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python-peps / pep-0328.txt

The default branch has multiple heads

PEP: 328
Title: Imports: Multi-Line and Absolute/Relative
Version: $Revision$
Last-Modified: $Date$
Author: Aahz <aahz@pythoncraft.com>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 21-Dec-2003
Python-Version: 2.4, 2,5, 2.6
Post-History: 8-Mar-2004


Abstract
========

The ``import`` statement has two problems:

* Long ``import`` statements can be difficult to write, requiring
  various contortions to fit Pythonic style guidelines.

* Imports can be ambiguous in the face of packages; within a package,
  it's not clear whether ``import foo`` refers to a module within the
  package or some module outside the package.  (More precisely, a local
  module or package can shadow another hanging directly off
  ``sys.path``.)

For the first problem, it is proposed that parentheses be permitted to
enclose multiple names, thus allowing Python's standard mechanisms for
multi-line values to apply.  For the second problem, it is proposed that
all ``import`` statements be absolute by default (searching ``sys.path``
only) with special syntax (leading dots) for accessing package-relative
imports.


Timeline
========

In Python 2.5, you must enable the new absolute import behavior with ::

    from __future__ import absolute_import

You may use relative imports freely.  In Python 2.6, any ``import``
statement that results in an intra-package import will raise
``DeprecationWarning`` (this also applies to ``from <> import`` that
fails to use the relative import syntax).


Rationale for Parentheses
=========================

Currently, if you want to import a lot of names from a module or
package, you have to choose one of several unpalatable options:

* Write a long line with backslash continuations::

    from Tkinter import Tk, Frame, Button, Entry, Canvas, Text, \
        LEFT, DISABLED, NORMAL, RIDGE, END

* Write multiple ``import`` statements::

      from Tkinter import Tk, Frame, Button, Entry, Canvas, Text
      from Tkinter import LEFT, DISABLED, NORMAL, RIDGE, END

(``import *`` is *not* an option ;-)

Instead, it should be possible to use Python's standard grouping
mechanism (parentheses) to write the ``import`` statement::

    from Tkinter import (Tk, Frame, Button, Entry, Canvas, Text,
        LEFT, DISABLED, NORMAL, RIDGE, END)

This part of the proposal had BDFL approval from the beginning.

Parentheses support was added to Python 2.4.


Rationale for Absolute Imports
==============================

In Python 2.4 and earlier, if you're reading a module located inside a
package, it is not clear whether ::

    import foo

refers to a top-level module or to another module inside the package.
As Python's library expands, more and more existing package internal
modules suddenly shadow standard library modules by accident.  It's a
particularly difficult problem inside packages because there's no way to
specify which module is meant.  To resolve the ambiguity, it is proposed
that ``foo`` will always be a module or package reachable from
``sys.path``.  This is called an absolute import.

The python-dev community chose absolute imports as the default because
they're the more common use case and because absolute imports can provide
all the functionality of relative (intra-package) imports  -- albeit at
the cost of difficulty when renaming package pieces higher up in the
hierarchy or when moving one package inside another.

Because this represents a change in semantics, absolute imports will
be optional in Python 2.5 and 2.6 through the use of ::

    from __future__ import absolute_import

This part of the proposal had BDFL approval from the beginning.


Rationale for Relative Imports
==============================

With the shift to absolute imports, the question arose whether
relative imports should be allowed at all.  Several use cases were
presented, the most important of which is being able to rearrange the
structure of large packages without having to edit sub-packages.  In
addition, a module inside a package can't easily import itself without
relative imports.

Guido approved of the idea of relative imports, but there has been a
lot of disagreement on the spelling (syntax).  There does seem to be
agreement that relative imports will require listing specific names to
import (that is, ``import foo`` as a bare term will always be an
absolute import).

Here are the contenders:

* One from Guido::

      from .foo import bar

  and ::

      from ...foo import bar

  These two forms have a couple of different suggested semantics.  One
  semantic is to make each dot represent one level.  There have been
  many complaints about the difficulty of counting dots.  Another
  option is to only allow one level of relative import.  That misses a
  lot of functionality, and people still complained about missing the
  dot in the one-dot form.  The final option is to define an algorithm
  for finding relative modules and packages; the objection here is
  "Explicit is better than implicit".  (The algorithm proposed is
  "search up from current package directory until the ultimate package
  parent gets hit".)

  Some people have suggested other punctuation as the separator, such
  as "-" or "^".

