python/exercises/concept/ellens-alien-game/.docs/introduction.md

# Introduction

## Object Oriented Programming in Python

If you have been programming in a [functional][functional], [declarative][declarative], or [imperative][imperative] style, shifting focus to [object oriented programming][oop] (OOP) may feel a bit foreign.
An OOP approach asks the programmer to think about modeling a problem as one or more `objects` that interact with one another, keep state, and act upon data.
Objects can represent real world entities (_such as cars or cats_) - or more abstract concepts (_such as integers, vehicles, or mammals_).
Each object becomes a unique instance in computer memory and represents some part of the overall model.

## Classes

`Classes` are the definitions of new object types, and from which new `instances` of objects are created.
They often bundle data with code or functions that operate on that data.
In this sense, classes are _blueprints_ or sets of instructions from which many objects of a similar type can be built and used.
A complex program can have many classes, each building many different flavors of objects.
The process of building an object from a class is known as `instantiation` (_or creating an instance of the class_).

A class definition in Python is straightforward:

```python
class MyClass:
    # Class body goes here
```

### Class Attributes

Class fields (_otherwise known as `properties`, `attributes`, `data members`, or `variables`_) can be added to the body of the class:

```python
class MyClass:
    number = 5
    string = "Hello!"
```

An instance (_object_) of `MyClass` can be created and bound to a name by [_calling_][calling] the class (_in the same way a function is called_):

```python
>>> new_object = MyClass()

# Class is instantiated and resulting object is bound to the "new_object" name.
# Note: the object address 'at 0x15adc55b0' will vary by computer.
>>> new_object
<__main__.MyClass at 0x15adc55b0>
```

`Class attributes` are shared across all objects (_or instances_) created from a class, and can be accessed via [`dot notation`][dot notation]  -  a `.` placed after the object name and before the attribute name:

```python
>>> new_object = MyClass()

# Accessing the class attribute "number" via dot-notation.
>>> new_object.number
5

# Accessing the class attribute "string" via dot-notation.
>>> new_object.string
'Hello!'

# Instantiating an additional object and binding it to the "second_new_object" name.
>>> second_new_object = MyClass()

>>> second_new_object
# Note: the object address "at 0x15ad99970" will vary by computer.
<__main__.MyClass at 0x15ad99970>

# Second_new_object shares the same class attributes as new_object.
>>> new_object.number == second_new_object.number
True
```

Class attributes are defined in the body of the class itself, before any other methods.
They are owned by the class - allowing them to be shared across instances of the class.
Because these attributes are shared, their value can be accessed and manipulated from the class _directly_.
Altering the value of class attributes alters the value _**for all objects instantiated from the class**_:

```python
>>> obj_one = MyClass()
>>> obj_two = MyClass()

# Accessing a class attribute from an object.
>>> obj_two.number
5

# Accessing the class attribute from the class itself.
>>> MyClass.number
5

# Modifying the value of the "number" class attribute.
>>> MyClass.number = 27

# Modifying the "number" class attribute changes the "number" attribute for all objects.
>>> obj_one.number
27

>>> obj_two.number
27
```

Having a bunch of objects with synchronized data at all times is not particularly useful.
Fortunately, objects created from a class can be customized with their own `instance attributes` (_or instance properties, variables, or fields_).
As their name suggests, instance attributes are unique to each object, and can be modified independently.


## Customizing Object Instantiation with `__init__()`

The special ["dunder method"][dunder] (_short for "double underscore method"_) `__init__()` is used to customize class instances, and can be used to initialize instance attributes or properties for objects.
For its role in initializing instance attributes, `__init__()` is also referred to as a `class constructor` or `initializer`.
`__init__()` takes one required parameter called `self`, which refers to the newly initialized or created object.
Data for instance attributes or properties can then be passed as arguments of `__init__()`, following the `self` parameter.

Below, `MyClass` now has instance attributes called `location_x` and `location_y`.
As you can see, the two attributes have been assigned to the first and second indexes of the `location` (_a tuple_) argument that has been passed to `__init__()`.
The `location_x` and `location_y` attributes for a class instance will now be initialized when you instantiate the class, and an object is created:

```python
class MyClass:
    # These are class attributes, variables, or fields.
    number = 5
    string = "Hello!"

    # This is the class "constructor", with a "location" parameter that is a tuple.
    def __init__(self, location):

        # This is an instance or object property, attribute, or variable.
        # Note that we are unpacking the tuple argument into two separate instance variables.
        self.location_x = location[0]
        self.location_y = location[1]

# Create a new object "new_object_one", with object property (1, 2).
>>> new_object_one = MyClass((1, 2))

# Create a second new object "new_object_two" with object property (-8, -9).
>>> new_object_two = MyClass((-8, -9))

# Note that new_object_one.location_x and new_object_two.location_x two different values.
>>> new_object_one.location_x
1

>>> new_object_two.location_x
-8
```

Note that you only need to pass one argument when initializing `MyClass` above -- Python takes care of passing `self` when the class is called.


