Expose Python Interpreter for rules and actions

[HDembinski]

Make sure you completed the following tasks

• I searched the discussions
• I searched the closed and open issues

Describe your use case

In the build of boost.histogram, I am running Python scripts to build the tests and documentation. Currently, I use actions with

/usr/bin/python3 my_python_script.py

This assumes that we are on Linux or MacOS, it won't work on Windows. I need the path of the Python interpreter.

The python module in B2 detects the path to the Python interpreter, but AFAIU, it does not provide a way for me to access that detected path.

Describe the solution you'd like

I want a way to access the Python interpreter that is found by the python module. Pseudocode:

import python ;

actions doxygen-postprocessing
{
    $(PYTHON_INTERPRETER) $(THIS_PATH)/doxygen_postprocessing.py "$(>)"
}

Or similar, I don't know how modules export path-constants.

Alternatives, if applicable

This is the brittle alternative is what we are currently using:

if [ os.name ] = NT
  {
    actions doxygen-postprocessing
    {
      python $(THIS_PATH)/doxygen_postprocessing.py "$(>)"
    }
  }
  else
  {
    actions doxygen-postprocessing
    {
      python3 $(THIS_PATH)/doxygen_postprocessing.py "$(>)"
    }
  }

Clearly, this is just a heuristic and begs to fail; detecting the interpreter is much more complicated than that and should be done by the code in the python module.

[grisumbras]

See this:
https://github.com/boostorg/docca/blob/develop/docca.jam#L403
https://github.com/boostorg/docca/blob/develop/docca.jam#L435

So, for your rule this would become:

import python ;
import toolset ;

actions doxygen-postprocessing
{
    "$(PYTHON_INTERPRETER:E=python)" $(THIS_PATH)/doxygen_postprocessing.py "$(>)"
}
toolset.flags $(__name__).doxygen-postprocessing PYTHON_INTERPRETER <python.interpreter> ;