COP 1000c Lab Assignment 7

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GaddisPythonChapter6FilesandExceptions.ppt

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C H A P T E R 6

Files and Exceptions

Topics

Introduction to File Input and Output
Using Loops to Process Files
Processing Records
Exceptions

Introduction to File Input and Output

For program to retain data between the times it is run, you must save the data

Data is saved to a file, typically on computer disk

Saved data can be retrieved and used at a later time

“Writing data to”: saving data on a file
Output file: a file that data is written to

Introduction to File Input and Output (cont’d.)

“Reading data from”: process of retrieving data from a file
Input file: a file from which data is read
Three steps when a program uses a file

Open the file

Process the file

Close the file

Types of Files and File Access Methods

In general, two types of files

Text file: contains data that has been encoded as text

Binary file: contains data that has not been converted to text

Two ways to access data stored in file

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

Filenames and File Objects

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

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

Filenames and File Objects (cont’d.)

Opening a File

open function: used to open a file

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

General format:

file_object = open(filename, mode)

Mode: string specifying how the file will be opened

Example: reading only ('r'), writing ('w'), and appending ('a')

Specifying the Location of a File

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

Can specify alternative path and file name in the open function argument

Prefix the path string literal with the letter r

Writing Data to a File

Method: a function that belongs to an object

Performs operations using that object

File object’s write method used to write data to the file

Format: file_variable.write(string)

File should be closed using file object close method

Format: file_variable.close()

Reading Data From a File

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

readline method: file object method that reads a line from the file

Line returned as a string, including '\n'

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

Concatenating a Newline to and Stripping it From a String

In most cases, data items written to a file are values referenced by variables

Usually necessary to concatenate a '\n' to data before writing it

Carried out using the + operator in the argument of the write method

In many cases need to remove '\n' from string after it is read from a file

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

Appending Data to an Existing File

When open file with 'w' mode, if the file already exists it is overwritten
To append data to a file use the 'a' mode

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

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

Writing and Reading Numeric Data

Numbers must be converted to strings before they are written to a file
str function: converts value to string
Number are read from a text file as strings

Must be converted to numeric type in order to perform mathematical operations

Use int and float functions to convert string to numeric value

Using Loops to Process Files

Files typically used to hold large amounts of data

Loop typically involved in reading from and writing to a file

Often the number of items stored in file is unknown

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

Can write a while loop with the condition

while line != ''

Using Python’s for Loop to Read Lines

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

Format: for line in file_object:

statements

The loop iterates once over each line in the file

Processing Records

Record: set of data that describes one item
Field: single piece of data within a record
Write record to sequential access file by writing the fields one after the other
Read record from sequential access file by reading each field until record complete

Processing Records (cont’d.)

When working with records, it is also important to be able to:

Add records

Display records

Search for a specific record

Modify records

Delete records

Exceptions

Exception: error that occurs while a program is running

Usually causes program to abruptly halt

Traceback: 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 exception to be raised

Exceptions (cont’d.)

Many exceptions can be prevented by careful coding

Example: input validation

Usually involve a simple decision construct

Some exceptions cannot be avoided by careful coding

Examples

Trying to convert non-numeric string to an integer

Trying to open for reading a file that doesn’t exist

Exceptions (cont’d.)

Exception handler: code that responds when exceptions are raised and prevents program from crashing

In Python, written as try/except statement

General format: try:

statements

except exceptionName:

statements

Try suite: statements that can potentially raise an exception

Handler: statements contained in except block

Exceptions (cont’d.)

If statement in try suite raises exception:

Exception specified in except clause:

Handler immediately following except clause executes

Continue program after try/except statement

Other exceptions:

Program halts with traceback error message

If no exception is raised, handlers are skipped

Handling Multiple Exceptions

Often code in try suite can throw more than one type of exception

Need to write except clause for each type of exception that needs to be handled

An except clause that does not list a specific exception will handle any exception that is raised in the try suite

Should always be last in a series of except clauses

Displaying an Exception’s Default Error Message

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

Example: except ValueError as err:

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

The else Clause

try/except statement may include an optional else clause, which appears after all the except clauses

Aligned with try and except clauses

Syntax similar to else clause in decision structure

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

The finally Clause

try/except statement may include an optional finally clause, which appears after all the except clauses

Aligned with try and except clauses

General format: finally:

statements

Finally suite: block of statements after the finally clause

Execute whether an exception occurs or not

Purpose is to perform cleanup before exiting

What If an Exception Is Not Handled?

Two ways for exception to go unhandled:

No except clause specifying exception of the right type

Exception raised outside a try suite

In both cases, exception will cause the program to halt

Python documentation provides information about exceptions that can be raised by different functions

Summary

This chapter covered:

Types of files and file access methods

Filenames and file objects

Writing data to a file

Reading data from a file and determining when the end of the file is reached

Processing records

Exceptions, including:

Traceback messages

Handling exceptions