## Methods

A `method` is a `function` that is bound to either the class itself (_known as a [class method][class method], which will be discussed in a later exercise_) or an _instance_ of the class (object).
Methods that operate on an object (instance) need to be defined with `self` as the first parameter.
You can then define the rest of the parameters as you would for a "normal" or non-bound function:

```python
class MyClass:
    number = 5
    string = "Hello!"

    # Class constructor.
    def __init__(self, location):
        # Instance properties
        self.location_x = location[0]
        self.location_y = location[1]

    # Instance method. Note "self" as first parameter.
    def change_location(self, amount):
        self.location_x += amount
        self.location_y += amount
        return self.location_x, self.location_y
```

Like attribute access, calling a method simply requires putting a `.` after the object name, and before the method name.
The called method does not need a copy of the object as a first parameter -- Python fills in `self` automatically:

```python
class MyClass:
    number = 5
    string = "Hello!"

    def __init__(self, location):
        self.location_x = location[0]
        self.location_y = location[1]

    def change_location(self, amount):
        self.location_x += amount
        self.location_y += amount
        return  self.location_x, self.location_y

# Make a new test_object with location (3,7)
>>> test_object = MyClass((3,7))
>>> (test_object.location_x, test_object.location_y)
(3,7)

# Call change_location to increase location_x and location_y by 7.
>>> test_object.change_location(7)
(10, 14)
```

Class attributes can be accessed from within instance methods in the same way that they are accessed outside of the class:

```python
class MyClass:
    number = 5
    string = "Hello!"

    def __init__(self, location):
        self.location_x = location[0]
        self.location_y = location[1]

    # Alter instance variable location_x and location_y
    def change_location(self, amount):
        self.location_x += amount
        self.location_y += amount
        return  self.location_x, self.location_y

    # Alter class variable number for all instances from within an instance.
    def increment_number(self):
        # Increment the 'number' class variable by 1.
        MyClass.number += 1


>>> test_object_one = MyClass((0,0))
>>> test_object_one.number
5

>>> test_object_two = MyClass((13, -3))
>>> test_object_two.increment_number()
>>> test_object_one.number
6
```

## Placeholding or Stubbing Implementation with Pass

In previous concept exercises and practice exercise stubs, you will have seen the `pass` keyword used within the body of  functions in place of actual code.
The `pass` keyword is a syntactically valid placeholder - it prevents Python from throwing a syntax error for an empty function or class definition.
Essentially, it is a way to say to the Python interpreter, 'Don't worry! I _will_ put code here eventually, I just haven't done it yet.'

```python
class MyClass:
    number = 5
    string = "Hello!"

    def __init__(self, location):
        self.location_x = location[0]
        self.location_y = location[1]

    # Alter instance variable location_x and location_y
    def change_location(self, amount):
        self.location_x += amount
        self.location_y += amount
        return  self.location_x, self.location_y

    # Alter class variable number for all instances
    def increment_number(self):
        # Increment the 'number' class variable by 1.
        MyClass.number += 1

    # This will compile and run without error, but has no current functionality.
    def pending_functionality(self):
       # Stubbing or placholding the body of this method.
       pass
```

[calling]: https://www.pythonmorsels.com/topics/calling-a-function
[class method]: https://stackoverflow.com/questions/17134653/difference-between-class-and-instance-methods
[dunder]: https://www.dataindependent.com/python/python-glossary/python-dunder/
[imperative]: https://en.wikipedia.org/wiki/Imperative_programming
[declarative]: https://en.wikipedia.org/wiki/Declarative_programming
[oop]: https://www.digitalocean.com/community/tutorials/how-to-construct-classes-and-define-objects-in-python-3
[functional]: https://en.wikipedia.org/wiki/Functional_programming
[dot notation]: https://stackoverflow.com/questions/45179186/understanding-the-dot-notation-in-python