[Python-tut] Private Instance Variables

[wtd]

So you've created an object in Python:

Python:

class StudentRecord(object):    def __init__(self, name, grades = []):       self.name = name       self.grades = grades    def add_grade(self, new_grade):       self.grades.append(new_grade)    def max_grade(self):       return max(self.grades)    def min_grade(self):       return min(self.grades)    def avg_grade(self):       return float(sum(self.grades)) / len(self.grades)

And you create an instance:

Python:

bob = StudentRecord("Bob", [100, 76, 89, 34])

Now, it's pretty obvious that if you call:

Python:

bob.max_grade()

You should get 100. But what if I was an idiot and wrote:

Python:

bob.grades = []

What will Python do to stop me?

Python won't do a darn thing, because the grades list was public.

So, the question is how we go about making it private.

Python:

class StudentRecord(object):    def __init__(self, name, grades = []):       self.name = name       self.__grades = grades    def add_grade(self, new_grade):       self.__grades.append(new_grade)    def max_grade(self):       return max(self.__grades)    def min_grade(self):       return min(self.__grades)    def avg_grade(self):       return float(sum(self.__grades)) / len(self.__grades)

That's great, but now how do I get to that list in a read-only fashion?

Python:

class StudentRecord(object):    def __init__(self, name, grades = []):       self.name = name       self.__grades = grades    def add_grade(self, new_grade):       self.__grades.append(new_grade)    def max_grade(self):       return max(self.__grades)    def min_grade(self):       return min(self.__grades)    def avg_grade(self):       return float(sum(self.__grades)) / len(self.__grades)    def __get_grades(self):       return self.__grades    grades = property(__get_grades)

Or alternatively:

Python:

class StudentRecord(object):    def __init__(self, name, grades = []):       self.name = name       self.__grades = grades    def add_grade(self, new_grade):       self.__grades.append(new_grade)    def max_grade(self):       return max(self.__grades)    def min_grade(self):       return min(self.__grades)    def avg_grade(self):       return float(sum(self.__grades)) / len(self.__grades)    grades = property(lambda self: self.__grades)

The property function can also be used to create "setters" as well as "getters".

[Mazer]

This is great! I was hoping that something like this existed.

[[Gandalf]]

Something I can learn . I'll go through it once I learn a bit more of the basics...