+        - [Stack Overflow](http://stackoverflow.com/questions/56860/what-is-the-liskov-substitution-principle)

+        - **Moral of the story: model your classes based on behaviours not on properties; model your data based on properties and not on behaviours. If it behaves like a duck, it's certainly a bird.**

+        - **This strongly suggests that inheritance should never be used when the sub-class restricts the freedom implicit in the base class, but should only be used when the sub-class adds extra detail to the concept represented by the base class as in 'Monkey' is-an 'Animal'.**

+- [Clean Coder](http://cleancoders.com/category/fundamentals)

+#!/usr/bin/env python

+# -*- coding: utf-8 -*-

+# The violation of LSP here is that a `Prisoner` is not a suitable 

+# substitution of `Person` since they "behave" differently.

+# Remember, the principle is that you should model your class according

+# to their behavior rather than porperties. See "Circle-Ecllipse Problem"

+# for detail.

+import copy

+

+class Person(object):

+

+    def __init__(self, position):

+        self.position = position

+

+    def walk_North(self, dist):

+        self.position[1] += dist

+

+    def walk_East(self, dist):

+        self.position[0] += dist

+

+# `Prisoner` is a logicall natural extension of `Person`

+# since they fulfill the "is-a" relation: a `Prisoner` is a `Person`.

+# However, such extension violate LSP in this case.

+class Prisoner(Person):

+    PRISON_LOCATION = [3, 3]

+

+    def __init__(self):

+        super(Prisoner, self).__init__(copy.copy(self.PRISON_LOCATION))

+        self.is_free = False

+

+# The issue here is that `Prisoner` inherite `walk_North` and `walk_East` methods

+# from the `Person` which is not logically correct for the `Prisoner` class.

+

+def main():

+    prisoner = Prisoner()

+    print "The prisoner trying to walk to north by 10 and east by -3."

+    

+    try:

+        prisoner.walk_North(10)

+        prisoner.walk_East(-3)

+    except:

+        pass

+    

+    print "The location of the prison: {}".format(prisoner.PRISON_LOCATION)

+    print "The current position of the prisoner: {}".format(prisoner.position)

+

+if __name__ == "__main__":

+    main()(파일 끝에 줄바꿈 문자 없음)

+#!/usr/bin/env python

+# -*- coding: utf-8 -*-

+# As we can see in `python_code.bad.LSP` module where a violation

+# of LSP may lead to an unexpected behaviour of sub-types. In our 

+# example, "is-a" relation can not directly applied to `Person` and 

+# `Prisoner`. The cause is that these two classes "behave" differently.

+# How to fix it? Maybe a better naming will do the trick:

+

+class FreeMan(object):

+

+    def __init__(self, position):

+        self.position = position

+

+    def walk_North(self, dist):

+        self.position[1] += dist

+

+    def walk_East(self, dist):

+        self.position[0] += dist

+

+# "is-a" relationship no longer holds since a `Prisoner` is not a `FreeMan`.

+class Prisoner(object):

+    PRISON_LOCATION = (3, 3)

+

+    def __init__(self):

+        self.position = type(self).PRISON_LOCATION

+

+def main():

+

+    prisoner = Prisoner()

+    print "The prisoner trying to walk to north by 10 and east by -3."

+    

+    try:

+        prisoner.walk_North(10)

+        prisoner.walk_East(-3)

+    except:

+        pass

+    

+    print "The location of the prison: {}".format(prisoner.PRISON_LOCATION)

+    print "The current position of the prisoner: {}".format(prisoner.position)

+

+if __name__ == "__main__":

+    main()(파일 끝에 줄바꿈 문자 없음)