Is variable default value expansion is published and reliable

[dustdfg]

Hello I've just thought about expansion and want to know if following code is reliable and deterministic.

import SCons
from SCons.Variables import *

env = Environment()

options = Variables(["custom.py"], ARGUMENTS)

options.Add("my_option", "", "some_string")
options.Add("my_option2", "", "$my_option/${2+3}")

options.Update(env)

print(options.GenerateHelpText(env))

Following code prints as expected:

my_option: 
    default: some_string
    actual: test

my_option2: 
    default: $my_option/${2+3}
    actual: test/5

But inside docs I saw following https://scons.org/doc/production/HTML/scons-user.html#b-Substfile that Substfile can be nondeterministic in some cases. So are there similar limitations for this feature and is it published? I couldn't find it in the docs so... But looks like very intended beahavior so maybe it is published?

[mwichmann]

Sorry, never commented on this... I think the Substfile wording in the User Guide you point to is a remnant of when Python dicts were not order-preserving.

In the case of Variables, ones that come from a file are processed by doing an exec on the script as if it were Python code, so that behavior should be well-known; nothing particular is done up front for values that come from the command line. Then, once a declared variable has a value, if it has a converter it is substituted before the converter is called; if it has a validator it may be substituted, depending on the type of the value - both also controlled by a flag which can be set to not do substitution (that's because of a specific request to cover a case where a value needed to hold something starting with a $ which was intended for the external tool, not as an SCons variable reference).

So there seem to be plenty of ways for substitution not to happen on the value/default of a variable...