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<!--
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-->
<para>
&SCons; supports a lot of additional functionality
that doesn't readily fit into the other chapters.
</para>
<section>
<title>Verifying the Python Version: the &EnsurePythonVersion; Function</title>
<para>
Although the &SCons; code itself will run
on any Python version 1.5.2 or later,
you are perfectly free to make use of
Python syntax and modules from more modern versions
(for example, Python 2.4 or 2.5)
when writing your &SConscript; files
or your own local modules.
If you do this, it's usually helpful to
configure &SCons; to exit gracefully with an error message
if it's being run with a version of Python
that simply won't work with your code.
This is especially true if you're going to use &SCons;
to build source code that you plan to distribute publicly,
where you can't be sure of the Python version
that an anonymous remote user might use
to try to build your software.
</para>
<para>
&SCons; provides an &EnsurePythonVersion; function for this.
You simply pass it the major and minor versions
numbers of the version of Python you require:
</para>
<!--
TODO: Figure out how to generate the error message
regardless of executing Python version by faking out
the infrastructure in some way.
<scons_example name="EnsurePythonVersion">
<file name="SConstruct" printme="1">
EnsurePythonVersion(2, 5)
</file>
</scons_example>
-->
<sconstruct>
EnsurePythonVersion(2, 5)
</sconstruct>
<para>
And then &SCons will exit with the following error
message when a user runs it with an unsupported
earlier version of Python:
</para>
<!--
TODO: Figure out how to generate the error message
regardless of executing Python version by faking out
the infrastructure in some way.
<scons_output example="EnsurePythonVersion">
<scons_output_command>scons -Q</scons_output_command>
</scons_output>
-->
<screen>
% <userinput>scons -Q</userinput>
Python 2.5 or greater required, but you have Python 2.3.6
</screen>
</section>
<section>
<title>Verifying the SCons Version: the &EnsureSConsVersion; Function</title>
<para>
You may, of course, write your &SConscript; files
to use features that were only added in
recent versions of &SCons;.
When you publicly distribute software that is built using &SCons;,
it's helpful to have &SCons;
verify the version being used and
exit gracefully with an error message
if the user's version of &SCons; won't work
with your &SConscript; files.
&SCons; provides an &EnsureSConsVersion; function
that verifies the version of &SCons;
in the same
the &EnsurePythonVersion; function
verifies the version of Python,
by passing in the major and minor versions
numbers of the version of SCons you require:
</para>
<!--
TODO: Figure out how to generate the error message
regardless of executing SCons version by faking out
the infrastructure in some way.
<scons_example name="EnsureSConsVersion">
<file name="SConstruct" printme="1">
EnsureSConsVersion(1, 0)
</file>
</scons_example>
-->
<sconstruct>
EnsureSConsVersion(1, 0)
</sconstruct>
<para>
And then &SCons will exit with the following error
message when a user runs it with an unsupported
earlier version of &SCons;:
</para>
<!--
TODO: Figure out how to generate the error message
regardless of executing SCons version by faking out
the infrastructure in some way.
<scons_output example="EnsureSConsVersion">
<scons_output_command>scons -Q</scons_output_command>
</scons_output>
-->
<screen>
% <userinput>scons -Q</userinput>
SCons 1.0 or greater required, but you have SCons 0.98.5
</screen>
</section>
<section>
<title>Explicitly Terminating &SCons; While Reading &SConscript; Files: the &Exit; Function</title>
<para>
&SCons; supports an &Exit; function
which can be used to terminate &SCons;
while reading the &SConscript; files,
usually because you've detected a condition
under which it doesn't make sense to proceed:
</para>
<scons_example name="Exit">
<file name="SConstruct" printme="1">
if ARGUMENTS.get('FUTURE'):
print "The FUTURE option is not supported yet!"
Exit(2)
env = Environment()
env.Program('hello.c')
</file>
<file name="hello.c">
hello.c
</file>
</scons_example>
<scons_output example="Exit">
<scons_output_command>scons -Q FUTURE=1</scons_output_command>
<scons_output_command>scons -Q</scons_output_command>
</scons_output>
<para>
The &Exit; function takes as an argument
the (numeric) exit status that you want &SCons; to exit with.
