7.5. Modifying a Construction Environment

7.5.1. Replacing Values in a Construction Environment

You can replace existing construction variable values using the Replace method:

        env = Environment(CCFLAGS = '-DDEFINE1')
        env.Replace(CCFLAGS = '-DDEFINE2')
        env.Program('foo.c')
     

The replacing value (-DDEFINE2 in the above example) completely replaces the value in the construction environment:

        % scons -Q
        cc -o foo.o -c -DDEFINE2 foo.c
        cc -o foo foo.o
     

You can safely call Replace for construction variables that don't exist in the construction environment:

        env = Environment()
        env.Replace(NEW_VARIABLE = 'xyzzy')
        print "NEW_VARIABLE =", env['NEW_VARIABLE']
     

In this case, the construction variable simply gets added to the construction environment:

        % scons -Q
        NEW_VARIABLE = xyzzy
        scons: `.' is up to date.
     

Because the variables aren't expanded until the construction environment is actually used to build the targets, and because SCons function and method calls are order-independent, the last replacement "wins" and is used to build all targets, regardless of the order in which the calls to Replace() are interspersed with calls to builder methods:

        env = Environment(CCFLAGS = '-DDEFINE1')
        print "CCFLAGS =", env['CCFLAGS']
        env.Program('foo.c')

        env.Replace(CCFLAGS = '-DDEFINE2')
        print "CCFLAGS =", env['CCFLAGS']
        env.Program('bar.c')
     

The timing of when the replacement actually occurs relative to when the targets get built becomes apparent if we run scons without the -Q option:

        % scons
        scons: Reading SConscript files ...
        CCFLAGS = -DDEFINE1
        CCFLAGS = -DDEFINE2
        scons: done reading SConscript files.
        scons: Building targets ...
        cc -o bar.o -c -DDEFINE2 bar.c
        cc -o bar bar.o
        cc -o foo.o -c -DDEFINE2 foo.c
        cc -o foo foo.o
        scons: done building targets.
     

Because the replacement occurs while the SConscript files are being read, the $CCFLAGS variable has already been set to -DDEFINE2 by the time the foo.o target is built, even though the call to the Replace method does not occur until later in the SConscript file.

7.5.2. Appending to the End of Values in a Construction Environment

You can append a value to an existing construction variable using the Append method:

        env = Environment(CCFLAGS = '-DMY_VALUE')
        env.Append(CCFLAGS = ' -DLAST')
        env.Program('foo.c')
     

SCons then supplies both the -DMY_VALUE and -DLAST flags when compiling the object file:

        % scons -Q
        cc -o foo.o -c -DMY_VALUE -DLAST foo.c
        cc -o foo foo.o
     

If the construction variable doesn't already exist, the Append method will create it:

        env = Environment()
        env.Append(NEW_VARIABLE = 'added')
        print "NEW_VARIABLE =", env['NEW_VARIABLE']
     

Which yields:

        % scons -Q
        NEW_VARIABLE = added
        scons: `.' is up to date.
     

7.5.3. Appending to the Beginning of Values in a Construction Environment

You can append a value to the beginning of an existing construction variable using the Prepend method:

        env = Environment(CCFLAGS = '-DMY_VALUE')
        env.Prepend(CCFLAGS = '-DFIRST ')
        env.Program('foo.c')
     

SCons then supplies both the -DFIRST and -DMY_VALUE flags when compiling the object file:

        % scons -Q
        cc -o foo.o -c -DFIRST -DMY_VALUE foo.c
        cc -o foo foo.o
     

If the construction variable doesn't already exist, the Prepend method will create it:

        env = Environment()
        env.Prepend(NEW_VARIABLE = 'added')
        print "NEW_VARIABLE =", env['NEW_VARIABLE']
     

Which yields:

        % scons -Q
        NEW_VARIABLE = added
        scons: `.' is up to date.