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Welcome back.

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So I'm going to start by defining
a class to represent a person.

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So I'm going to say class Person.

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And let's say that every person has
a name and year that they were born.

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So I'm going to create a constructor,
def __ init.

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That accepts a name and a year_born and

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assigns instance variables for
name and year_born.

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Self.name = name.

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Self.year_born = year_born.

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Now let's suppose that I also
want to have a method getAge.

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So getAge that takes this year_born and

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let's suppose that we have a current year.

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So I'll define CURRENT_YEAR to be 2019.

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So getAge is going to
subtract CURRENT_YEAR.

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So I'll say return
CURRENT_YEAR- self.year_born.

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And let's suppose that we also want
to override the __str__ method,

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so I'll say def __str__,
to print out self.name.

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So, I'll use the dot format method.

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And .format self.name and

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self.getAge.

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Okay, so now we have a class
to represent a person and

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every person has a name and
a year that they were born.

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So if I create something like alice

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equals a new person named Alice Smith.

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And I print out the value of Alice, so

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after passing the year_born so
I'll just say 1990.

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Oops, I need one more
closing parenthesis here.

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Okay, so now I can see that
Alice Smith is the name,

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and 29 is the age given
that the year born is 1990.

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Of course, we're just going to ignore
things like exact birthdays for

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this purpose.

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Now, suppose that I wanted to define a new
class to represent a student, and what I

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want a student class to have is everything
that a person has, plus one more thing.

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So I want students to have
this notion of knowledge.

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So what I'm going to do,
is I'm going to start by just copying and

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pasting my person class.

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So I can this, and then let's suppose that
I want to have another instance variable,

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named knowledge,
which I'll start out as 0.

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So I'll say, self.knowledge = 0.

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And I want this to be student,
instead of person.

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And let's suppose that I want
to have this other method on

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a student named study, so def study.

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And whenever a student studies,
then their knowledge should go up,

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so I'll say self.knowledge +=1 okay.

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So, now if I define Alice to be
a student instead of just a person.

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By saying, alice equals a new student.

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Then you can see that I still
get Alice Smith, age 29.

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If I print out alice.knowledge,
then I should get 0.

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If I tell alice to study,
and then print out alice's

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knowledge level, then I'm going to get 1.

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Now, this is a fine way of defining both
the person class and the stream class.

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But there is a much easier in more
concise way of actually doing this using

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something called inheritance.

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The idea of inheritance is that you can
define classes that inherit from other

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classes.

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So, in other words,
they take all of the instance variables,

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all of the methods that
that other class has, and

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then they can add more instance
variables and more methods.

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So, in this case,
every student is a person.

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It's not true that necessarily
every person is a student, but

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every student is a person.

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So, what I'm going to do is I'm going
to say that this student class should

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inherit from person.

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So a student should have
everything a person has plus more.

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Now, the way that I say I want a student
to inherit from person is by putting

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parenthesis after
the class declaration here.

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And putting the class I want to inherit
from inside of those parentheses.

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So I'll say that every
student is a person.

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So now that I do this I can get rid
of some of the repetitive parts

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of the student class.

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So here for example getAge,
is a little bit repetitive.

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I don't need getAge in both student and

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person because getAge is
already inside of person.

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So I'm going to remove getAge
from the student class.

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I'm going to do the same for
the def__ str__ method.

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And I'm going to do one
more thing as well.

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So this setting self.name and
self.year_born is a little bit repetitive

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with the Person class setting
self.name and self.year_born.

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So in the constructor for the student,

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I'm going to say that I want to call
the constructor, this method, of Person.

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And the way that I'll do that,

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is I'll say Person.__init__, and

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I'm going to pass in self,
name, and year_born.

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Now, when I do this I'm going
to call this init method, or

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the Person constructor,
on this new student.

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So this is going to call this method.

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And then this method is going
to set instance variables for

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name and for year_born automatically.

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So let's test this out with Alice,
and be sure that our code still works.

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So you can see again we have our
student Alice Smith born in 1990.

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And if I still run this code you can see
that after Alice studies then Alice's

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knowledge is 1.

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But now if I print out alice.getAge.

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You'll see that Alice is 29.

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If I print out Alice, Then you'll
see that we get Alice Smith, age 29.

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And the important part here is that
even though Alice is a student,

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and even though the student class didn't
directly define __str__ or getAge.

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I'm able to call both of
these methods on alice,

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because alice is a Student, and
Student inherits from Person, and

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the Person class has these methods,
getAge, and __str__.

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That's all for now, until next time.