python-peps/pep-0221.txt

PEP: 221
Title: Import As
Version: $Revision$
Owner: thomas@xs4all.net (Thomas Wouters)
Python-Version: 2.0
Status: Accepted
Created: 15-Aug-2000
Type: Standard


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 a small extention of current 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. Currently, a
    round-about way has to be used to achieve this:

        import os
        real_os = os
        del os
    
    And similar 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.
    
    To avoid confusion, importing a submodule `as' another module is
    not allowed. When importing a submodule in the normal way,
    
        import os.path
        
    The actual name stored locally is `os', not `path' (so that the
    newly imported module can be referenced as `os.path'.) When
    introducing the `as' keyword, it is unclear whether the `os'
    module or the `path' sub-module should be stored `as' the
    requested local name.


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.

    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.
    
    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 proves[2], and
    the resulting generalization allows a number of new constructs
    that run completely parallel with other Python assignment
    constructs.

        import sys as x['sys']

        from MyFastcPickle import Pickler as shelve.Pickler
        
        from sys import version_info as (maj, min, pl, relnam, relno)
        
        from sys import path as mypath[-1:]


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



Local Variables:
mode: indented-text
indent-tabs-mode: nil
End: