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<chapter id="chap-build-install"
         xmlns="http://www.scons.org/dbxsd/v1.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://www.scons.org/dbxsd/v1.0 http://www.scons.org/dbxsd/v1.0/scons.xsd">

<title>Building and Installing &SCons;</title>

  <para>

  This chapter will take you through the basic steps
  of installing &SCons; so you can use it for your projects.
  Before that, however, this chapter will also describe the basic steps
  involved in installing &Python; on your system,
  in case that is necessary.
  Fortunately, both &SCons; and &Python;
  are easy to install on almost any system,
  and &Python; already comes installed on many systems.

  </para>

  <!--

  <para>

  Lastly, this chapter also contains a section that
  provides a brief overview of the &Python; programming language,
  which is the language used to implement &SCons;,
  and which forms the basis of the &SCons; configuration files.
  Becoming familiar with some &Python; concepts will make it easier
  to understand many of the examples in this User's Guide.
  Nevertheless, it <emphasis>is</emphasis> possible
  to configure simple &SCons; builds without knowing &Python;,
  so you can skip this section if you
  want to dive in and pick up things
  by example- -or, of course, if you are
  already familiar with &Python;.

  </para>

  -->

  <section id="sect-install-python">
  <title>Installing Python</title>

    <para>
    Because &SCons; is written in the &Python; programming language,
    you need to have a &Python; interpreter available on your system
    to use &SCons;.
    Before you try to install &Python;,
    check to see if &Python; is already
    available on your system  by typing
    <userinput>python -V</userinput>
    (capital 'V')
    or
    <userinput>python --version</userinput>
    at your system's command-line prompt.
    For Linux/Unix/MacOS/BSD type systems this looks like:
    </para>

    <screen>
$ <userinput>python -V</userinput>
Python 3.9.15
    </screen>

    <para>
    If you get a version like 2.7.x, you may need to try using the
    name <command>python3</command> - current &SCons; no longer
    works with &Python; 2.
    </para>

    <para>
    Note to Windows users: there are a number of different ways &Python;
    can be installed or invoked on Windows, it is beyond the scope
    of this guide to unravel all of them. Some have an additional
    program called the <firstterm>Python launcher</firstterm> (described,
    somewhat technically, in
    <ulink url="https://www.python.org/dev/peps/pep-0397/">PEP 397</ulink>):
    try using the command name <command>py</command> instead of
    <command>python</command>, if that is not available drop
    back to trying <command>python</command>
    </para>

    <screen>
C:\><userinput>py -V</userinput>
Python 3.9.15
    </screen>

    <para>
    If &Python; is not installed on your system,
    or is not findable in the current search path,
    you will see an error message
    stating something like <computeroutput>"command not found"</computeroutput>
    (on UNIX or Linux)
    or <computeroutput>"'python' is not recognized as an internal
    or external command, operable program or batch file"</computeroutput>
    (on Windows <command>cmd</command>).
    In that case, you need to either install &Python;
    or fix the search path
    before you can install &SCons;.
    </para>

    <para>
    The link for downloading &Python; installers (Windows and Mac)
    from the project's own website is:
    <ulink url="https://www.python.org/download">https://www.python.org/download</ulink>.
    There are useful system-specific entries on setup and
    usage to be found at:
    <ulink url="https://docs.python.org/3/using">https://docs.python.org/3/using</ulink>
    </para>

    <para>
    For Linux systems, &Python; is
    almost certainly available as a supported package, probably
    installed by default; this is often preferred over installing
    by other means as the system package will be built with
    carefully chosen optimizations, and will be kept up to date
    with bug fixes and security patches. In fact, the &Python;
    project itself does not build installers for Linux for this reason.
    Many such systems have separate packages for
    &Python; 2 and &Python; 3 - make sure the &Python; 3 package is
    installed, as the latest &SCons; requires it.
    Building from source may still be a
    useful option if you need a specific version that is not offered by
    the distribution you are using.
    </para>

    <para>
    Recent versions of the Mac no longer come with &Python;
    pre-installed; older versions came with a rather out-of-date
    version (based on &Python; 2.7) which is insufficient to run
    current &SCons;.
    The python.org installer can be used on the Mac, but there are
    also other sources such as MacPorts and Homebrew.
    The Anaconda installation also comes with a bundled &Python;.
    </para>

