so there is a frequent desire to package all needed modules in a single file.
So frequent in fact that multiple solutions have been implemented over the
years.

The oldest one is included with the Python source code: Freeze [2]_.  It puts
marshalled byte code into static objects in C source code.  Freeze's "import
hook" is hard wired into ``import.c``, and has a couple of issues.  Later
solutions include Fredrik Lundh's Squeeze, Gordon McMillan's Installer, and
Thomas Heller's py2exe [3]_.  MacPython ships with a tool called
``BuildApplication``.

Squeeze, Installer and py2exe use an ``__import__`` based scheme (py2exe
currently uses Installer's ``iu.py``, Squeeze used ``ihooks.py``), MacPython
has two Mac-specific import hooks hard wired into ``import.c``, that are
similar to the Freeze hook.  The hooks proposed in this PEP enables us (at
least in theory; it's not a short-term goal) to get rid of the hard coded
hooks in ``import.c``, and would allow the ``__import__``-based tools to get
rid of most of their ``import.c`` emulation code.

Before work on the design and implementation of this PEP was started, a new
``BuildApplication``-like tool for Mac OS X prompted one of the authors of
this PEP (JvR) to expose the table of frozen modules to Python, in the ``imp``
module.  The main reason was to be able to use the freeze import hook
(avoiding fancy ``__import__`` support), yet to also be able to supply a set
of modules at runtime.  This resulted in issue #642578 [4]_, which was
mysteriously accepted (mostly because nobody seemed to care either way ;-).
Yet it is completely superfluous when this PEP gets accepted, as it offers a
much nicer and general way to do the same thing.


Rationale
=========

While experimenting with alternative implementation ideas to get built-in Zip
import, it was discovered that achieving this is possible with only a fairly
small amount of changes to ``import.c``.  This allowed to factor out the
Zip-specific stuff into a new source file, while at the same time creating a
*general* new import hook scheme: the one you're reading about now.

An earlier design allowed non-string objects on ``sys.path``.  Such an object
would have the necessary methods to handle an import.  This has two
disadvantages: 1) it breaks code that assumes all items on ``sys.path`` are
strings; 2) it is not compatible with the ``PYTHONPATH`` environment variable.
The latter is directly needed for Zip imports.  A compromise came from Jython:
allow string *subclasses* on ``sys.path``, which would then act as importer
objects.  This avoids some breakage, and seems to work well for Jython (where
it is used to load modules from ``.jar`` files), but it was perceived as an
"ugly hack".

This led to a more elaborate scheme, (mostly copied from McMillan's
``iu.py``) in which each in a list of candidates is asked whether it can
handle the ``sys.path`` item, until one is found that can.  This list of
candidates is a new object in the ``sys`` module: ``sys.path_hooks``.

Traversing ``sys.path_hooks`` for each path item for each new import can be
expensive, so the results are cached in another new object in the ``sys``
module: ``sys.path_importer_cache``.  It maps ``sys.path`` entries to importer
objects.

To minimize the impact on ``import.c`` as well as to avoid adding extra
overhead, it was chosen to not add an explicit hook and importer object for
the existing file system import logic (as ``iu.py`` has), but to simply fall
back to the built-in logic if no hook on ``sys.path_hooks`` could handle the
path item.  If this is the case, a ``None`` value is stored in
``sys.path_importer_cache``, again to avoid repeated lookups.  (Later we can
go further and add a real importer object for the built-in mechanism, for now,
the ``None`` fallback scheme should suffice.)

A question was raised: what about importers that don't need *any* entry on
``sys.path``? (Built-in and frozen modules fall into that category.)  Again,
Gordon McMillan to the rescue: ``iu.py`` contains a thing he calls the
*metapath*.  In this PEP's implementation, it's a list of importer objects
that is traversed *before* ``sys.path``.  This list is yet another new object
in the ``sys`` module: ``sys.meta_path``.  Currently, this list is empty by
default, and frozen and built-in module imports are done after traversing
``sys.meta_path``, but still before ``sys.path``.


