Use isinstance to check if o is an instance of str or any subclass of str:

if isinstance(o, str):

To check if the type of o is exactly str, excluding subclasses of str:

if type(o) is str:

See Built-in Functions in the Python Library Reference for relevant information.

---

Checking for strings in Python 2

For Python 2, this is a better way to check if o is a string:

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. In Python 3, basestring no longer exists since there's a strict separation of strings (str) and binary data (bytes).

Alternatively, isinstance accepts a tuple of classes. This will return True if o is an instance of any subclass of any of (str, unicode):

if isinstance(o, (str, unicode)):