Python Language OOP

Internal

• Python Language

TODO

• How to call a method from inside the constructor. If I try, the compiler says "Unresolved reference"
• PyOOP "Manager objects"
• PyOOP "Removing duplicate code" + "In practice" + "Case study"
• PyOOP "Empty objects"

Overview

Everything in Python is an object, from numbers to modules. The language offers possibility of declaring custom types, and creating object instances for those types. Unlike modules, there can be multiple instances of the same type. Creation of new types is achieved by using the class syntax. The custom types such declared are no different, intrinsically, from the types already existing in the runtime. The existing types can be extended.

An object contains data, in form of variables called attributes and behavior, in form of functions called methods.

Class

A class is the definition of an object. A new type (class) is defined with the class keyword. "Class" and "type" mean pretty much the same thing.

class MyClass:
  def __init__(self):
    pass

The class may be declared with parentheses, but the IDE static checks find those as "redundant":

class MyClass():
  ...

Attributes

An object contains data in form of variables called attributes.

The attributes can be accessed inside the class definition using self.<attribute-name>. Outside the class definition, they can be accessed via the variable holding the reference to the class instance: my_instance.<attribute-name>. Unlike in other languages, all attributes are public. If an attribute was not explicitly declared inside the instance with self.some_attribute, even just to be assigned to None, an attempt to access the attribute will end up in:

AttributeError: 'MyClass' object has no attribute 'some_attribute'

Python offers two syntactical patterns to expose internal class instance state, either represented by an actual attributes, or computed values: properties and @property/@<attribute-name>.setter decorators.

The state associated with a computed value can be prevented from being written by omitting the corresponding setter.

Another advantage of using properties over direct attribute access is that if the definition of the attribute changes, only the code within the class definition needs to be changed, not in all the callers,.

Properties

• TO PROCESS: PyOOP "Adding behavior to class data with properties", "Properties in detail"
• TO PROCESS: PyOOP "Deciding when to use properties"

Properties are class construct that associate getter and setter methods to an attribute. The attribute name can the be used to access and write state inside the class by invoking the associated "property" methods. In other words, the getter and setter methods are "properties" of the attribute with the given name.

class A:
  def __init__(self, c):
    self.internal_color = c

  def get_color(self):
    return self.internal_color

  def set_color(self, c):
    self.internal_color = c

  # 'color' is an attribute, though not explicitly declared on self, and get_color() and set_color() are properties of the attribute
  color = property(get_color, set_color)

The first argument to property() is the getter method, and the second argument is the setter method.

Internal class instance state can be accesses and written through the attribute, though there is no actual attribute with that name initialized on self:

a = A('red')
assert 'red' == a.color
a.color = 'blue'
assert 'blue' == a.color

Defining Properties with Decorators

TO PROCESS: PyOOP "Decorators - another way to create properties"

Another way to define properties is with decorators. The same attribute color, which is not declared on self, can defined by two different property methods, one getter and one setter, preceded by corresponding decorators (annotations):

• @property: it annotates the getter method. ⚠️ The name of the method must match the name of the attribute.
• @<attribute-name>.setter: it annotates the setter method. ⚠️ The <attribute-name> that is part of the annotation must match the name of the attribute and the name of the setter method.

class A:
    def __init__(self, c):
        self.internal_color = c

    @property
    def color(self):
        return self.internal_color

    # not exposing a setter prevents the attribute from being written
    @color.setter
    def color(self, c):
        self.internal_color = c

The interaction with the internal state is done identically as in the case of the property() declaration:

a = A('red')
assert 'red' == a.color
a.color = 'blue'
assert 'blue' == a.color

For more details on decorators see:

Python Decorators

Mangling Attribute Names for Privacy

TODO

Methods

An object contains behavior, in form of functions called methods. Unlike in other languages, all methods are public.

Calling class methods from inside class methods:

class MyClass:
  def __init__(self):
    self.internal()

  def internal(self):
     pass

Special Methods

__init__()

The object instance initialization method, which is used to do anything that's needed to distinguish an object instance from other object instances of the same class. The self argument specifies that it refers to the individual object itself. self is not a reserved word in Python, but it is used conventionally, and it's a good idea to stick with the convention.

class MyClass:
  def __init__(self, name, color):
    self.name = name
    self.color = color

An __init__() method is not required.

It is good practice to define all attributes in a contiguous block in __init(), even if we have to assign them to None, and possibly document them. If we call other "private" methods from __init__() that initialize attributes, an attempt to initialize an attribute not declared in __init__() will be signaled as a warning: "Instance attribute key defined outside __init__".

Can there be more than one __init__() methods, each with a different set of arguments, like overloaded constructors?

__str__()

__str__() is used by print(), str() and the string formatters to produce a string representation of an instance.

class MyClass:
  ...
  def __str__(self):
      return self.name + ', ' + self.color

Properties, Getters and Setters

In Python, all attributes are public, they can be accessed from outside the class using the variable name that holds a reference to a class' instance.

However, Python has syntax that allows declaring getter and setter methods to access instance state that is not available in form of an actual instance attribute. The instance state in question is known as a property, which is exposed via getter and setter methods.

Static Initialization at Class Level

To perform an initialization once, upon creation of the type in the class' namespace, place the code directly under the code definition: All methods placed into the class are also "executed" - like when defining a function in the global namespace - but they are not calling.

class MyClass:
  # executed only once upon creation of the type
  pattern = re.compile(r'^(\w+):(\w+)-(\w+)$')

  def __init__(self):
    ...
  def my_method(self):
    ...

The static variable can be accessed inside the class (and outside) by prefixing it with the class name, or, within an instance method, by prefixing it with self:

class MyClass:
  some_var = 'something'

  def some_method(self):
    print('some_var value', MyClass.some_var)
    print('some_var value', self.some_var)

Type Checking

The type of a class instance can be displayed with type().

class MyClass:
  ...

i = MyClass()
print(type(c))

will display:

<class '__main__.MyClass'>

Appurtenance to a specific type can be verified with:

assert type(i) is MyClass

Objects

In Python, everything is an object. To create an instance of a class, use the class name followed by left parenthesis, followed by constructor arguments, followed by the right parenthesis.

o2 = SomeOtherClass('something')

If only the class name without parentheses is used, the code will work, but the variable will contain a reference to the class, not the instance of the class.

The ID of any object can be obtained with the built-in function id()

Duck Typing

.