Specification part 1: The Importer Protocol
===========================================

This PEP introduces a new protocol: the "Importer Protocol".  It is important
to understand the context in which the protocol operates, so here is a brief
overview of the outer shells of the import mechanism.

When an import statement is encountered, the interpreter looks up the
``__import__`` function in the built-in name space.  ``__import__`` is then
called with four arguments, amongst which are the name of the module being
imported (may be a dotted name) and a reference to the current global
namespace.

The built-in ``__import__`` function (known as ``PyImport_ImportModuleEx()``
in ``import.c``) will then check to see whether the module doing the import is
a package or a submodule of a package.  If it is indeed a (submodule of a)
package, it first tries to do the import relative to the package (the parent
package for a submodule).  For example, if a package named "spam" does "import
eggs", it will first look for a module named "spam.eggs".  If that fails, the
import continues as an absolute import: it will look for a module named
"eggs".  Dotted name imports work pretty much the same: if package "spam" does
"import eggs.bacon" (and "spam.eggs" exists and is itself a package),
"spam.eggs.bacon" is tried.  If that fails "eggs.bacon" is tried.  (There are
more subtleties that are not described here, but these are not relevant for
implementers of the Importer Protocol.)

Deeper down in the mechanism, a dotted name import is split up by its
components.  For "import spam.ham", first an "import spam" is done, and only
when that succeeds is "ham" imported as a submodule of "spam".

The Importer Protocol operates at this level of *individual* imports.  By the
time an importer gets a request for "spam.ham", module "spam" has already been
imported.

The protocol involves two objects: a *finder* and a *loader*.  A finder object
has a single method::

    finder.find_module(fullname, path=None)

This method will be called with the fully qualified name of the module.  If
the finder is installed on ``sys.meta_path``, it will receive a second
argument, which is ``None`` for a top-level module, or ``package.__path__``
for submodules or subpackages [5]_.  It should return a loader object if the
module was found, or ``None`` if it wasn't.  If ``find_module()`` raises an
exception, it will be propagated to the caller, aborting the import.

A loader object also has one method::

    loader.load_module(fullname)

This method returns the loaded module or raises an exception, preferably
``ImportError`` if an existing exception is not being propagated.  If
``load_module()`` is asked to load a module that it cannot, ``ImportError`` is
to be raised.

In many cases the finder and loader can be one and the same object:
``finder.find_module()`` would just return ``self``.

The ``fullname`` argument of both methods is the fully qualified module name,
for example "spam.eggs.ham".  As explained above, when
``finder.find_module("spam.eggs.ham")`` is called, "spam.eggs" has already
been imported and added to ``sys.modules``.  However, the ``find_module()``
method isn't necessarily always called during an actual import: meta tools
that analyze import dependencies (such as freeze, Installer or py2exe) don't
actually load modules, so a finder shouldn't *depend* on the parent package
being available in ``sys.modules``.

The ``load_module()`` method has a few responsibilities that it must fulfill
*before* it runs any code:

* If there is an existing module object named 'fullname' in ``sys.modules``,
  the loader must use that existing module.  (Otherwise, the ``reload()``
  builtin will not work correctly.)  If a module named 'fullname' does not
  exist in ``sys.modules``, the loader must create a new module object and
  add it to ``sys.modules``.

  Note that the module object *must* be 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 the load fails, the loader needs to remove any module it may have
  inserted into ``sys.modules``. If the module was already in ``sys.modules``
  then the loader should leave it alone.

* The ``__file__`` attribute must be set.  This must be a string, but it may
  be a dummy value, for example "<frozen>".  The privilege of not having a
  ``__file__`` attribute at all is reserved for built-in modules.

* The ``__name__`` attribute must be set.  If one uses ``imp.new_module()``
  then the attribute is set automatically.

* If it's a package, the ``__path__`` variable must be set.  This must be a
  list, but may be empty if ``__path__`` has no further significance to the
  importer (more on this later).

