Chapter 6 - Filenames and exceptions

Question 1

Define Persistence

Answer 1

Programs that run for a long time, or all the time.
Some data is in the permanent storage(hard drive)
A shutdown/restart sequence has them picking up where the program ended.

Examples: operating systems, web servers, apps on phone

Question 2

Define Transient

Answer 2

These programs run for a short time and produce some output but when they end the data disappears. If you run the program again it starts with a clean slate.

Question 3

Output File

Answer 3

A file that data is written to it

Question 4

Input file

Answer 4

a file from which data is read

Question 5

What are the 3 steps when a program uses a file?

Answer 5

Open file, process file, close file

Question 6

What are the two general types of files?

Answer 6

Text file
Binary File

Question 7

What is a text file?

Answer 7

contains data that has been encoded as text

Question 8

What is a binary file?

Answer 8

Contains data that has not been converted to text. (faster for computer because it is already in binary)

Question 9

What are the two ways to access data stored in a file:

Answer 9

sequential access, direct access

Question 10

Define Sequential access

Answer 10

File read sequentially from beginning to end, can’t skip ahead (cassette tape)

Question 11

Define Direct access

Answer 11

can jump directly to any piece of data in a file ( a CD)

Question 12

What are filename extensions?

Answer 12

short sequences of characters that appear at the end of a filename preceded by a period.

Extension indicates type of data stored in the file.

Question 13

What is a file object?

Answer 13

object associated with a specific file

provides a way for a program to work with the file. file object referenced BY A VARIABLE.

File objects allow us to use,access, and. manipulate all the user accessible files.

Question 14

What does the open function do?

Answer 14

used to open a file.
creates a file object and associates it with a file on the disc.

Format:
file_object = open(‘filename’,’mode’)

Question 15

What is a mode? what are the three types that you can use when opening a file?

Answer 15

A mode is a string specifiying how the file will be opened.

‘r’ = read
‘w’ = write
‘a’ = append

Question 16

What is a method?

Answer 16

A function that belongs to an object.

performs operations using that object.

example: file objects write method used to write data to a file
format: file_variable.write(string)

Question 17

What is the read method?

Answer 17

file object method that reads entire file contents into memory.
Only works if the file was opened for reading.
The contents are returned as a string

Question 18

What is the readline method?

Answer 18

file object method that reads a line from the file

Line returned as a string, including ‘\n’

Question 19

Define read position:

Answer 19

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

Question 20

What is the rstrip method?

Answer 20

string method that strips specific characters from the end of the string

Question 21

What happens if you open the file in ‘w’ mode if the file already exists

Answer 21

the file will be overwritten

Question 22

How do you write numbers to a file

Answer 22

You must first convert them to a string

Question 23

What does the str function do:

Answer 23

converts a value to a string

numbers are read from a text file as strings.
Must be converted to numeric type in order to perform mathematical calculations.

Use int and float functions to convert strings to numeric values.

Question 24

Which method uses an empty string as a sentinel when end of file is reached?

Answer 24

readline method.

while line != ‘ ‘ #does not equal empty

Question 25

Python allows the programmer to write a for loop that automatically reads lines in a file and stops when end of the file is reached.

Format:
for lin in file_object:
statements

How many times will the loop iterate over each line in the file?

Answer 25

Once

Question 26

Define record:

Answer 26

a set of data that describes one item

Question 27

describe field:

Answer 27

a single piece of data within a record

Question 28

How do you write a record to a sequential access file?

Answer 28

By writing the fields one after the other

Question 29

How do you read a record from a sequential access file?

Answer 29

By reading each field until record complete

Question 30

When working with records, it is important to be able to do what (5) things?

Answer 30

1.) Add records
2.) display records
3.) search for a specific record
4.) modify records
5.) delete records

Question 31

Define a directory: (folders)

Answer 31

they organize files within memory. Each time a program runs it has a current directory which is the default directory for most operations.

Question 32

What does the os(operating system) library provide?

Answer 32

functions for working with files and directories.

Question 33

what does the following code snippet do:

import os
cwd = os.getcwd()
cwd

Answer 33

returns the name of the current working directory

Question 34

define a path:

Answer 34

it identifies a file or directory where a file is stored in memory.

Example c:/mydocuments/python/practice.py

Question 35

os.path.isfile does what?

Answer 35

checks whether it is a file

Question 36

os.path.join does what

Answer 36

takes a directory and file name and joins into a complete path

Question 37

os.walk does what?

Answer 37

generates the file names in a directory tree by walking the tree either top-down or bottom-up. For each directory in the tree rooted at directory top(including top itself), it yields a 3-tuple (dirpath, dirnames, filenames)

Question 38

define exception:

Answer 38

error that occurs while program is running, usually causes a program to abruptly halt

Question 39

define trace back:

Answer 39

error message that gives information regarding line numbers that caused the exception. indicates the type of exception and brief description of the error that caused the exception to be raised.

Question 40

How can many exceptions be prevented

Answer 40

careful coding
ex: input validation

some exceptions cannot be avoided by careful coding
ex: trying to convert non-numeric string to an integer
ex: trying to open for reading a file that doesn’t exist

Question 41

What is an exception handler?

Answer 41

code that represents when exceptions are raised and prevents program from crashing.

in python written as try/except statement

try:
statements
except exceptionName:
statements

Question 42

What is a try suite?

Answer 42

statements that can potentially raise an exception

Question 43

What is a handler:

Answer 43

statements contained in an except block

Question 44

exception object:

Answer 44

object created in memory when an exception is thrown.

usually contains default error message pertaining to the exception

can assign the exception object to a variable in an except clause.

can pass exception object to variable to print function to display the default error message.

Question 45

Try/ Except statement may include and optional else clause which:

Answer 45

which appears after all the except clauses.

Is aligned with try and except clauses
syntax similar to else clause in decision structure.

Question 46

What is an else suite:

Answer 46

block of statements executed after statements in try suite only if no exceptions were raised.

If exception was raised, the else suite is skipped.

Question 47

Try/except statements may include an optional finally clause:

Answer 47

which appears after all the except clauses

aligned with try and except clauses.

format:
finally:
statements

Question 48

what is the finally suite

Answer 48

block of statements after the finally clause.

execute whether an exception occurs or not

purpose is to perform cleanup before exiting

Question 49

What are the two ways an exception can go unhandled?

Answer 49

1.) no except clause specifying exception of the right type

2.) exception raised outside a try suite

In both cases exception will cause program to halt.