[Python-checkins] peps: Add PEP 221: "Import As", as posted on python-dev.

Wed Mar 23 21:24:59 CET 2011

http://hg.python.org/peps/rev/ea3bbd855a3d
changeset: 100:ea3bbd855a3d
user: Thomas Wouters <thomas at python.org>
date: Tue Aug 15 13:42:44 2000 +0000
summary:
 Add PEP 221: "Import As", as posted on python-dev.
files:
 pep-0221.txt | 109 +++++++++++++++++++++++++++++++++++++++
 1 files changed, 109 insertions(+), 0 deletions(-)

diff --git a/pep-0221.txt b/pep-0221.txt
new file mode 100644
--- /dev/null
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+PEP: 221
+Title: Import As
+Version: $Revision$
+Owner: thomas at xs4all.net (Thomas Wouters)
+Python-Version: 2.0
+Status: Draft
+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.
+
+
+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
+
+
+
+Local Variables:
+mode: indented-text
+indent-tabs-mode: nil
+End:
-- 
Repository URL: http://hg.python.org/peps
