How does SCons inject symbols into global scope?

[abitrolly]

How does SCons make global symbols available without imports?

$ cat SConstruct 
print(BoolVariable)

$ scons         
scons: Reading SConscript files ...
<function BoolVariable at 0x7f5c57846f80>
scons: done reading SConscript files.
scons: Building targets ...
scons: `.' is up to date.
scons: done building targets.

Is it possible to inject symbols dynamically? Like when a custom tool is loaded.

[bdbaddog]

A custom tool doesn't inject symbols into the global symbols.
It add's itself into the provided Environment()

There's not a SCons provided way to add to the globals() in the context of a SConscript/SConstruct.

You can Import("*") which might solve your issue depending on what you're trying to do.
Perhaps explain your use model and we can provide a workable solution.

[abitrolly]

I am just interested to know how SCons works. How different parts of it can be augmented without committing them to the core. Might reuse the same approach for my own Python scripts.

You can Import("*") which might solve your issue depending on what you're trying to do.

Is that it? https://scons.org/doc/latest/HTML/scons-api/_modules/SCons/Script/SConscript/#SConsEnvironment.Import Not sure what it does.

Perhaps explain your use model and we can provide a workable solution.

I don't need a workable solution, but the use model I'd like to see is this. Given a custom dir for SCons files, like .scons, I want to drop my Sophisticated variable into .scons/Variables/Sophisticated.py so that it became available to my scripts and I can use it the same way BoolVariable is used. If SCons doesn't work this way, I am interested to know how to get it there.

[bdbaddog]

Add it in .site_scons/__init__.py
Have you read the users guide or manpage thoroughly?

Search for site_scons in the manpage, and look at : https://scons.org/doc/production/HTML/scons-user.html#idm46358254405712

[abitrolly]

Yes, I am aware of site_scons/site_tools, and "where to put tools" is not what I am asking.

I'd like to understand how/where SCons injects symbols in the global scope of SConstruct? Is it a static or dynamic process? If process is static, then the injection is probably done by Python itself at import time before SConstruct is read. If dynamic, then it must be possible to inject new symbols after SConstruct started to execute.

[mwichmann]

As far as the implementation, that "magic" happens in SCons/Script. The sconscript files are successively exec'd using the existing globals after some stack fiddling.

[abitrolly]

Found only one exec here.

scons/SCons/Script/__init__.py

Line 379 in bc959a9

exec ("%s = _SConscript.DefaultEnvironmentCall(%s)" % (name, repr(name)))

It doesn't looks like it executes SConscript files. Rather sets global variables in itself.

[bdbaddog]

I wasn't talking about site_scons/site_tools I was talking about site_scons/site_init.py
Any symbols set in there should end up in the global name space.
Which the docs explain...

Before reading the SConscript files, scons looks for a directory named site_scons in various system directories and in the directory containing the SConstruct file or, if specified, the directory from the [--site-dir](https://scons.org/doc/production/HTML/scons-man.html#opt-site-dir) option instead, and prepends the ones it finds to the Python module search path (sys.path), thus allowing modules in such directories to be imported in the normal Python way in SConscript files. For each found site directory, (1) if it contains a file site_init.py that file is evaluated, and (2) if it contains a directory site_tools the path to that directory is prepended to the default toolpath. See the [--site-dir](https://scons.org/doc/production/HTML/scons-man.html#opt-site-dir) and [--no-site-dir](https://scons.org/doc/production/HTML/scons-man.html#opt-no-site-dir) options for details on default paths and controlling the site directories.

[bdbaddog]

The source code is there for all to read. :)

[abitrolly]

Too much code. An entrypoint would be welcome. )

[bdbaddog]

That's what grep's for!

SCons/Script/SConscript.py
Beyond that you're on your own.

[bdbaddog]

SConscript() calls end up building a set of globals based on previous globals. This happens at each SConscript call. So parent SConscript calls would have different globals from children. It's a bit complicated, and likely this implementation is not that useful in other contexts.

Python's eval() can be passed a set of globals. That's the basis for SConscript/SConstruct processing.

[abitrolly]

Looks like this is it.

scons/SCons/Script/SConscript.py

Line 196 in bc959a9

exec(sys.stdin.read(), call_stack[-1].globals)

And here is the first frame in stack where globals are supplied by BuildDefaultGlobals().

scons/SCons/Script/SConscript.py

Line 108 in bc959a9

self.globals = BuildDefaultGlobals()

For SConscript that is somehow piped to stdin, if I read this correctly.

1. First a Frame object is created on call_stack, with BuildDefaultGlobals()
2. Then SConscript is executed passing it globals from Frame
3. Then the Frame is popped of the stack.

So if no globals are shared once script starts execution, then SCons can not dynamically insert new symbols after exec started.