How do I distinguish a string from a sequence?

[Paul Moore]

Yes, I know, a string is a sequence...

I'm thinking of writing a function which can take either a string, or a sequence
of strings, as an argument. A simplified example would be treating a single
string as a 1-tuple - something like

   def foo(args):
      if # args is a string:
         args = (args,)
      for arg in args:
         print arg

But I'm not sure how best to distinguish a string from a sequence - after all, a
string *is* a sequence. I want to avoid explicit type tests, as that seems prone
to errors (for a start, I should trap both strings and Unicode strings, and what
about 2.2 subclasses of strings?)

But I can't think of a suitable test.

The best I can come up with is a type-test along the lines of

    if isinstance(args, type('')) or isinstance(args, type(u'')):

but as I say, that's annoyingly "explicit".

[If it matters, my "real" example is a popen-type function, which can take one
string argument - a command to execute - or a sequence of strings - a pipe
specification, where each element of the sequence would be a stage in the
pipeline]

Has anyone got any alternative suggestions?

Paul.