pythonでモジュールをダイナミックにロードする

コード：

import sys
if __name__ == "__main__":
     if len(sys.argv) > 1:
        for a in sys.argv[1:]:
            try:
                exec "import " + a
                print "module has successfully loaded:", a
            except: pass

実行結果：

% python hello.py unittest imp hage hoge
module has successfully loaded: unittest
module has successfully loaded: imp
%