Chapter 6 - Filenames and exceptions Flashcards
Define Persistence
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
Define Transient
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.
Output File
A file that data is written to it
Input file
a file from which data is read
What are the 3 steps when a program uses a file?
Open file, process file, close file
What are the two general types of files?
Text file
Binary File
What is a text file?
contains data that has been encoded as text
What is a binary file?
Contains data that has not been converted to text. (faster for computer because it is already in binary)
What are the two ways to access data stored in a file:
sequential access, direct access
Define Sequential access
File read sequentially from beginning to end, can’t skip ahead (cassette tape)
Define Direct access
can jump directly to any piece of data in a file ( a CD)
What are filename extensions?
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.
What is a file object?
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.
What does the open function do?
used to open a file.
creates a file object and associates it with a file on the disc.
Format:
file_object = open(‘filename’,’mode’)
What is a mode? what are the three types that you can use when opening a file?
A mode is a string specifiying how the file will be opened.
‘r’ = read
‘w’ = write
‘a’ = append
What is a method?
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)
What is the read method?
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
What is the readline method?
file object method that reads a line from the file
Line returned as a string, including ‘\n’
Define read position:
marks the location of the next item to be read from a file
What is the rstrip method?
string method that strips specific characters from the end of the string
What happens if you open the file in ‘w’ mode if the file already exists
the file will be overwritten
How do you write numbers to a file
You must first convert them to a string
What does the str function do:
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.
Which method uses an empty string as a sentinel when end of file is reached?
readline method.
while line != ‘ ‘ #does not equal empty
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?
Once
Define record:
a set of data that describes one item
describe field:
a single piece of data within a record
How do you write a record to a sequential access file?
By writing the fields one after the other
How do you read a record from a sequential access file?
By reading each field until record complete
When working with records, it is important to be able to do what (5) things?
1.) Add records
2.) display records
3.) search for a specific record
4.) modify records
5.) delete records
Define a directory: (folders)
they organize files within memory. Each time a program runs it has a current directory which is the default directory for most operations.
What does the os(operating system) library provide?
functions for working with files and directories.
what does the following code snippet do:
import os
cwd = os.getcwd()
cwd
returns the name of the current working directory
define a path:
it identifies a file or directory where a file is stored in memory.
Example c:/mydocuments/python/practice.py
os.path.isfile does what?
checks whether it is a file
os.path.join does what
takes a directory and file name and joins into a complete path
os.walk does what?
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)
define exception:
error that occurs while program is running, usually causes a program to abruptly halt
define trace back:
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.
How can many exceptions be prevented
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
What is an exception handler?
code that represents when exceptions are raised and prevents program from crashing.
in python written as try/except statement
try:
statements
except exceptionName:
statements
What is a try suite?
statements that can potentially raise an exception
What is a handler:
statements contained in an except block
exception object:
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.
Try/ Except statement may include and optional else clause which:
which appears after all the except clauses.
Is aligned with try and except clauses
syntax similar to else clause in decision structure.
What is an else suite:
block of statements executed after statements in try suite only if no exceptions were raised.
If exception was raised, the else suite is skipped.
Try/except statements may include an optional finally clause:
which appears after all the except clauses
aligned with try and except clauses.
format:
finally:
statements
what is the finally suite
block of statements after the finally clause.
execute whether an exception occurs or not
purpose is to perform cleanup before exiting
What are the two ways an exception can go unhandled?
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.
