foo.py

from datetime import datetime


class Person(object):
    pass


person = Person()


################################################################################
# Procedural

if getattr(person, 'spouse') and person.spouse.birthday == datetime.today():
    print "Happy Procedural birthday to your Spouse!"


################################################################################
# Duck-typing
try:
    if person.spouse.birthday == datetime.today():
        print "Happy Duck-Typed birthday to your Spouse!"
except AttributeError:
    pass  # person does not have birthday


################################################################################
# Null-Object

class NullObject(object):
    # Imagine a class that returns more NullObjects for every method call and
    # property access.
    #
    # http://en.wikipedia.org/wiki/Null_Object_pattern
    pass


def maybe(value):
    if value is None:
        return NullObject()
    else:
        return value

if maybe(person.spouse).birthday == datetime.today():
    print "Happy Null Object birthday to your Spouse!"


################################################################################
# Sensible defaults

# Ensure that a person.spouse is always SOMETHING, never None.
class Nobody(object):
    birthday = None


class Person(object):

    @property
    def spouse(self):
        if not self._spouse:
            return Nobody()
        else:
            return self._spouse

if person.spouse.birthday == datetime.today():
    print "Happy Sensible Defaults birthday to your Spouse!"

################################################################################
# Law of demeter

class Person(object):
   # Add method to person to handle spouse birthday interface
    def spouse_birthday(self):
        if hasattr(self, 'spouse'):
            return self.spouse.birthday


if person.spouse_birthday == datetime.today():
    print "Happy Law of Demeter birthday to your Spouse!"