[Scons-users] Custom environment tools evaluated before Configure contexts?

[Mats Wichmann]

On 11/5/23 22:42, Ben Farmer wrote:
> Hi scons mailing list,
> 
> I am currently trying to understand the intended usage of the Configure 
> context vs Environment 'tools'. Here's my scenario: the codebase I am 
> working on has a bunch of custom 'tools' defined, which create various 
> "actions" to run, one of which is running some unit tests via pytest. 
> These are passed to the construction Environment in it's constructor in 
> the project SConstruct file:
> 
>      env = Environment(tools=our_custom_tools, ...)
> 
> However, I would now like to change the behaviour of these tools 
> depending on the build environment. It's a simple change, I just want to 
> pass `-n <N>` to pytest to set it to run in parallel if pytest-xdist is 
> installed. So I figured I would check this somewhere, then in the custom 
> tool build the pytest Builder action with or without this -n flag 
> depending on the result. Checking if stuff is installed seemed like a 
> job for the Configure context, so I made some custom checks for that 
> following 
> https://scons.org/doc/production/HTML/scons-man.html#configure_contexts 
> <https://scons.org/doc/production/HTML/scons-man.html#configure_contexts>, and do this:
> 
>      conf = Configure(env, custom_tests={"CheckPytest": check_pytest,
>                                          "CheckPytestXdist": 
> check_pytest_xdist})
>      env["PYTEST_INSTALLED"] = 'true' if conf.CheckPytest() else 'false'
>      env["PYTEST_XDIST_INSTALLED"] = 'true' if conf.CheckPytestXdist() 
> else 'false'
>      conf.Finish()
> 
> This works totally fine, it checks if `pytest-xdist` is available and 
> adds some construction variables to communicate this. Unfortunately it 
> seems like it does this too late. The tools have already been "built" 
> when Environment was called, so I can't use them to build the "action" 
> differently depending on the environment

It's  true that for tools specified in the Environment() call (or if 
none, the default tools list) their generate() method is called as the 
environment is instantiated. But that doesn't have to be the end of the 
story.  Here are three avenues you can explore:

* don't specify the tool in the Environment call, but after it is 
instantiated using env.Tool() - that gives you control over when the 
initialization happens.
* if the action is a function, it is passed the environment, so it can 
make decisions based on settings in construction variables at the time 
it triggers.
* generation of the action itself can be deferred by using a generator 
in the tool's builder. Some more of that topic here:

https://scons.org/doc/production/HTML/scons-user.html#idp105549030489544