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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 | |
| Content-Type: text/x-rst | |
| Created: 15-Aug-2000 | |
| Python-Version: 2.0 | |
| 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 extension 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 accommodate 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 accommodate 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] https://hg.python.org/cpython/rev/18385172fac0 | |
| .. [2] http://sourceforge.net/patch/?func=detailpatch&patch_id=101234&group_id=5470 | |
| .. | |
| Local Variables: | |
| mode: indented-text | |
| indent-tabs-mode: nil | |
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