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<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></a>

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<LI><A href="alt-install-windows.html#SECTION000310000000000000000">3.1 Alternate installation: the home scheme</a>
<LI><A href="alt-install-windows.html#SECTION000320000000000000000">3.2 Alternate installation: <span class="Unix">Unix</span> (the prefix scheme)</a>
<LI><A href="alt-install-windows.html#SECTION000330000000000000000">3.3 Alternate installation: Windows (the prefix scheme)</a>
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<H1><A NAME="SECTION000300000000000000000"></A>
<A NAME="alt-install"></A>
<BR>
3 Alternate Installation
</H1>

<P>
Often, it is necessary or desirable to install modules to a location
other than the standard location for third-party Python modules.  For
example, on a <span class="Unix">Unix</span> system you might not have permission to write to the
standard third-party module directory.  Or you might wish to try out a
module before making it a standard part of your local Python
installation.  This is especially true when upgrading a distribution
already present: you want to make sure your existing base of scripts
still works with the new version before actually upgrading.

<P>
The Distutils <code class="du-command">install</code> command is designed to make installing
module distributions to an alternate location simple and painless.  The
basic idea is that you supply a base directory for the installation, and
the <code class="du-command">install</code> command picks a set of directories (called an
<em>installation scheme</em>) under this base directory in which to
install files.  The details differ across platforms, so read whichever
of the following sections applies to you.

<P>

<H2><A NAME="SECTION000310000000000000000"></A>
<A NAME="alt-install-prefix"></A>
<BR>
3.1 Alternate installation: the home scheme
</H2>

<P>
The idea behind the ``home scheme'' is that you build and maintain a
personal stash of Python modules.  This scheme's name is derived from
the idea of a ``home'' directory on <span class="Unix">Unix</span>, since it's not unusual for
a <span class="Unix">Unix</span> user to make their home directory have a layout similar to
<span class="file">/usr/</span> or <span class="file">/usr/local/</span>.  This scheme can be used by
anyone, regardless of the operating system their installing for.

<P>
Installing a new module distribution is as simple as

<P>
<div class="verbatim"><pre>
python setup.py install --home=&lt;dir&gt;
</pre></div>

<P>
where you can supply any directory you like for the
<b class="programopt">--home</b> option.  On <span class="Unix">Unix</span>, lazy typists can just type a
tilde (<code>~</code>); the <code class="du-command">install</code> command will
expand this to your home directory:

<P>
<div class="verbatim"><pre>
python setup.py install --home=~
</pre></div>

<P>
The <b class="programopt">--home</b> option defines the installation base
directory.  Files are installed to the following directories under the
installation base as follows:
<div class="center"><table class="realtable">
  <thead>
    <tr>
      <th class="left"  >Type of file</th>
      <th class="left"  >Installation Directory</th>
      <th class="left"  >Override option</th>
      </tr>
    </thead>
  <tbody>
    <tr><td class="left"   valign="baseline">pure module distribution</td>
        <td class="left"  ><span class="du-filevar">home</span><span class="file">/lib/python</span></td>
        <td class="left"  ><b class="programopt">--install-purelib</b></td></tr>
    <tr><td class="left"   valign="baseline">non-pure module distribution</td>
        <td class="left"  ><span class="du-filevar">home</span><span class="file">/lib/python</span></td>
        <td class="left"  ><b class="programopt">--install-platlib</b></td></tr>
    <tr><td class="left"   valign="baseline">scripts</td>
        <td class="left"  ><span class="du-filevar">home</span><span class="file">/bin</span></td>
        <td class="left"  ><b class="programopt">--install-scripts</b></td></tr>
    <tr><td class="left"   valign="baseline">data</td>
        <td class="left"  ><span class="du-filevar">home</span><span class="file">/share</span></td>
        <td class="left"  ><b class="programopt">--install-data</b></td></tr></tbody>
</table></div>

<P>

<span class="versionnote">Changed in version 2.4:
The <b class="programopt">--home</b> option used to be supported
                only on <span class="Unix">Unix</span>.</span>

<P>

<H2><A NAME="SECTION000320000000000000000"></A>
<A NAME="alt-install-home"></A>
<BR>
3.2 Alternate installation: <span class="Unix">Unix</span> (the prefix scheme)
</H2>

<P>
The ``prefix scheme'' is useful when you wish to use one Python
installation to perform the build/install (i.e., to run the setup
script), but install modules into the third-party module directory of a
different Python installation (or something that looks like a different
Python installation).  If this sounds a trifle unusual, it is--that's
why the ``home scheme'' comes first.  However, there are at least two
known cases where the prefix scheme will be useful.

<P>
First, consider that many Linux distributions put Python in <span class="file">/usr</span>,
rather than the more traditional <span class="file">/usr/local</span>.  This is entirely
appropriate, since in those cases Python is part of ``the system''
rather than a local add-on.  However, if you are installing Python
modules from source, you probably want them to go in
<span class="file">/usr/local/lib/python2.<span class="du-filevar">X</span></span> rather than
<span class="file">/usr/lib/python2.<span class="du-filevar">X</span></span>.  This can be done with

<P>
<div class="verbatim"><pre>
/usr/bin/python setup.py install --prefix=/usr/local
</pre></div>

<P>
Another possibility is a network filesystem where the name used to write
to a remote directory is different from the name used to read it: for
example, the Python interpreter accessed as <span class="file">/usr/local/bin/python</span>
might search for modules in <span class="file">/usr/local/lib/python2.<span class="du-filevar">X</span></span>,
but those modules would have to be installed to, say,
<span class="file">/mnt/<span class="du-filevar">@server</span>/export/lib/python2.<span class="du-filevar">X</span></span>.  This
could be done with

