Search for file give in "executable". If it is not found, we try the environment PATH.
        Returns either the absolute path to the found executable, or None if the executable
        couldn't be found.

def _search_for_executable(self, executable):
        """
        Search for file give in "executable". If it is not found, we try the environment PATH.
        Returns either the absolute path to the found executable, or None if the executable
        couldn't be found.
        """
        if os.path.isfile(executable):
            return os.path.abspath(executable)
        else:
            envpath = os.getenv('PATH')
            if envpath is None:
                return
            for path in envpath.split(os.pathsep):
                exe = os.path.join(path, executable)
                if os.path.isfile(exe):
                    return os.path.abspath(exe)