How to escape os.system() calls?

When using os.system() it's often necessary to escape filenames and other arguments passed as parameters to commands. How can I do this? Preferably something that would work on multiple operating systems/shells but in particular for bash.

I'm currently doing the following, but am sure there must be a library function for this, or at least a more elegant/robust/efficient option:

def sh_escape(s):
   return s.replace("(","\\(").replace(")","\\)").replace(" ","\\ ")

os.system("cat %s | grep something | sort > %s" 
          % (sh_escape(in_filename), 
             sh_escape(out_filename)))

Edit: I've accepted the simple answer of using quotes, don't know why I didn't think of that; I guess because I came from Windows where ' and " behave a little differently.

Regarding security, I understand the concern, but, in this case, I'm interested in a quick and easy solution which os.system() provides, and the source of the strings is either not user-generated or at least entered by a trusted user (me).