summaryrefslogtreecommitdiffstats
path: root/Doc/lib/libfuture.tex
blob: a37459181ac34ce96f37c0d190d2ae73b38d51e0 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
\section{\module{__future__} ---
         Future statement definitions}

\declaremodule[future]{standard}{__future__}
\modulesynopsis{Future statement definitions}

\module{__future__} is a real module, and serves three purposes:

\begin{itemize}

\item To avoid confusing existing tools that analyze import statements
      and expect to find the modules they're importing.

\item To ensure that future_statements run under releases prior to 2.1
      at least yield runtime exceptions (the import of
      \module{__future__} will fail, because there was no module of
      that name prior to 2.1). 

\item To document when incompatible changes were introduced, and when they
      will be --- or were --- made mandatory.  This is a form of executable
      documentation, and can be inspected programatically via importing
      \module{__future__} and examining its contents.

\end{itemize}

Each statement in \file{__future__.py} is of the form:

\begin{alltt}
FeatureName = "_Feature(" \var{OptionalRelease} "," \var{MandatoryRelease} ","
                        \var{CompilerFlag} ")"
\end{alltt}

where, normally, \var{OptionalRelease} is less than
\var{MandatoryRelease}, and both are 5-tuples of the same form as
\code{sys.version_info}:

\begin{verbatim}
    (PY_MAJOR_VERSION, # the 2 in 2.1.0a3; an int
     PY_MINOR_VERSION, # the 1; an int
     PY_MICRO_VERSION, # the 0; an int
     PY_RELEASE_LEVEL, # "alpha", "beta", "candidate" or "final"; string
     PY_RELEASE_SERIAL # the 3; an int
    )
\end{verbatim}

\var{OptionalRelease} records the first release in which the feature
was accepted.

In the case of a \var{MandatoryRelease} that has not yet occurred,
\var{MandatoryRelease} predicts the release in which the feature will
become part of the language.

Else \var{MandatoryRelease} records when the feature became part of
the language; in releases at or after that, modules no longer need a
future statement to use the feature in question, but may continue to
use such imports.

\var{MandatoryRelease} may also be \code{None}, meaning that a planned
feature got dropped.

Instances of class \class{_Feature} have two corresponding methods,
\method{getOptionalRelease()} and \method{getMandatoryRelease()}.

\var{CompilerFlag} is the (bitfield) flag that should be passed in the
fourth argument to the builtin function \function{compile()} to enable
the feature in dynamically compiled code.  This flag is stored in the
\member{compiler_flag} attribute on \class{_Future} instances.

No feature description will ever be deleted from \module{__future__}.