Python Midterm Prep

Text file

contains data that has been encoded as text

Binary file

contains data that has not been converted to text

Sequential access

file read sequentially from beginning to end, can’t skip ahead

Direct access

can jump directly to any piece of data in the file

what are the two general types of files

Text File

Binary file

what are the two ways to access data stored in file ?

sequential access

direct access

open function

used to open a file ,

creates a file object and associates it with a file on the disk

General format of the open function

file_object = open(filename, mode)

Mode

string specifying how the file will be opened

reading (‘r’), writing (‘w’), appending (‘a’)

what does this do ? customer_file = open('customers.csv', 'r’)

opens the file customers.csv and it is stored in the object variable ‘customer_file’

what does this do ? sales_file = open('sales.txt', 'w’)

creates a file named sale.txt and can write data to it through the object variable ‘sales_file’

If a file with the same name exists in the current directory, it will be overwritten

true

If open function receives a filename that does not contain a path, it assumes that file is in same directory as program

True

Method

a function that belongs to an object

performs operations using that object

What is an example of a method?

name = input(“what is your name? “)

print(name.capitalize())

how would you write data to a file ?

file_variable.write(string)

How would you close a file ?

file_variable.close()

what does the rstrip method do ?

Strips specific characters from end of the string

Is it necessary to concatenate a “\n’ to data before writing it ?

Yes and you do so by adding a + to it.

read method

file object method that reads entire file contents into memory

• only works if file has been opened for reading

• contents returned as a string

What is the format of the read method ?

file_variable.read()

readline method

file object method that reads a line from the

line returned as a strong, including ‘\n’

Read Position

marks the location of the next item to be read from a file

when open file with ‘w’ mode, if the file exists it is overwritten

true

Appending attributes

If file exists, it is not erased, and if it does not exists it is created

Data is written to the file at the end of the current contents

Numbers must be converted to strings before they are written to a file

True

str function

converts value to string

How do you convert a string to a numeric value ?

use int and float functions

the readline method uses an empty string as a sentinel when the end of file is reached

True

Dictionary

object that stores a collection of data

each element consists of a key and a value

-often referred to as mapping of key to value

- key must be an immutable object

To retrieve a specific value, use the key associated with it

true

Example of a dictionary

phonebook = {‘Chris’: ‘555-’1111’, ‘Katie’ : ‘555-2222’}

How to retrieve a value from a dictionary

dictionary[key]

If key in the dictionary, associated value is returned, otherwise,

KeyError exception is raised

How do you test whether a key is in a dictionary?

Use the in and not in operators

how do you add a key value to a dictionary?

dictionary[key] = value

if key exists in the dictionary, the value associated with it will be changed

how do you delete a key-value pair":

del dictionary[key]

• del phonebook[‘Chris’]

what is purpose of the len function ?

used to obtain number of elements in a dictionary

Can values be stored in a single dictionary as different types ?

Yes

to create an empty dictionary:

Use { }

Use built-in function dict ()

what do items methods do ?

returns all the dictionaries keys and associated values

What does the clear method do?

deletes all the elements in a dictionary, leaving it empty,

Format: dictionary.clear()

What does the get method do?

gets a value associated with specified key from the dictionary

dictionary.get(key, default)

What does the key method do ?

Returns all the dictionaries keys as a sequence

dictonary.keys()

What does the pop method ?

returns value associated with specified key and removes that key-value pair from the dictionary

dictionary.pop(key, default)

what does the value method do ?

returns all the dictionary values as a sequence

dictionary.values()

what is a set in dictionary ?

an object that stores a collection of data in same way as mathematical set

• all items must be unique

• set is unordered

• Elements can be of different data types

what does the set function do ?

used to create a set

what does the add method do ?

adds an element to a set

what does an update method do ?

adds a group of elements to a set

What do remove and discard methods do ?

remove the specified item from the set

• the item that should be removed is passed to both methods as an argument

• behave differently when the specified item is not found in the set

What does the clear method do ?

clears all the elements of the set

intersection of two sets

a set that contains only the elements found in both sets

difference of two sets

a set that contains the elements that appear in the first set do no not appear in the second set

what does it mean to serialize an Object ?

convert the object to a stream of bytes that can easily be stored in a file

what does pickling mean ?

serializing an object

Chapter 10 Classes and OOP

Chapter 10

Procedural Programming

Writing programs made of functions that perform specific tasks

Procedures typically operate on data items that are separate from the procedures,

Data items commonly passed from one procedure to another

What is the Focus of Procedural Programming?

to create procedures that operate on the program’s data

Object-oriented programming

focused on creating objects

Data is known as data attributes

Procedures are known as methods

what is an object?

an entity that contains data and procedures

What is encapsulation?

