|Author:||Thomas Wouters <thomas at python.org>|
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.
This PEP proposes an extention 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
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. 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 accomodate 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 accomodate this is minimal, as the patch 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'.
This document has been placed in the Public Domain.