Object Oriented Programming (OOP)
---------------------------------

OOP allows you to encapsulate specific behaviour into so called classes.
Instances of those classes can be used to write intuitive program code.
Below is an example using classes defined in rock.py to create instances
of fruits and rocks and to determine their cumulative weight on a scale.
Classes can represent real world objects as in this example but also
abstract things like a button in a graphical user interface or a list
with special sorting algorithms.

Normally you would import the classes from the module rock like this:

.. code:: python

    #from rock import Rock, Sediment, MagneticMatter, MagneticSediment, Scale, Fruit

Alternatively you can use the magic %load command to get the whole file
into this ipython notebook:

.. code:: python

    # %load rock.py
    # object oriented programming with Python
    # Michael Wack 2015
    
    class Fruit:
        def __init__(self, weight):
            self.weight = weight
    
        def __repr__(self):
            return( "fruit with weight {:.2e}".format( self.weight))
    
    
    class Rock:
        # class variable
        rockcounter=0
    
    
        @classmethod
        def BlueRock(cls, weight, volume):
            return cls( color="blue", weight=weight, volume=volume)
    
    
        @staticmethod
        def density( weight, volume):
            return weight / volume
    
        def __init__(self, color, weight, volume=11e-6):
            self.color = color
            self.weight = weight  # SI: kg
            self.volume = volume  # SI: m^3
    
            Rock.rockcounter += 1
            self.no = Rock.rockcounter
    
            print("{}. rock created".format(Rock.rockcounter))
    
        def calculate_density(self):
            return Rock.density( self.weight, self.volume)  # SI: kg / m^3
    
    
        def __repr__(self):
            return( "{} rock (No {}) with a density of {:.2f}".format( self.color, self.no, self.calculate_density()))
    
    
    class Sediment( Rock):
        def __init__(self, color, weight, volume=11e-6, grainsize=0):
            super().__init__(color, weight, volume)
    
            self.grainsize = grainsize
    
    
        def double_grainsize(self):
            self.grainsize *= 2
    
        def __repr__(self):
            return( "{} sediment (No {}) with a density of {:.2e} and a grainsize of {:.2e}".format( self.color, self.no, self.calculate_density(), self.grainsize))
    
    
    
    class MagneticMatter(): # all volume normalized
        def __init__(self, magnetization=0, susceptibiltiy=0):
            self.magnetization = magnetization # Am^2 / m^3 = A/m
            self.susceptibility = susceptibiltiy # volume normailzed -> no units
    
        def induced_magnetization(self, external_field_H): # SI A/m
            return self.susceptibility * external_field_H
    
        def total_magnetization(self, external_field_H): # SI A/m
            return self.induced_magnetization(external_field_H) + self.magnetization
    
    
    
    class MagneticSediment( MagneticMatter, Sediment):
        def __init__(self, color, weight, volume, grainsize=0, magnetization=0, susceptibility=0):
            MagneticMatter.__init__( self, magnetization, susceptibility)
            Sediment.__init__( self, color, weight, volume)
    
        def magnetic_moment(self):
            return self.magnetization * self.volume
    
    
    # class to determine weight of other object instances
    class Scale():
        def __init__(self, weight_limit=100):
            self.weight_limit = weight_limit
            self.instances = []
    
        def put_on(self, inst):
            if inst in self.instances:
                print("{} is already on scale.".format(inst))
            else:
                self.instances.append(inst)
                print("{} placed on scale.".format(inst))
    
        def take_off(self, inst):
            try:
                self.instances.remove(inst)
                print("{} removed.".format(inst))
            except KeyError:
                print("{} was not on scale.".format(inst))
    
        @property
        def weight(self):
            weight = 0
            for i in self.instances:
                weight += i.weight
            if weight > self.weight_limit:
                print("Scale overloaded.")
    
            return weight


.. code:: python

    apple = Fruit(weight=0.2)

.. code:: python

    apple




.. parsed-literal::

    fruit with weight 2.00e-01



.. code:: python

    my_first_rock = Rock(color='red', weight=10)


.. parsed-literal::

    1. rock created


.. code:: python

    my_first_rock




.. parsed-literal::

    red rock (No 1) with a density of 909090.91



.. code:: python

    my_second_rock = Sediment(color='yellow', weight=12)


.. parsed-literal::

    2. rock created


.. code:: python

    blue_rock = Rock.BlueRock(weight=3, volume = 0.1)


.. parsed-literal::

    3. rock created


.. code:: python

    blue_rock




.. parsed-literal::

    blue rock (No 3) with a density of 30.00



.. code:: python

    my_last_rock = MagneticSediment(color='purple', weight=1, volume=0.01, magnetization=1e-3, susceptibility=5e-3)


.. parsed-literal::

    4. rock created


.. code:: python

    my_last_rock.induced_magnetization(external_field_H = 40)




.. parsed-literal::

    0.2



.. code:: python

    my_last_rock.total_magnetization(external_field_H = 40)




.. parsed-literal::

    0.201



.. code:: python

    myscale = Scale(weight_limit = 20)

.. code:: python

    myscale




.. parsed-literal::

    <__main__.Scale at 0x7f45b01a36d8>



.. code:: python

    myscale.put_on( blue_rock)


.. parsed-literal::

    blue rock (No 3) with a density of 30.00 placed on scale.


.. code:: python

    myscale.weight




.. parsed-literal::

    3



.. code:: python

    myscale.put_on( my_second_rock)


.. parsed-literal::

    yellow sediment (No 2) with a density of 1.09e+06 and a grainsize of 0.00e+00 placed on scale.


.. code:: python

    myscale.weight




.. parsed-literal::

    15



.. code:: python

    myscale.put_on( my_second_rock)


.. parsed-literal::

    yellow sediment (No 2) with a density of 1.09e+06 and a grainsize of 0.00e+00 is already on scale.


.. code:: python

    myscale.weight




.. parsed-literal::

    15



.. code:: python

    myscale.put_on( my_first_rock)


.. parsed-literal::

    red rock (No 1) with a density of 909090.91 placed on scale.


.. code:: python

    myscale.weight


.. parsed-literal::

    Scale overloaded.




.. parsed-literal::

    25



.. code:: python

    myscale.take_off( blue_rock)


.. parsed-literal::

    blue rock (No 3) with a density of 30.00 removed.


.. code:: python

    myscale.weight


.. parsed-literal::

    Scale overloaded.




.. parsed-literal::

    22