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annotate Doc/lib/libfuture.tex @ 33091:48100a01ac7c 2.3

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author Martin v. Löwis <martin@v.loewis.de>
date Thu, 12 Aug 2004 13:14:20 +0000
parents 5b1ddc4b49b9
children 562089a8678f
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949f41829f36 Rename lib__future__.tex to libfuture.tex and add link in lib.tex.
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1 \section{\module{__future__} ---
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2 Future statement definitions}
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4 \declaremodule[future]{standard}{__future__}
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5 \modulesynopsis{Future statement definitions}
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7 \module{__future__} is a real module, and serves three purposes:
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9 \begin{itemize}
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11 \item To avoid confusing existing tools that analyze import statements
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12 and expect to find the modules they're importing.
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14 \item To ensure that future_statements run under releases prior to 2.1
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15 at least yield runtime exceptions (the import of
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16 \module{__future__} will fail, because there was no module of
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17 that name prior to 2.1).
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19 \item To document when incompatible changes were introduced, and when they
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20 will be --- or were --- made mandatory. This is a form of executable
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21 documentation, and can be inspected programatically via importing
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22 \module{__future__} and examining its contents.
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24 \end{itemize}
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25
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26 Each statement in \file{__future__.py} is of the form:
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28 \begin{verbatim}
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29 FeatureName = "_Feature(" OptionalRelease "," MandatoryRelease ","
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30 CompilerFlag ")"
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31 \end{verbatim}
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33 where, normally, OptionalRelease is less then MandatoryRelease, and
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34 both are 5-tuples of the same form as \code{sys.version_info}:
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36 \begin{verbatim}
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37 (PY_MAJOR_VERSION, # the 2 in 2.1.0a3; an int
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38 PY_MINOR_VERSION, # the 1; an int
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39 PY_MICRO_VERSION, # the 0; an int
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40 PY_RELEASE_LEVEL, # "alpha", "beta", "candidate" or "final"; string
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41 PY_RELEASE_SERIAL # the 3; an int
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42 )
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43 \end{verbatim}
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45 OptionalRelease records the first release in which the feature was
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46 accepted.
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48 In the case of MandatoryReleases that have not yet occurred,
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49 MandatoryRelease predicts the release in which the feature will become
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50 part of the language.
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52 Else MandatoryRelease records when the feature became part of the
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53 language; in releases at or after that, modules no longer need a
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54 future statement to use the feature in question, but may continue to
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55 use such imports.
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57 MandatoryRelease may also be \code{None}, meaning that a planned
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58 feature got dropped.
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60 Instances of class \class{_Feature} have two corresponding methods,
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61 \method{getOptionalRelease()} and \method{getMandatoryRelease()}.
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63 CompilerFlag is the (bitfield) flag that should be passed in the
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64 fourth argument to the builtin function \function{compile()} to enable
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65 the feature in dynamically compiled code. This flag is stored in the
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66 \member{compiler_flag} attribute on \class{_Future} instances.
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68 No feature description will ever be deleted from \module{__future__}.
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69