If you don't specify a value,
the default is to exit with <literal>0</literal>,
which indicates successful execution.
</para>
<para>
Note that the &Exit; function
is equivalent to calling the Python
<function>sys.exit</function> function
(which the it actually calls),
but because &Exit; is a &SCons; function,
you don't have to import the Python
<literal>sys</literal> module to use it.
</para>
</section>
<section>
<title>Handling Nested Lists: the &Flatten; Function</title>
<para>
&SCons; supports a &Flatten; function
which takes an input Python sequence
(list or tuple)
and returns a flattened list
containing just the individual elements of
the sequence.
This can be handy when trying to examine
a list composed of the lists
returned by calls to various Builders.
For example, you might collect
object files built in different ways
into one call to the &Program; Builder
by just enclosing them in a list, as follows:
</para>
<scons_example name="Flatten1">
<file name="SConstruct" printme="1">
objects = [
Object('prog1.c'),
Object('prog2.c', CCFLAGS='-DFOO'),
]
Program(objects)
</file>
<file name="prog1.c">
prog1.c
</file>
<file name="prog2.c">
prog2.c
</file>
</scons_example>
<para>
Because the Builder calls in &SCons;
flatten their input lists,
this works just fine to build the program:
</para>
<scons_output example="Flatten1">
<scons_output_command>scons -Q</scons_output_command>
</scons_output>
<para>
But if you were debugging your build
and wanted to print the absolute path
of each object file in the
<varname>objects</varname> list,
you might try the following simple approach,
trying to print each Node's
<literal>abspath</literal>
attribute:
</para>
<scons_example name="Flatten2">
<file name="SConstruct" printme="1">
objects = [
Object('prog1.c'),
Object('prog2.c', CCFLAGS='-DFOO'),
]
Program(objects)
for object_file in objects:
print object_file.abspath
</file>
<file name="prog1.c">
prog1.c
</file>
<file name="prog2.c">
prog2.c
</file>
</scons_example>
<para>
This does not work as expected
because each call to <function>str</function>
is operating an embedded list returned by
each &Object; call,
not on the underlying Nodes within those lists:
</para>
<scons_output example="Flatten2">
<scons_output_command>scons -Q</scons_output_command>
</scons_output>
<para>
The solution is to use the &Flatten; function
so that you can pass each Node to
the <function>str</function> separately:
</para>
<scons_example name="Flatten3">
<file name="SConstruct" printme="1">
objects = [
Object('prog1.c'),
Object('prog2.c', CCFLAGS='-DFOO'),
]
Program(objects)
for object_file in Flatten(objects):
print object_file.abspath
</file>
<file name="prog1.c">
prog1.c
</file>
<file name="prog2.c">
prog2.c
</file>
</scons_example>
<!--
TODO: can't use this now because it displays the temporary path name
<scons_output example="Flatten3">
<scons_output_command>scons -Q</scons_output_command>
</scons_output>
-->
<screen>
% <userinput>scons -Q</userinput>
/home/me/project/prog1.o
/home/me/project/prog2.o
cc -o prog1.o -c prog1.c
cc -o prog2.o -c -DFOO prog2.c
cc -o prog1 prog1.o prog2.o
</screen>
</section>
<section>
<title>Finding the Invocation Directory: the &GetLaunchDir; Function</title>
<para>
If you need to find the directory from
which the user invoked the &scons; command,
you can use the &GetLaunchDir; function:
</para>
<sconstruct>
env = Environment(
LAUNCHDIR = GetLaunchDir(),
)
env.Command('directory_build_info',
'$LAUNCHDIR/build_info'
Copy('$TARGET', '$SOURCE'))
</sconstruct>
<para>
Because &SCons; is usually invoked from the top-level
directory in which the &SConstruct; file lives,
the Python <function>os.getcwd()</function>
is often equivalent.
However, the &SCons;
<literal>-u</literal>,
<literal>-U</literal>
and
<literal>-D</literal>
command-line options,
when invoked from a subdirectory,
will cause &SCons; to change to the directory
in which the &SConstruct; file is found.
When those options are used,
&GetLaunchDir; will still return the path to the
user's invoking subdirectory,
allowing the &SConscript; configuration
to still get at configuration (or other) files
from the originating directory.
</para>
</section>
|