    <para>
    Windows has even more choices.  The Python.org installer is
    a traditional <filename>.exe</filename> style;
    the same software is also released as a Windows application through
    the Microsoft Store.  Several alternative builds also exist
    such as Chocolatey and ActiveState, and, again,
    a version of Python comes with Anaconda.
    </para>

    <para>
    &SCons; will work with &Python; 3.7 or later.
    If you need to install &Python; and have a choice,
    we recommend using the most recent &Python; version available.
    Newer &Python; versions have significant improvements
    that help speed up the performance of &SCons;.
    </para>

  </section>

  <section id="sect-install-scons">
  <title>Installing &SCons;</title>

    <para>
    The recommended way to install &SCons; is from the &Python; Package
    Index (<ulink url="https://pypi.org/project/SCons/">PyPI</ulink>):
    </para>

    <screen>
% <userinput>python -m pip install scons</userinput>
    </screen>

    <para>
    If you prefer not to install to the &Python; system location,
    or do not have privileges to do so, you can add a flag to install
    to a location specific to your own account and &Python; version:
    </para>

    <screen>
% <userinput>python -m pip install --user scons</userinput>
    </screen>

    <para>
    For those users using Anaconda or Miniconda, use the
    <command>conda</command> installer instead, so the &scons;
    install location will match the version of &Python; that
    system will be using. For example:
    </para>

    <screen>
% <userinput>conda install -c conda-forge scons</userinput>
    </screen>

    <para>
    If you need a specific
    version of &SCons; that is different from the current version,
    <systemitem>pip</systemitem> has a version option
    (e.g. <userinput>python -m pip install scons==3.1.2</userinput>),
    or you can follow the instructions in the following sections.
    </para>

    <para>
    &SCons; does comes pre-packaged for installation on many Linux systems.
    Check your package installation system
    to see if there is an up-to-date &SCons; package available.
    Many people prefer to install distribution-native packages if available,
    as they provide a central point for management and updating;
    however not all distributions update in a timely fashion.
    During the still-ongoing &Python; 2 to 3 transition,
    some distributions may still have two &SCons; packages available,
    one which uses &Python; 2 and one which uses &Python; 3.  Since
    the latest &scons; only runs on &Python; 3, to get the current version
    you should choose the &Python; 3 package.
    </para>

  </section>

  <section id="sect-scons-no-install">
  <title>Using &SCons; Without Installing</title>

    <para>
    You don't actually need to "install" &SCons; to use it.
    Nor do you need to "build" it, unless you are interested in
    producing the &SCons; documentation, which does use several
    tools to produce HTML, PDF and other output formats from
    files in the source tree.
    All you need to do is
    call the <filename>scons.py</filename> driver script in a
    location that contains an &SCons; tree, and it will figure out
    the rest. You can test that like this:
    </para>

    <screen>
$ <userinput>python <replaceable>/path/to/unpacked</replaceable>/scripts/scons.py --version</userinput>
    </screen>

    <para>
    To make use of an uninstalled &SCons;,
    the first step is to download either the
    <filename>scons-&buildversion;.tar.gz</filename>
    or <filename>scons-&buildversion;.zip</filename>,
    which are available from the SCons download page at
    <ulink url="https://scons.org/pages/download.html">https://scons.org/pages/download.html</ulink>.
    There is also a <literal>scons-local</literal> bundle you can make
    use of.  It is arranged a little bit differently, with the idea
    that you can include it with your own project if you want people
    to be able to do builds without having to download or install &SCons;.
    Finally, you can also use a checkout of the git tree from GitHub
    at a location to point to.
    </para>

    <para>
    Unpack the archive you downloaded,
    using a utility like <application>tar</application>
    on Linux or UNIX,
    or <application>WinZip</application> on Windows.
    This will create a directory called
    <filename>scons-&buildversion;</filename>,
    usually in your local directory.  The driver script
    will be in a subdirectory named <filename>scripts</filename>,
    unless you are using <literal>scons-local</literal>,
    in which case it will be in the top directory.
    Now you only need to call <filename>scons.py</filename> by
    giving a full or relative path to it in order to use that
    &SCons; version.
    </para>

