def load(self, requirements_file=None):
        """Load or reload requirements from a requirements.txt file.

        :param requirements_file: if not given, the filename used from
            initialization will be read again.
        """
        if requirements_file is None:
            requirements_file = self.filename
            if requirements_file is None:
                raise ValueError("no filename provided")
        elif isinstance(requirements_file, text_type):
            requirements_file = Path(requirements_file)
        self.clear()
        with requirements_file.open() as f:
            self.loads(f.read())

        if isinstance(requirements_file, (text_type, Path)):
            self.filename = requirements_file
def build(self, spec, prefix):
        """Build using the OpenFOAM Allwmake script, with a wrapper to source
        its environment first.
        Only build if the compiler is known to be supported.
        """
        self.foam_arch.has_rule(self.stage.source_path)
        self.foam_arch.create_rules(self.stage.source_path, self)

        args = []
        if self.parallel:  # Build in parallel? - pass via the environment
            os.environ['WM_NCOMPPROCS'] = str(make_jobs)
        builder = Executable(self.build_script)
        builder(*args)