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: 08-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
-   Re: Python-Dev Digest, Vol 5, Issue 57
-   Relative import
-   Another Strategy for Relative Import

Copyright

This document has been placed in the public domain.



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

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