File: //usr/share/doc/python-docs-2.4.3/html/lib/module-future.html
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<link rel="STYLESHEET" href="lib.css" type='text/css' />
<link rel="SHORTCUT ICON" href="../icons/pyfav.png" type="image/png" />
<link rel='start' href='../index.html' title='Python Documentation Index' />
<link rel="first" href="lib.html" title='Python Library Reference' />
<link rel='contents' href='contents.html' title="Contents" />
<link rel='index' href='genindex.html' title='Index' />
<link rel='last' href='about.html' title='About this document...' />
<link rel='help' href='about.html' title='About this document...' />
<link rel="prev" href="module-main.html" />
<link rel="parent" href="python.html" />
<link rel="next" href="strings.html" />
<meta name='aesop' content='information' />
<title>3.34 __future__ -- Future statement definitions</title>
</head>
<body>
<DIV CLASS="navigation">
<div id='top-navigation-panel' xml:id='top-navigation-panel'>
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="3.33 __main__ "
href="module-main.html"><img src='../icons/previous.png'
border='0' height='32' alt='Previous Page' width='32' /></A></td>
<td class='online-navigation'><a rel="parent" title="3. Python Runtime Services"
href="python.html"><img src='../icons/up.png'
border='0' height='32' alt='Up One Level' width='32' /></A></td>
<td class='online-navigation'><a rel="next" title="4. String Services"
href="strings.html"><img src='../icons/next.png'
border='0' height='32' alt='Next Page' width='32' /></A></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
href="contents.html"><img src='../icons/contents.png'
border='0' height='32' alt='Contents' width='32' /></A></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
border='0' height='32' alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
href="genindex.html"><img src='../icons/index.png'
border='0' height='32' alt='Index' width='32' /></A></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="module-main.html">3.33 __main__ </A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="python.html">3. Python Runtime Services</A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="strings.html">4. String Services</A>
</div>
<hr /></div>
</DIV>
<!--End of Navigation Panel-->
<H1><A NAME="SECTION0053400000000000000000">
3.34 <tt class="module">__future__</tt> --
Future statement definitions</A>
</H1>
<P>
<A NAME="module-future"></A>
<P>
<tt class="module">__future__</tt> is a real module, and serves three purposes:
<P>
<UL>
<LI>To avoid confusing existing tools that analyze import statements
and expect to find the modules they're importing.
<P>
</LI>
<LI>To ensure that future_statements run under releases prior to 2.1
at least yield runtime exceptions (the import of
<tt class="module">__future__</tt> will fail, because there was no module of
that name prior to 2.1).
<P>
</LI>
<LI>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
<tt class="module">__future__</tt> and examining its contents.
<P>
</LI>
</UL>
<P>
Each statement in <span class="file">__future__.py</span> is of the form:
<P>
<div class="verbatim"><pre><TT>
FeatureName = "_Feature(" <var>OptionalRelease</var> "," <var>MandatoryRelease</var> ","
<var>CompilerFlag</var> ")"
</TT></pre></div>
<P>
where, normally, <var>OptionalRelease</var> is less than
<var>MandatoryRelease</var>, and both are 5-tuples of the same form as
<code>sys.version_info</code>:
<P>
<div class="verbatim"><pre>
(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
)
</pre></div>
<P>
<var>OptionalRelease</var> records the first release in which the feature
was accepted.
<P>
In the case of a <var>MandatoryRelease</var> that has not yet occurred,
<var>MandatoryRelease</var> predicts the release in which the feature will
become part of the language.
<P>
Else <var>MandatoryRelease</var> 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.
<P>
<var>MandatoryRelease</var> may also be <code>None</code>, meaning that a planned
feature got dropped.
<P>
Instances of class <tt class="class">_Feature</tt> have two corresponding methods,
<tt class="method">getOptionalRelease()</tt> and <tt class="method">getMandatoryRelease()</tt>.
<P>
<var>CompilerFlag</var> is the (bitfield) flag that should be passed in the
fourth argument to the builtin function <tt class="function">compile()</tt> to enable
the feature in dynamically compiled code. This flag is stored in the
<tt class="member">compiler_flag</tt> attribute on <tt class="class">_Feature</tt> instances.
<P>
No feature description will ever be deleted from <tt class="module">__future__</tt>.
<P>
<DIV CLASS="navigation">
<div class='online-navigation'>
<p></p><hr />
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="3.33 __main__ "
href="module-main.html"><img src='../icons/previous.png'
border='0' height='32' alt='Previous Page' width='32' /></A></td>
<td class='online-navigation'><a rel="parent" title="3. Python Runtime Services"
href="python.html"><img src='../icons/up.png'
border='0' height='32' alt='Up One Level' width='32' /></A></td>
<td class='online-navigation'><a rel="next" title="4. String Services"
href="strings.html"><img src='../icons/next.png'
border='0' height='32' alt='Next Page' width='32' /></A></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
href="contents.html"><img src='../icons/contents.png'
border='0' height='32' alt='Contents' width='32' /></A></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
border='0' height='32' alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
href="genindex.html"><img src='../icons/index.png'
border='0' height='32' alt='Index' width='32' /></A></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="module-main.html">3.33 __main__ </A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="python.html">3. Python Runtime Services</A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="strings.html">4. String Services</A>
</div>
</div>
<hr />
<span class="release-info">Release 2.4.3, documentation updated on 29 March 2006.</span>
</DIV>
<!--End of Navigation Panel-->
<ADDRESS>
See <i><a href="about.html">About this document...</a></i> for information on suggesting changes.
</ADDRESS>
</BODY>
</HTML>