* The ``__loader__`` attribute must be set to the loader object.  This is
  mostly for introspection and reloading, but can be used for
  importer-specific extras, for example getting data associated with an
  importer.

* The ``__package__`` attribute must be set (:pep:`366`).

  If the module is a Python module (as opposed to a built-in module or a
  dynamically loaded extension), it should execute the module's code in the
  module's global name space (``module.__dict__``).

  Here is a minimal pattern for a ``load_module()`` method::

       # Consider using importlib.util.module_for_loader() to handle
       # most of these details for you.
       def load_module(self, fullname):
           code = self.get_code(fullname)
           ispkg = self.is_package(fullname)
           mod = sys.modules.setdefault(fullname, imp.new_module(fullname))
           mod.__file__ = "<%s>" % self.__class__.__name__
           mod.__loader__ = self
           if ispkg:
               mod.__path__ = []
               mod.__package__ = fullname
           else:
               mod.__package__ = fullname.rpartition('.')[0]
           exec(code, mod.__dict__)
           return mod


Specification part 2: Registering Hooks
=======================================

There are two types of import hooks: *Meta hooks* and *Path hooks*.  Meta
hooks are called at the start of import processing, before any other import
processing (so that meta hooks can override ``sys.path`` processing, frozen
modules, or even built-in modules).  To register a meta hook, simply add the
finder object to ``sys.meta_path`` (the list of registered meta hooks).

Path hooks are called as part of ``sys.path`` (or ``package.__path__``)
processing, at the point where their associated path item is encountered.  A
path hook is registered by adding an importer factory to ``sys.path_hooks``.

``sys.path_hooks`` is a list of callables, which will be checked in sequence
to determine if they can handle a given path item.  The callable is called
with one argument, the path item.  The callable must raise ``ImportError`` if
it is unable to handle the path item, and return an importer object if it can
handle the path item.  Note that if the callable returns an importer object
for a specific ``sys.path`` entry, the builtin import machinery will not be
invoked to handle that entry any longer, even if the importer object later
fails to find a specific module.  The callable is typically the class of the
import hook, and hence the class ``__init__()`` method is called.  (This is
also the reason why it should raise ``ImportError``: an ``__init__()`` method
can't return anything.  This would be possible with a ``__new__()`` method in
a new style class, but we don't want to require anything about how a hook is
implemented.)

The results of path hook checks are cached in ``sys.path_importer_cache``,
which is a dictionary mapping path entries to importer objects.  The cache is
checked before ``sys.path_hooks`` is scanned.  If it is necessary to force a
rescan of ``sys.path_hooks``, it is possible to manually clear all or part of
``sys.path_importer_cache``.

Just like ``sys.path`` itself, the new ``sys`` variables must have specific
types:

* ``sys.meta_path`` and ``sys.path_hooks`` must be Python lists.
* ``sys.path_importer_cache`` must be a Python dict.

Modifying these variables in place is allowed, as is replacing them with new
objects.


Packages and the role of ``__path__``
=====================================

If a module has a ``__path__`` attribute, the import mechanism will treat it
as a package.  The ``__path__`` variable is used instead of ``sys.path`` when
importing submodules of the package.  The rules for ``sys.path`` therefore
also apply to ``pkg.__path__``.  So ``sys.path_hooks`` is also consulted when
``pkg.__path__`` is traversed.  Meta importers don't necessarily use
``sys.path`` at all to do their work and may therefore ignore the value of
``pkg.__path__``.  In this case it is still advised to set it to list, which
can be empty.


Optional Extensions to the Importer Protocol
============================================

The Importer Protocol defines three optional extensions.  One is to retrieve
data files, the second is to support module packaging tools and/or tools that
analyze module dependencies (for example Freeze), while the last is to support
execution of modules as scripts.  The latter two categories of tools usually
don't actually *load* modules, they only need to know if and where they are
available.  All three extensions are highly recommended for general purpose
importers, but may safely be left out if those features aren't needed.

