[Python-Dev] New PEP for import-as

[Thomas Wouters]

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I wrote a quick PEP describing the 'import as' proposal I posted a patch for
last week. Mostly since I was bored in the train to work (too many kids running
around to play Diablo II or any other game -- I hate it when those brats go
'oh cool' and keep hanging around looking over my shoulder ;-) but also a
bit because Tim keeps insisting it should be easy to write a PEP. Perhaps
lowering the standard by providing a few *small* PEPs helps with that ;)
Just's 'indexing-for' PEP would be a good one, too, in that case.

Anyway, the proto-PEP is attached. It's in draft status as far as I'm
concerned, but the PEP isn't really necessary if the feature is accepted by
acclamation.

-- 
Thomas Wouters <thomas@xs4all.net>

Hi! I'm a .signature virus! copy me into your .signature file to help me spread!

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PEP: 2XX
Title: Import As
Version: $Revision: 1.0 $
Owner: thomas@xs4all.net (Thomas Wouters)
Python-Version: 2.0
Status: Draft


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.


Implementation details

    A proposed implementation of this new clause can be found in the
    SourceForge patch manager[XX]. The patch uses a NAME field in the
    grammar rather than a bare string, to avoid the keyword issue. It
    also introduces a new bytecode, IMPORT_FROM_AS, which loads an
    object from a module and pushes it onto the stack, so it can be
    stored by a normal STORE_NAME opcode.
    
    The special case of `from module import *' remains a special case,
    in that it cannot accomodate an `as' clause. Also, the current
    implementation does not use IMPORT_FROM_AS for the old form of
    from-import, even though it would make sense to do so. The reason
    for this is that the current IMPORT_FROM bytecode loads objects
    directly from a module into the local namespace, in one bytecode
    operation, and also handles the special case of `*'. As a result
    of moving to the IMPORT_FROM_AS bytecode, two things would happen:
    
    - Two bytecode operations would have to be performed, per symbol,
      rather than one.
      
    - The names imported through `from-import' would be susceptible to
      the `global' keyword, which they currently are not. This means
      that `from-import' outside of the `*' special case behaves more
      like the normal `import' statement, which already follows the
      `global' keyword. It also means, however, that the `*' special
      case is even more special, compared to the ordinary form of
      `from-import'

    However, for consistency and for simplicity of implementation, it
    is probably best to split off the special case entirely, making a
    separate bytecode `IMPORT_ALL' that handles the special case of
    `*', and handle all other forms of `from-import' the way the
    proposed `IMPORT_FROM_AS' bytecode does.

    This dilemma does not apply to the normal `import' statement,
    because this is alread split into two opcodes, a `LOAD_MODULE' and a
    `STORE_NAME' opcode. Supporting the `import as' syntax is a slight
    change to the compiler only.


Copyright

    This document has been placed in the Public Domain.


References

    [1]
http://sourceforge.net/patch/?func=detailpatch&patch_id=101135&group_id=5470



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