2000-10-26 17:22:26 -04:00
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PEP: 5
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Title: Guidelines for Language Evolution
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Version: $Revision$
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2006-03-23 15:13:19 -05:00
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Last-Modified: $Date$
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2000-10-26 17:22:26 -04:00
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Author: paul@prescod.net (Paul Prescod)
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Status: Active
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Type: Informational
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Created: 26-Oct-2000
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Abstract
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In the natural evolution of programming languages it is sometimes
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necessary to make changes that modify the behavior of older
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programs. This PEP proposes a policy for implementing these
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changes in a manner respectful of the installed base of Python
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users.
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Implementation Details
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Implementation of this PEP requires the addition of a formal
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warning and deprecation facility that will be described in another
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proposal.
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Scope
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These guidelines apply to future versions of Python that introduce
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backward-incompatible behavior. Backward incompatible behavior is
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a major deviation in Python interpretation from an earlier
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behavior described in the standard Python documentation. Removal
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of a feature also constitutes a change of behavior.
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This PEP does not replace or preclude other compatibility
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strategies such as dynamic loading of backwards-compatible
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parsers. On the other hand, if execution of "old code" requires a
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special switch or pragma then that is indeed a change of behavior
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from the point of view of the user and that change should be
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implemented according to these guidelines.
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In general, common sense must prevail in the implementation of
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these guidelines. For instance changing "sys.copyright" does not
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constitute a backwards-incompatible change of behavior!
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Steps For Introducing Backwards-Incompatible Features
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1. Propose backwards-incompatible behavior in a PEP. The PEP must
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include a section on backwards compatibility that describes in
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detail a plan to complete the remainder of these steps.
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2. Once the PEP is accepted as a productive direction, implement
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an alternate way to accomplish the task previously provided by
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the feature that is being removed or changed. For instance if
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the addition operator were scheduled for removal, a new version
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of Python could implement an "add()" built-in function.
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3. Formally deprecate the obsolete construct in the Python
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documentation.
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4. Add an an optional warning mode to the parser that will inform
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users when the deprecated construct is used. In other words,
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all programs that will behave differently in the future must
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trigger warnings in this mode. Compile-time warnings are
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preferable to runtime warnings. The warning messages should
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steer people from the deprecated construct to the alternative
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construct.
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5. There must be at least a one-year transition period between the
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release of the transitional version of Python and the release
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of the backwards incompatible version. Users will have at
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least a year to test their programs and migrate them from use
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of the deprecated construct to the alternative one.
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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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