To check if o is an instance of str or any subclass of str, use isinstance (this would be the "canonical" way):
if isinstance(o, str):
To check if the type of o is exactly str:
if type(o) is str:
The following also works, and can be useful in some cases:
if issubclass(type(o), str):
See Built-in Functions in the Python Library Reference for relevant information.
One more note: in this case, if you're using python 2, you may actually want to use:
if isinstance(o, basestring):
because this will also catch Unicode strings (unicode is not a subclass of str; both str and unicode are subclasses of basestring). Note that basestring no longer exists in python 3, where there's a strict separation of strings (str) and binary data (bytes).
Alternatively, isinstance accepts a tuple of classes. This will return True if x is an instance of any subclass of any of (str, unicode):
if isinstance(o, (str, unicode)):