How to create a custom string representation for a class object?

Implement __str__() or __repr__() in the class's metaclass.

class MC(type):
  def __repr__(self):
    return 'Wahaha!'

class C(object):
  __metaclass__ = MC

print(C)

Use __str__ if you mean a readable stringification, use __repr__ for unambiguous representations.

If you have to choose between __repr__ or __str__ go for the first one, as by default implementation __str__ calls __repr__ when it wasn't defined.

Custom Vector3 example:

class Vector3(object):
    def __init__(self, args):
        self.x = args[0]
        self.y = args[1]
        self.z = args[2]

    def __repr__(self):
        return "Vector3([{0},{1},{2}])".format(self.x, self.y, self.z)

    def __str__(self):
        return "x: {0}, y: {1}, z: {2}".format(self.x, self.y, self.z)

In this example, repr returns again a string that can be directly consumed/executed, whereas str is more useful as a debug output.

v = Vector3([1,2,3])
print repr(v)    #Vector3([1,2,3])
print str(v)     #x:1, y:2, z:3

class foo(object):
    def __str__(self):
        return "representation"
    def __unicode__(self):
        return u"representation"