<P>
<div class="verbatim"><pre>
/usr/local/bin/python setup.py install --prefix=/mnt/@server/export
</pre></div>

<P>
In either case, the <b class="programopt">--prefix</b> option defines the
installation base, and the <b class="programopt">--exec-prefix</b> option defines
the platform-specific installation base, which is used for
platform-specific files.  (Currently, this just means non-pure module
distributions, but could be expanded to C libraries, binary executables,
etc.)  If <b class="programopt">--exec-prefix</b> is not supplied, it defaults to
<b class="programopt">--prefix</b>.  Files are installed as follows:

<P>
<div class="center"><table class="realtable">
  <thead>
    <tr>
      <th class="left"  >Type of file</th>
      <th class="left"  >Installation Directory</th>
      <th class="left"  >Override option</th>
      </tr>
    </thead>
  <tbody>
    <tr><td class="left"   valign="baseline">pure module distribution</td>
        <td class="left"  ><span class="du-filevar">prefix</span><span class="file">/lib/python2.<span class="du-filevar">X</span>/site-packages</span></td>
        <td class="left"  ><b class="programopt">--install-purelib</b></td></tr>
    <tr><td class="left"   valign="baseline">non-pure module distribution</td>
        <td class="left"  ><span class="du-filevar">exec-prefix</span><span class="file">/lib/python2.<span class="du-filevar">X</span>/site-packages</span></td>
        <td class="left"  ><b class="programopt">--install-platlib</b></td></tr>
    <tr><td class="left"   valign="baseline">scripts</td>
        <td class="left"  ><span class="du-filevar">prefix</span><span class="file">/bin</span></td>
        <td class="left"  ><b class="programopt">--install-scripts</b></td></tr>
    <tr><td class="left"   valign="baseline">data</td>
        <td class="left"  ><span class="du-filevar">prefix</span><span class="file">/share</span></td>
        <td class="left"  ><b class="programopt">--install-data</b></td></tr></tbody>
</table></div>

<P>
There is no requirement that <b class="programopt">--prefix</b> or
<b class="programopt">--exec-prefix</b> actually point to an alternate Python
installation; if the directories listed above do not already exist, they
are created at installation time.

<P>
Incidentally, the real reason the prefix scheme is important is simply
that a standard <span class="Unix">Unix</span> installation uses the prefix scheme, but with
<b class="programopt">--prefix</b> and <b class="programopt">--exec-prefix</b> supplied by
Python itself as <code>sys.prefix</code> and <code>sys.exec_prefix</code>.  Thus,
you might think you'll never use the prefix scheme, but every time you
run <code>python setup.py install</code> without any other options, you're
using it.

<P>
Note that installing extensions to an alternate Python installation has
no effect on how those extensions are built: in particular, the Python
header files (<span class="file">Python.h</span> and friends) installed with the Python
interpreter used to run the setup script will be used in compiling
extensions.  It is your responsibility to ensure that the interpreter
used to run extensions installed in this way is compatible with the
interpreter used to build them.  The best way to do this is to ensure
that the two interpreters are the same version of Python (possibly
different builds, or possibly copies of the same build).  (Of course, if
your <b class="programopt">--prefix</b> and <b class="programopt">--exec-prefix</b> don't even
point to an alternate Python installation, this is immaterial.)

<P>

<H2><A NAME="SECTION000330000000000000000"></A>
<A NAME="alt-install-windows"></A>
<BR>
3.3 Alternate installation: Windows (the prefix scheme)
</H2>

<P>
Windows has no concept of a user's home directory, and since the
standard Python installation under Windows is simpler than under
<span class="Unix">Unix</span>, the <b class="programopt">--prefix</b> option has traditionally been used
to install additional packages in separate locations on Windows.

<P>
<div class="verbatim"><pre>
python setup.py install --prefix="\Temp\Python"
</pre></div>

<P>
to install modules to the
<span class="file">&#92;Temp&#92;Python</span> directory on the
current drive.

<P>
The installation base is defined by the <b class="programopt">--prefix</b> option;
the <b class="programopt">--exec-prefix</b> option is not supported under Windows.
Files are installed as follows:
<div class="center"><table class="realtable">
  <thead>
    <tr>
      <th class="left"  >Type of file</th>
      <th class="left"  >Installation Directory</th>
      <th class="left"  >Override option</th>
      </tr>
    </thead>
  <tbody>
    <tr><td class="left"   valign="baseline">pure module distribution</td>
        <td class="left"  ><span class="du-filevar">prefix</span><span class="file"></span></td>
        <td class="left"  ><b class="programopt">--install-purelib</b></td></tr>
    <tr><td class="left"   valign="baseline">non-pure module distribution</td>
        <td class="left"  ><span class="du-filevar">prefix</span><span class="file"></span></td>
        <td class="left"  ><b class="programopt">--install-platlib</b></td></tr>
    <tr><td class="left"   valign="baseline">scripts</td>
        <td class="left"  ><span class="du-filevar">prefix</span><span class="file">&#92;Scripts</span></td>
        <td class="left"  ><b class="programopt">--install-scripts</b></td></tr>
    <tr><td class="left"   valign="baseline">data</td>
        <td class="left"  ><span class="du-filevar">prefix</span><span class="file">&#92;Data</span></td>
        <td class="left"  ><b class="programopt">--install-data</b></td></tr></tbody>
</table></div>

<P>

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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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