  Some people have suggested using "*"::

      from *.foo import bar

* The next set of options is conflated from several posters::

      from __pkg__.__pkg__ import

  and ::

      from .__parent__.__parent__ import

  Many people (Guido included) think these look ugly, but they *are*
  clear and explicit.  Overall, more people prefer ``__pkg__`` as the
  shorter option.

* One suggestion was to allow only sibling references.  In other words,
  you would not be able to use relative imports to refer to modules
  higher in the package tree.  You would then be able to do either ::

      from .spam import eggs

  or ::

      import .spam.eggs

* Some people favor allowing indexed parents::

      from -2.spam import eggs

  In this scenario, importing from the current directory would be a
  simple ::

      from .spam import eggs


* Finally, some people dislike the way you have to change ``import``
  to ``from ... import`` when you want to dig inside a package.  They
  suggest completely rewriting the ``import`` syntax::

      from MODULE import NAMES as RENAME searching HOW

  or ::

      import NAMES as RENAME from MODULE searching HOW
          [from NAMES] [in WHERE] import ...

  However, this most likely could not be implemented for Python 2.5
  (too big a change), and allowing relative imports is sufficiently
  critical that we need something now (given that the standard
  ``import`` will change to absolute import).  More than that, this
  proposed syntax has several open questions:

  - What is the precise proposed syntax?  (Which clauses are optional
    under which circumstances?)

  - How strongly does the ``searching`` clause bind?  In other words,
    do you write::

        import foo as bar searching XXX, spam as ham searching XXX

    or::

        import foo as bar, spam as ham searching XXX


Guido's Decision
================

Guido has Pronounced [1]_ that relative imports will use leading dots.
A single leading dot indicates a relative import, starting with the
current package.  Two or more leading dots give a relative import to the
parent(s) of the current package, one level per dot after the first.
Here's a sample package layout::

    package/
        __init__.py
        subpackage1/
            __init__.py
            moduleX.py
            moduleY.py
        subpackage2/
            __init__.py
            moduleZ.py
        moduleA.py

Assuming that the current file is either ``moduleX.py`` or
``subpackage1/__init__.py``, following are correct usages of the new
syntax::

    from .moduleY import spam
    from .moduleY import spam as ham
    from . import moduleY
    from ..subpackage1 import moduleY
    from ..subpackage2.moduleZ import eggs
    from ..moduleA import foo
    from ...package import bar
    from ...sys import path

Note that while that last case is legal, it is certainly discouraged
("insane" was the word Guido used).

Relative imports must always use ``from <> import``; ``import <>`` is
always absolute.  Of course, absolute imports can use ``from <> import``
by omitting the leading dots.  The reason ``import .foo`` is prohibited
is because after ::

    import XXX.YYY.ZZZ

then ::

    XXX.YYY.ZZZ

is usable in an expression.  But ::

    .moduleY

is not usable in an expression.


Relative Imports and __name__
=============================

Relative imports use a module's __name__ attribute to determine that
module's position in the package hierarchy. If the module's name does
not contain any package information (e.g. it is set to '__main__')
then relative imports are resolved as if the module were a top level
module, regardless of where the module is actually located on the file
system.


Relative Imports and Indirection Entries in sys.modules
=======================================================

When packages were introduced, the concept of an indirection entry in
sys.modules came into existence [2]_.  When an entry in sys.modules
for a module within a package had a value of None, it represented that
the module actually referenced the top-level module.  For instance,
'Sound.Effects.string' might have a value of None in sys.modules.
That meant any import that resolved to that name actually was to
import the top-level 'string' module.

This introduced an optimization for when a relative import was meant
to resolve to an absolute import.  But since this PEP makes a very
clear delineation between absolute and relative imports, this
optimization is no longer needed.  When absolute/relative imports
become the only import semantics available then indirection entries in
sys.modules will no longer be supported.



References
==========

For more background, see the following python-dev threads:

- `Re: Christmas Wishlist
  <http://mail.python.org/pipermail/python-dev/2003-December/040973.html>`__

- `Re: Python-Dev Digest, Vol 5, Issue 57
  <http://mail.python.org/pipermail/python-dev/2003-December/041078.html>`__

- `Relative import
  <http://mail.python.org/pipermail/python-dev/2003-December/041065.html>`__

- `Another Strategy for Relative Import
  <http://mail.python.org/pipermail/python-dev/2003-December/041418.html>`__

.. [1] http://mail.python.org/pipermail/python-dev/2004-March/043739.html

.. [2] http://www.python.org/doc/essays/packages.html


Copyright
=========

This document has been placed in the public domain.


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