python-peps/pep-0221.txt

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PEP: 221
Title: Import As
Version: $Revision$
Last-Modified: $Date$
Author: thomas@python.org (Thomas Wouters)
Status: Final
Type: Standards Track
Python-Version: 2.0
Created: 15-Aug-2000
Post-History:
Introduction
This PEP describes the `import as' proposal for Python 2.0. This
PEP tracks the status and ownership of this feature. It contains
a description of the feature and outlines changes necessary to
support the feature. The CVS revision history of this file
contains the definitive historical record.
Rationale
This PEP proposes an extention of Python syntax regarding the
`import' and `from <module> import' statements. These statements
load in a module, and either bind that module to a local name, or
binds objects from that module to a local name. However, it is
sometimes desirable to bind those objects to a different name, for
instance to avoid name clashes. This can currently be achieved
using the following idiom:
import os
real_os = os
del os
And similarly for the `from ... import' statement:
from os import fdopen, exit, stat
os_fdopen = fdopen
os_stat = stat
del fdopen, stat
The proposed syntax change would add an optional `as' clause to
both these statements, as follows:
import os as real_os
from os import fdopen as os_fdopen, exit, stat as os_stat
The `as' name is not intended to be a keyword, and some trickery
has to be used to convince the CPython parser it isn't one. For
more advanced parsers/tokenizers, however, this should not be a
problem.
A slightly special case exists for importing sub-modules. The
statement
import os.path
stores the module `os' locally as `os', so that the imported
submodule `path' is accessible as `os.path'. As a result,
import os.path as p
stores `os.path', not `os', in `p'. This makes it effectively the
same as
from os import path as p
Implementation details
This PEP has been accepted, and the suggested code change has been
checked in. The patch can still be found in the SourceForge patch
manager[1]. Currently, a NAME field is used in the grammar rather
than a bare string, to avoid the keyword issue. It introduces a
new bytecode, IMPORT_STAR, which performs the `from module import
*' behaviour, and changes the behaviour of the IMPORT_FROM
bytecode so that it loads the requested name (which is always a
single name) onto the stack, to be subsequently stored by a STORE
opcode. As a result, all names explicitly imported now follow the
`global' directives.
The special case of `from module import *' remains a special case,
in that it cannot accomodate an `as' clause, and that no STORE
opcodes are generated; the objects imported are loaded directly
into the local namespace. This also means that names imported in
this fashion are always local, and do not follow the `global'
directive.
An additional change to this syntax has also been suggested, to
generalize the expression given after the `as' clause. Rather
than a single name, it could be allowed to be any expression that
yields a valid l-value; anything that can be assigned to. The
change to accomodate this is minimal, as the patch[2] proves, and
the resulting generalization allows a number of new constructs
that run completely parallel with other Python assignment
constructs. However, this idea has been rejected by Guido, as
`hypergeneralization'.
Copyright
This document has been placed in the Public Domain.
References
[1] http://sourceforge.net/patch/?func=detailpatch&patch_id=101135&group_id=5470
[2] http://sourceforge.net/patch/?func=detailpatch&patch_id=101234&group_id=5470
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