[Scons-users] Why is resetting user-defined options with SetOption forbidden?

[Andrew C. Morrow]

Hi -

I'd like to be able to rewrite the value of an option I've defined with
AddOption, but SetOption appears to forbid this except for certain
intrinsic SCons options. This seems quite restrictive. I have many options
of this form:

    AddOption('build-fast',
        choices=['on', 'off', 'auto'],
        const='on',
        default='auto',
        help='faster dependency checking',
        nargs='?',
        type='choice',
    )

The 'auto' value is intended to allow the system to decide whether 'on' or
'off' is appropriate, given the value of other options. However, the end
result is that it should hold the value 'on' or 'off'. So, ideally, I'd
later write:

if GetOption('build-fast') == 'auto':
    if <conditions>:
        SetOption('build-fast') = 'on'
    else
        SetOption('build-fast') = 'off'

That way, once I've done this normalization step, I can just ask anywhere I
need:

if GetOption('build-fast') == 'on':
    do-stuff

However, it seems that this is forbidden by SCons internally, even for
user-defined options, failing with a message like:

scons: *** This option is not settable from a SConscript file: build-fast

As a result, I need to hoist the option value into a variable, and
normalize that, then Export it for others to see and use instead of the
option value:

build_fast = GetOption('build-fast')
if build_fast == 'auto':
    build_fast = 'on' if conditions else 'off'
Export('build_fast')

This is unfortunate, since now the value of GetOption('build-fast') isn't
canonical, this exported state variable is instead, they vary in their
values, and it is easy to query the wrong one yielding different results.

I understand that some intrinsic SCons options may not make sense to be
reset, but is there an explicit reason that it cannot be done for user
defined options?

Thanks,
Andrew
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