    <para>
    Note that instructions for older versions may have suggested
    running <userinput>python setup.py install</userinput> to
    "build and install" &SCons;. This is no longer recommended
    (in fact, it is not recommended by the wider &Python; packaging
    community for <emphasis>any</emphasis> end-user installations
    of &Python; software). There is a <filename>setup.py</filename> file,
    but it is only tested and used for the automated procedure which
    prepares  an &SCons; bundle for making a release on PyPI,
    and even that is not guaranteed to work in the future.
    </para>

  </section>

  <section id="sect-install-scons-multi">
  <title>Running Multiple Versions of &SCons; Side-by-Side</title>

      <para>
      In some cases you may need several versions of &SCons;
      present on a system at the same time - perhaps you have
      an older project to build that has not yet been "ported"
      to a newer &SCons; version, or maybe you want to test a
      new &SCons; release side-by-side with a previous one
      before switching over.
      The use of an "uninstalled" package as described in the
      previous section can be of use for this purpose.
      </para>

      <para>
      Another approach to multiple versions is to create
      &Python; virtualenvs, and install different &SCons; versions in each.
      A Python <firstterm>virtual environment</firstterm>
      is a directory with an isolated set of Python packages,
      where packages you install/upgrade/remove inside the
      environment do not affect anything outside it,
      and those you install/upgrade/remove outside of it
      do not affect anything inside it.
      In other words, anything you do with <command>pip</command>
      in the environment stays in that environment.
      The &Python; standard library provides a module called
      <systemitem>venv</systemitem> for creating these
      (<ulink url="https://docs.python.org/e/library/venv.html"/>),
      although there are also other tools which provide more precise
      control of the setup.
      </para>

      <para>
      Using a virtualenv can be useful even for a single version of
      &SCons;, to gain the advantages of having an isolated environment.
      It also gets around the problem of not having administrative
      privileges on a particular system to install a distribution
      package or use <command>pip</command> to install to a
      system location, as the virtualenv is completely under your control.
      </para>

      <para>
      The following outline shows how this could be set up
      on a Linux/POSIX system (the syntax will be a bit different
      on Windows):
      </para>

      <screen>
$ <emphasis>create virtualenv named scons3</emphasis>
$ <emphasis>create virtualenv named scons4</emphasis>
$ <userinput>source scons3/bin/activate</userinput>
$ <userinput>pip install scons==3.1.2</userinput>
$ <userinput>deactivate</userinput>
$ <userinput>source scons4/bin/activate</userinput>
$ <userinput>pip install scons</userinput>
$ <userinput>deactivate</userinput>
$ <emphasis>activate a virtualenv and run 'scons' to use that version</emphasis>
      </screen>

  </section>

  <!--

  <section id="sect-python-basics">
  <title>Python Basics</title>

    <para>

    This section will provide a brief overview of
    the Python programming language.
    Skip this section if you are already familiar with Python
    (or you're really intent on diving into &SCons;
    and just picking up things as you go).

    </para>

    <para>

    Python has a lot of good
    documentation freely available on-line
    to help you get started.
    The standard tutorial is available at XXX.


    </para>

    <para>

    Python is very easy to pick up.

    </para>

    <para>

    Python variables must be assigned to before they can be referenced.

    </para>

    <para>

    Assignment is like most programming languages:

    x = 1 + 2
    z = 3 * x

    </para>

    <para>

    Function calls look like most language function calls:

    a = f(g)

    </para>

    <para>

    Define functions like so:

        def func(arg1, arg2):
            return arg1 * arg 2

    The number of parameters

    </para>

    <para>

    Strings can be enclosed in single quotes or double quotes,
    backslashes are used to escape characters,
    triple-quote syntax lets you include quotes and newlines,
    raw strings begin with 'r'.

    </para>

    <para>

    Lists are enclosed in square brackets,
    list items are separated by commas.
    List references use square brackets and integer index values,
    slice notation lets you select, delete or replace a range.

    </para>

    <para>

    Dictionaries (hashes) are enclosed in curly brackets,
    : separates keys from values,
    , separates items.
    Dictionary values are referenced using square brackets.

    </para>

    <para>

    Access class attributes (including methods) using a '.'.

    </para>

    <para>

    if: statements look like

    elif: statements look like

    else: statements look like

    </para>

    <para>

    for: statements look like

    while: statements look like

    break statements look like

    continue statements look like

    </para>

    <para>

    pass

    </para>

  </section>

  -->

</chapter>
