117 lines
3.9 KiB
Plaintext
117 lines
3.9 KiB
Plaintext
PEP: 221
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Title: Import As
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Version: $Revision$
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Author: thomas@xs4all.net (Thomas Wouters)
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Status: Final
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Type: Standards Track
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Python-Version: 2.0
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Created: 15-Aug-2000
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Post-History:
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Introduction
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This PEP describes the `import as' proposal for Python 2.0. This
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PEP tracks the status and ownership of this feature. It contains
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a description of the feature and outlines changes necessary to
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support the feature. The CVS revision history of this file
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contains the definitive historical record.
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Rationale
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This PEP proposes an extention of Python syntax regarding the
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`import' and `from <module> import' statements. These statements
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load in a module, and either bind that module to a local name, or
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binds objects from that module to a local name. However, it is
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sometimes desirable to bind those objects to a different name, for
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instance to avoid name clashes. This can currently be achieved
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using the following idiom:
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import os
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real_os = os
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del os
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And similarly for the `from ... import' statement:
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from os import fdopen, exit, stat
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os_fdopen = fdopen
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os_stat = stat
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del fdopen, stat
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The proposed syntax change would add an optional `as' clause to
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both these statements, as follows:
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import os as real_os
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from os import fdopen as os_fdopen, exit, stat as os_stat
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The `as' name is not intended to be a keyword, and some trickery
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has to be used to convince the CPython parser it isn't one. For
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more advanced parsers/tokenizers, however, this should not be a
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problem.
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A slightly special case exists for importing sub-modules. The
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statement
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import os.path
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stores the module `os' locally as `os', so that the imported
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submodule `path' is accessible as `os.path'. As a result,
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import os.path as p
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stores `os.path', not `os', in `p'. This makes it effectively the
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same as
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from os import path as p
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Implementation details
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This PEP has been accepted, and the suggested code change has been
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checked in. The patch can still be found in the SourceForge patch
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manager[1]. Currently, a NAME field is used in the grammar rather
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than a bare string, to avoid the keyword issue. It introduces a
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new bytecode, IMPORT_STAR, which performs the `from module import
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*' behaviour, and changes the behaviour of the IMPORT_FROM
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bytecode so that it loads the requested name (which is always a
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single name) onto the stack, to be subsequently stored by a STORE
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opcode. As a result, all names explicitly imported now follow the
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`global' directives.
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The special case of `from module import *' remains a special case,
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in that it cannot accomodate an `as' clause, and that no STORE
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opcodes are generated; the objects imported are loaded directly
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into the local namespace. This also means that names imported in
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this fashion are always local, and do not follow the `global'
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directive.
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An additional change to this syntax has also been suggested, to
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generalize the expression given after the `as' clause. Rather
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than a single name, it could be allowed to be any expression that
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yields a valid l-value; anything that can be assigned to. The
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change to accomodate this is minimal, as the patch[2] proves, and
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the resulting generalization allows a number of new constructs
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that run completely parallel with other Python assignment
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constructs. However, this idea has been rejected by Guido, as
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`hypergeneralization'.
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Copyright
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This document has been placed in the Public Domain.
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References
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[1] http://sourceforge.net/patch/?func=detailpatch&patch_id=101135&group_id=5470
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[2] http://sourceforge.net/patch/?func=detailpatch&patch_id=101234&group_id=5470
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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End:
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