A self contained unit combining data and code into a single object

An object contains data attributes and methods

True

What is Data Hiding (making Private) ?

Objects data attributes are hidden from code outside of the object

Access restricted to the object’s methods

• protects from accidental corruption

  • outside code does not need to know internal structure of the object

What is object Reusability ?

The same object can be used in different programs

What does outside code interact with in OOP ?

the object methods

What is a purpose of a class ?

Code that specifies the data attributes and methods of a particular type of object

• similar to a blueprint of a house or a cookie cutter

What is an instance ?

an object created from a class

• similar to a specific house built according to the blueprint

  • there can by many instances of one class.

What is the definition of a class ?

set of statements that define a class’s methods and data attributes

What does the self parameter method do?

references the specific object that the method is working on

- required in every method in the class

What is the Object’s state ?

the values of the object’s attribute at a given moment

what is the __str_ method?

displays the objects state

• automatically called when the object is passed as an argument to the print function

  • Automatically called when the object is passed as an argument to the str function

What does the accessor method do ?

Returns a value from a class’s attribute without changing it

• safe way for code outside the class to retrieve the value of attributes

What does the mutator method do ?

Stores or changes the value of a data attribute

What is a Instance attribute?

belongs to a specific instance of a class

• created when a method uses the self parameter to create an attribute

What happens if many instances of a class are created?

Each would have its own set of attributes

What do methods and functions need ?

to accept objects and arguments

What are you passing when you pass an object as an argument ?

You are actually passing a reference to the object

• therefore the receiving method or function has access to the actual object.

what is the UML diagram ?

the standard diagrams for graphically depicting object-oriented systems

Chapter 11

Chapter 11 Inheritance

What do we need inheritance in coding?

In the real world, many objects are a specialized version of more general objects

What is the “is a” relationship ?

exists when one object is a specialized version of another object

• Daisy is a flower

• Rectangle is a shape

What is inheritance ?

used to create an “is a” relationship between classes

What is a superclass (base class) ?

A general class

What is a subclass (derived class) ?

A specialized class

• an extended version of the superclass

• inherits attributes and methods of the superclass

  • New attributes and methods can be added

How do you indicate inheritance in code ?

class Car(Automobile):

• the initializer method of a subclass calls the initializer method of the superclass and then initializes the unique data attributes

• Add method definitions for unique methods

What is polymorphism ?

It is an objects ability to take different forms

What are essential ingredients in polymorphic behavior ?

Ability to define a method in a superclass and override it in a subclass

Ability to call the correct version of overridden method depending on the type of object that called for it

AttributeError exception:

raised when a method receives an object which is not an instance of the right class

isinstance function:

determines whether object is an instance of a class

• isinstance(object, class)

Chapter 13

Chapter 13 GUI programming

What is the User interface ?

the part of the computer with which the user interacts

What is the Command line interface ?

displays a prompt and the user types a command that is then executed

Graphical User interface (GUI) ?

allows users to interact with a program through graphical elements on the screen

What is the difference between text-based and GUI environments ?

text-based has programs determining the order in which things happen, the GUI is event-driven.

tkinter module purpose ?

allows you to create simple GUI programs

Widget

graphical element that the user can interact with or view

presented by a GUI program

What builds the GUI ?

the __init__

Label Widget

displays a single line of text in a window

made by creating an instance of tkinter module’s Label class

pack method

determines where a widget should be positioned and makes it visible when the main window is displayed

Frame Widget

container that holds others widgets

• useful for organizing and arranging groups of widgets in a window

  • The contained widgets are added to the frame widget which contains them