The specified installation directories are relative to

\filevar{prefix}. Of course, you also have to ensure that these

directories are in Python's module search path, such as by putting a

\file{.pth} file in \filevar{prefix}. See section~\ref{search-path}

to find out how to modify Python's search path.

\subsection{Modifying Python's Search Path}

\label{search-path}

When the Python interpreter executes an \keyword{import} statement, it

searches for both Python code and extension modules along a search

path. A default value for the path is configured into the Python

binary when the interpreter is built. You can determine the path by

importing the \module{sys} module and printing the value of

\code{sys.path}.

\begin{verbatim}

\end{verbatim}

The null string in \code{sys.path} represents the current working

directory.

The expected convention for locally installed packages is to put them

in the \file{.../site-packages/} directory, but you may want to

install Python modules into some arbitrary directory. For example,

your site may have a convention of keeping all software related to the

web server under \file{/www}. Add-on Python modules might then belong

in \file{/www/python}, and in order to import them, this directory

must be added to \code{sys.path}. There are several different ways to

add the directory.

The most convenient way is to add a path configuration file to a

directory that's already on Python's path, usually to the

\file{.../site-packages/} directory. Path configuration files have an

extension of \file{.pth}, and each line must contain a single path

that will be added to \code{sys.path}. Paths can be absolute or

relative, in which case they're relative to the directory containing

the \file{.pth} file. Any directories added to the search path will

be scanned in turn for \file{.pth} files. See

\citetitle[http://www.python.org/dev/doc/devel/lib/module-site.html]{the

documentation for the \module{site} module} for more information.

A slightly less convenient way is to edit the \file{site.py} file in

Python's standard library, and modify \code{sys.path}. \file{site.py}

is automatically imported when the Python interpreter is executed,

unless the \programopt{-S} switch is supplied to suppress this

behaviour. So you could simply edit \file{site.py} and add two lines to it:

\begin{verbatim}

\end{verbatim}

However, if you reinstall the same major version of Python (perhaps

when upgrading from 2.2 to 2.2.2, for example) \file{site.py} will be

overwritten by the stock version. You'd have to remember that it was

modified and save a copy before doing the installation.

There are two environment variables that can modify \code{sys.path}.

\envvar{PYTHONHOME} sets an alternate value for the prefix of the

Python installation. For example, if \envvar{PYTHONHOME} is set to

\samp{/www/python}, the search path will be set to \code{['',

'/www/python/lib/python2.2/', '/www/python/lib/python2.3/plat-linux2',

...]}.

The \envvar{PYTHONPATH} variable can be set to a list of paths that

will be added to the beginning of \code{sys.path}. For example, if

\envvar{PYTHONPATH} is set to \samp{/www/python:/opt/py}, the search

path will begin with \code{['/www/python', '/opt/py']}. (Note that

directories must exist in order to be added to \code{sys.path}; the

\module{site} module removes paths that don't exist.)

Finally, \code{sys.path} is just a regular Python list, so any Python

application can modify it by adding or removing entries.