110 lines
3.8 KiB
Plaintext
110 lines
3.8 KiB
Plaintext
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PEP: 221
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Title: Import As
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Version: $Revision$
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Owner: thomas@xs4all.net (Thomas Wouters)
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Python-Version: 2.0
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Status: Draft
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Created: 15-Aug-2000
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Type: Standard
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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 a
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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 a small extention of current Python syntax
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regarding the `import' and `from <module> import' statements.
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These statements load in a module, and either bind that module to
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a local name, or binds objects from that module to a local name.
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However, it is sometimes desirable to bind those objects to a
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different name, for instance to avoid name clashes. Currently, a
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round-about way has to be used to achieve this:
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import os
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real_os = os
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del os
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And similar 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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Implementation details
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A proposed implementation of this new clause can be found in the
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SourceForge patch manager[XX]. The patch uses a NAME field in the
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grammar rather than a bare string, to avoid the keyword issue. It
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also introduces a new bytecode, IMPORT_FROM_AS, which loads an
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object from a module and pushes it onto the stack, so it can be
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stored by a normal STORE_NAME opcode.
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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. Also, the current
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implementation does not use IMPORT_FROM_AS for the old form of
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from-import, even though it would make sense to do so. The reason
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for this is that the current IMPORT_FROM bytecode loads objects
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directly from a module into the local namespace, in one bytecode
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operation, and also handles the special case of `*'. As a result
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of moving to the IMPORT_FROM_AS bytecode, two things would happen:
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- Two bytecode operations would have to be performed, per symbol,
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rather than one.
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- The names imported through `from-import' would be susceptible to
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the `global' keyword, which they currently are not. This means
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that `from-import' outside of the `*' special case behaves more
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like the normal `import' statement, which already follows the
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`global' keyword. It also means, however, that the `*' special
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case is even more special, compared to the ordinary form of
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`from-import'
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However, for consistency and for simplicity of implementation, it
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is probably best to split off the special case entirely, making a
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separate bytecode `IMPORT_ALL' that handles the special case of
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`*', and handle all other forms of `from-import' the way the
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proposed `IMPORT_FROM_AS' bytecode does.
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This dilemma does not apply to the normal `import' statement,
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because this is alread split into two opcodes, a `LOAD_MODULE' and a
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`STORE_NAME' opcode. Supporting the `import as' syntax is a slight
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change to the compiler only.
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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]
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http://sourceforge.net/patch/?func=detailpatch&patch_id=101135&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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