To retrieve the data for arbitrary "files" from the underlying storage
backend, loader objects may supply a method named ``get_data()``::

    loader.get_data(path)

This method returns the data as a string, or raise ``IOError`` if the "file"
wasn't found.  The data is always returned as if "binary" mode was used -
there is no CRLF translation of text files, for example.  It is meant for
importers that have some file-system-like properties.  The 'path' argument is
a path that can be constructed by munging ``module.__file__`` (or
``pkg.__path__`` items) with the ``os.path.*`` functions, for example::

    d = os.path.dirname(__file__)
    data = __loader__.get_data(os.path.join(d, "logo.gif"))

The following set of methods may be implemented if support for (for example)
Freeze-like tools is desirable.  It consists of three additional methods
which, to make it easier for the caller, each of which should be implemented,
or none at all::

    loader.is_package(fullname)
    loader.get_code(fullname)
    loader.get_source(fullname)

All three methods should raise ``ImportError`` if the module wasn't found.

The ``loader.is_package(fullname)`` method should return ``True`` if the
module specified by 'fullname' is a package and ``False`` if it isn't.

The ``loader.get_code(fullname)`` method should return the code object
associated with the module, or ``None`` if it's a built-in or extension
module.  If the loader doesn't have the code object but it *does* have the
source code, it should return the compiled source code.  (This is so that our
caller doesn't also need to check ``get_source()`` if all it needs is the code
object.)

The ``loader.get_source(fullname)`` method should return the source code for
the module as a string (using newline characters for line endings) or ``None``
if the source is not available (yet it should still raise ``ImportError`` if
the module can't be found by the importer at all).

To support execution of modules as scripts (:pep:`338`),
the above three methods for
finding the code associated with a module must be implemented. In addition to
those methods, the following method may be provided in order to allow the
``runpy`` module to correctly set the ``__file__`` attribute::

    loader.get_filename(fullname)

This method should return the value that ``__file__`` would be set to if the
named module was loaded. If the module is not found, then ``ImportError``
should be raised.


Integration with the 'imp' module
=================================

The new import hooks are not easily integrated in the existing
``imp.find_module()`` and ``imp.load_module()`` calls.  It's questionable
whether it's possible at all without breaking code; it is better to simply add
a new function to the ``imp`` module.  The meaning of the existing
``imp.find_module()`` and ``imp.load_module()`` calls changes from: "they
expose the built-in import mechanism" to "they expose the basic *unhooked*
built-in import mechanism".  They simply won't invoke any import hooks.  A new
``imp`` module function is proposed (but not yet implemented) under the name
``get_loader()``, which is used as in the following pattern::

    loader = imp.get_loader(fullname, path)
    if loader is not None:
        loader.load_module(fullname)

In the case of a "basic" import, one the ``imp.find_module()`` function would
handle, the loader object would be a wrapper for the current output of
``imp.find_module()``, and ``loader.load_module()`` would call
``imp.load_module()`` with that output.

Note that this wrapper is currently not yet implemented, although a Python
prototype exists in the ``test_importhooks.py`` script (the ``ImpWrapper``
class) included with the patch.


Forward Compatibility
=====================

Existing ``__import__`` hooks will not invoke new-style hooks by magic, unless
they call the original ``__import__`` function as a fallback.  For example,
``ihooks.py``, ``iu.py`` and ``imputil.py`` are in this sense not forward
compatible with this PEP.


Open Issues
===========

Modules often need supporting data files to do their job, particularly in the
case of complex packages or full applications.  Current practice is generally
to locate such files via ``sys.path`` (or a ``package.__path__`` attribute).
This approach will not work, in general, for modules loaded via an import
hook.

There are a number of possible ways to address this problem:

* "Don't do that".  If a package needs to locate data files via its
  ``__path__``, it is not suitable for loading via an import hook.  The
  package can still be located on a directory in ``sys.path``, as at present,
  so this should not be seen as a major issue.

