def _expandSourceFiles(self, files, recurse, addIextIfMissing):
        C, D, W = self.chatter, self.debug, self.warn
        idir = self.opts.idir
        iext = self.opts.iext
        files = [] 
        for f in self.pathArgs:
            oldFilesLen = len(files)
            D("Expanding %s", f)
            path = os.path.join(idir, f)
            pathWithExt = path + iext # May or may not be valid.
            if os.path.isdir(path):
                if recurse:
                    os.path.walk(path, self._expandSourceFilesWalk, files)
                else:
                    raise Error("source file '%s' is a directory" % path)
            elif os.path.isfile(path):
                files.append(path)
            elif (addIextIfMissing and not path.endswith(iext) and 
                  os.path.isfile(pathWithExt)):
                files.append(pathWithExt)
                # Do not recurse directories discovered by iext appending.
            elif os.path.exists(path):
                W("Skipping source file '%s', not a plain file.", path)
            else:
                W("Skipping source file '%s', not found.", path)
            if len(files) > oldFilesLen:
                D("  ... found %s", files[oldFilesLen:])
        return files
Calculate source paths from 'files' by applying the 
dataset/ETHPy150Open skyostil/tracy/src/generator/Cheetah/CheetahWrapper.py/CheetahWrapper._expandSourceFiles