How do I check if an object is an instance of a given class or of a subclass of it?

Use the built-in function isinstance.

if isinstance(obj, MyClass):
     print "obj is my object"

You can check if an object is an instance of any of a number of classes by providing a tuple instead of a single class, e.g. isinstance(obj, (class1, class2, …)), and can also check whether an object is one of Python’s built-in types. Examples:

if isinstance(obj, str):
     print repr(obj), "is an 8-bit string"

if isinstance(obj, basestring):
     print repr(obj), "is some kind of string"

if isinstance(obj, (int, long, float, complex)):
     print obj, "is a built-in number type"

if isinstance(obj, (tuple, list, dict, set)):
     print "object is a built-in container type"

Note that most programs do not use isinstance on user-defined classes very often. If you are developing the classes yourself, a more proper object-oriented style is to define methods on the classes that encapsulate a particular behaviour, instead of checking the object’s class and doing a different thing based on what class it is. For example, if you have a function that does something:

def search(obj):
    if isinstance(obj, Mailbox):
        print "search a mailbox"
    elif isinstance(obj, Document):
        print "search a document"
    else:
        print "unknown object"

A better approach is to define a search() method on all the classes and just call it:

class Mailbox:
    def search(self):
        print "search a mailbox"

class Document:
    def search(self):
        print "search a document"

obj.search()