* Locate data files from a standard location, rather than relative to the
  module file.  A relatively simple approach (which is supported by
  distutils) would be to locate data files based on ``sys.prefix`` (or
  ``sys.exec_prefix``).  For example, looking in
  ``os.path.join(sys.prefix, "data", package_name)``.

* Import hooks could offer a standard way of getting at data files relative
  to the module file.  The standard ``zipimport`` object provides a method
  ``get_data(name)`` which returns the content of the "file" called ``name``,
  as a string.  To allow modules to get at the importer object, ``zipimport``
  also adds an attribute ``__loader__`` to the module, containing the
  ``zipimport`` object used to load the module.  If such an approach is used,
  it is important that client code takes care not to break if the
  ``get_data()`` method is not available, so it is not clear that this
  approach offers a general answer to the problem.

It was suggested on python-dev that it would be useful to be able to receive a
list of available modules from an importer and/or a list of available data
files for use with the ``get_data()`` method.  The protocol could grow two
additional extensions, say ``list_modules()`` and ``list_files()``.  The
latter makes sense on loader objects with a ``get_data()`` method.  However,
it's a bit unclear which object should implement ``list_modules()``: the
importer or the loader or both?

This PEP is biased towards loading modules from alternative places: it
currently doesn't offer dedicated solutions for loading modules from
alternative file formats or with alternative compilers.  In contrast, the
``ihooks`` module from the standard library does have a fairly straightforward
way to do this.  The Quixote project [7]_ uses this technique to import PTL
files as if they are ordinary Python modules.  To do the same with the new
hooks would either mean to add a new module implementing a subset of
``ihooks`` as a new-style importer, or add a hookable built-in path importer
object.

There is no specific support within this PEP for "stacking" hooks.  For
example, it is not obvious how to write a hook to load modules from ``tar.gz``
files by combining separate hooks to load modules from ``.tar`` and ``.gz``
files.  However, there is no support for such stacking in the existing hook
mechanisms (either the basic "replace ``__import__``" method, or any of the
existing import hook modules) and so this functionality is not an obvious
requirement of the new mechanism.  It may be worth considering as a future
enhancement, however.

It is possible (via ``sys.meta_path``) to add hooks which run before
``sys.path`` is processed.  However, there is no equivalent way of adding
hooks to run after ``sys.path`` is processed.  For now, if a hook is required
after ``sys.path`` has been processed, it can be simulated by adding an
arbitrary "cookie" string at the end of ``sys.path``, and having the required
hook associated with this cookie, via the normal ``sys.path_hooks``
processing.  In the longer term, the path handling code will become a "real"
hook on ``sys.meta_path``, and at that stage it will be possible to insert
user-defined hooks either before or after it.


Implementation
==============

The :pep:`302` implementation has been integrated with Python as of 2.3a1.  An
earlier version is available as patch #652586 [9]_, but more interestingly,
the issue contains a fairly detailed history of the development and design.

:pep:`273` has been implemented using :pep:`302`'s import hooks.


References and Footnotes
========================

.. [1] imputil module
   http://docs.python.org/library/imputil.html

.. [2] The Freeze tool.
   See also the ``Tools/freeze/`` directory in a Python source distribution

.. [3] py2exe by Thomas Heller
   http://www.py2exe.org/

.. [4] imp.set_frozenmodules() patch
   http://bugs.python.org/issue642578

.. [5] The path argument to ``finder.find_module()`` is there because the
   ``pkg.__path__`` variable may be needed at this point.  It may either come
   from the actual parent module or be supplied by ``imp.find_module()`` or
   the proposed ``imp.get_loader()`` function.

.. [7] Quixote, a framework for developing Web applications
   http://www.mems-exchange.org/software/quixote/

.. [9] New import hooks + Import from Zip files
   http://bugs.python.org/issue652586

.. [10] Language reference for imports
   http://docs.python.org/3/reference/import.html

.. [11] importlib documentation
   http://docs.python.org/3/library/importlib.html#module-importlib


Copyright
=========

This document has been placed in the public domain.



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