Computer Science 112

Fundamentals of Programming IIInterfaces and Implementations

What Is an Interface?

• The public or external view of a resource

• What you see when you run help on a resource

• Example: Python’s math module – a set of function names and constant names

What Is an Implementation?

• The private or internal view of a resource

• What you see when you examine the full code of a resource

def fib(n): """Returns the nth Fibonacci number.""" if n == 1 or n == 2: return 1 else: return fib(n – 1) + fib(n – 2)

Interface

Implementation

Types of Interfaces

• Function – The function’s header and docstring

• Class – The class and method headers and docstrings

• Module – The class and function headers

Interface and Implementation

• Users and implementers have different responsibilities

• A interface provides an abstraction barrier between users and implementers

User’s code Interface Implementer’s code

Why the Barrier?

• The resource is easier to learn

• Can plug and play resources

• Can provide several implementations of the same resource (polymorphism)

• Can maintain a resource without disturbing users’ code

Example Resource: A Bag Collection

• A bag is a container where you can put things, see if they’re there, visit them all, or remove each one

• There is one interface or set of methods for all bag implementations

• Two common implementations use an array or a linked structure

UML Class Diagram for Bags

LinkedBag ArrayBag

BagInterface

The Unified Modeling Language is a way of describing relationships among resources visually

means implements an interface

b.isEmpty() Returns True if empty, False otherwise

len(b) Returns the number of items in the bag

str(b) Returns a string representation of the bag

item in b Returns True if item is present in the bag, or False otherwise

for item in b: Visit each item in the bag

b.add(item) Adds item to the bag

b.remove(item) Removes item from the bag

b.clear() Removes all items from the bag

b1 + b2 Combines items in a new bag and returns it

b == anything True if equal, False otherwise

The Interface for All Bags

b.isEmpty() isEmpty(self)

len(b) __len__(self)

str(b) __str__(self)

item in b __contains__(self, item)

for item in b: __iter__(self)

b.add(item) add(self, item)

b.remove(item) remove(self, item)

b.clear() clear(self)

b1 + b2 __add__(self, other)

b == anything __eq__(self, other)

Actual Methods in Python

Bag Implementations

• Array-based

• Linked structure

• Always a single set of abstract operations

bag1 = LinkedBag()

bag2 = ArrayBag([1, 2, 3, 4])

bag = LinkedBag() # or ArrayBag()

bag.add("A string")bag.add("Another string")

for item in bag: print(item)

secondBag = LinkedBag(bag)thirdBag = bag + secondBag

print(secondBag == thirdBag)

for item in bag: secondBag.remove(item)

print(len(secondBag))

Example Use of a Bag

We have a single set of abstract operations and two implementing classes, ArrayBag and LinkedBag

Docstrings and Preconditions

def remove(self, item): """Removes item from self. Precondition: item is in self. Raises: KeyError if item is not in self."""

A docstring states what the method does.

A precondition states what must be true for a method to run correctly.

The method raises an exception if a precondition is not true.

Implementation: Data

• The first step is to decide what the attributes are and represent these using the appropriate data

• Will name these data with class or instance variables

• The __init__ method sets these up

Composition and Aggregation

means “is composed of”

LinkedBagNode*

ArrayBag

BagInterface

Array

* means “aggregates zero or more within”

from arrays import Array

class ArrayBag(object): """An array-based bag implementation."""

# Class variable DEFAULT_CAPACITY = 10

# Constructor def __init__(self, sourceCollection = None): """Sets the initial state of self, which includes the contents of sourceCollection, if it's present.""" self._items = Array(ArrayBag.DEFAULT_CAPACITY) self._size = 0 if sourceCollection: for item in sourceCollection: self.add(item)

Data for ArrayBag

# Accessor methodsdef isEmpty(self): """Returns True if len(self) == 0, or False otherwise.""" return len(self) == 0 def __len__(self): """Returns the number of items in self.""" return self._size

def __str__(self): """Returns the string representation of self.""" return "{" + ", ".join(map(str, self)) + "}"

Some Accessor Methods

Try to run methods within an implementation, rather than accessing variables directly.

# Mutator methodsdef clear(self): """Makes self become empty.""" self._size = 0 self._items = Array(ArrayBag.DEFAULT_CAPACITY)

def add(self, item): """Adds item to self.""" # Resize the array here if necessary self._items[len(self)] = item self._size += 1

Some Mutator Methods

from node import Node

class LinkedBag(object): """A link-based bag implementation."""

# Constructor def __init__(self, sourceCollection = None): """Sets the initial state of self, which includes the contents of sourceCollection, if it's present.""" self._items = None self._size = 0 if sourceCollection: for item in sourceCollection: self.add(item)

Data for LinkedBag

# Accessor methodsdef isEmpty(self): """Returns True if len(self) == 0, or False otherwise.""" return len(self) == 0 def __len__(self): """Returns the number of items in self.""" return self._size

def __str__(self): """Returns the string representation of self.""" return "{" + ", ".join(map(str, self)) + "}"

Some Accessor Methods

Hooray! No changes from the same methods in ArrayBag!

# Mutator methodsdef clear(self): """Makes self become empty.""" self._size = 0 self._items = None

def add(self, item): """Adds item to self.""" self._items = Node(item, self._items) self._size += 1

Some Mutator Methods

The container is now a linked structure, so it must be manipulated differently.

For Friday

Iterators