ProgFund_Lecture_6_Files_and_Exception_Handling-3.pdf

Programming Fundamentals
Lecture 6: Files and Exception Handling

This Lecture
• Files
– Opening, using and closing files
– File opening modes
– Writing to files
– Reading entire files, reading lines and bytes
• Exception handling
– Exception handlers, termination and continuation
– Exception names
– Try and catch/except
– Else and finally
2

Textbook
• Starting Out with Python, 4th Edition (Global)
– Tony Gaddis
• Reading the chapter(s) is required
– Read the indicated chapter(s) before class
• This week covers the following textbook chapter(s):
– Chapter 6 – Files and Exceptions
• Entire chapter
3

Files
• So far, our programs have been entirely “self-contained”
– Nothing is stored or kept between each run of the program
– Input we provide to the program is stored in RAM, output is shown on the screen…
Nothing lasts beyond end of program
• To retain data, receive input or store output, programs often use files on the
computer’s hard drive:
– Settings and configuration files of programs
– “Office Suite” files such as word processed documents, spreadsheets and slideshows
– Media files created and used by image, video and music apps
– Cache and cookie files created and used by web browsers
– Assets, save files and replays of games…
5

Files
• Files can be used for both input and output
– Reading data from a file allows you to use it as input
– Writing data to a file allows you to store/save output in it
• Files come in two main types:
– Text files simply contain text, encoded in ASCII or Unicode, and can be opened by
programs like Notepad
• Text can be formatted or structured in specific ways to make it more useful, e.g. CSV, JSON
• The extension is not necessarily “.txt” – e.g. Python programs (“.py”) are just text files
– Binary files are not text-based and are designed to only be opened by a program that
can use the data in that format
• Includes everything that isn’t a text file (images, exes, audio files, video files…)
6

Files
• Working with files in a program involves the same general process, regardless of
the programming language you are using:
1. Open the file in the program:
• If reading from a file, it must obviously exist
• If writing to a file, it must be created if it doesn’t exist. If it does, you can either overwrite its
contents or modify/append to them
– Opening a file in a program leaves you with a variable representing it (a “file object”),
which you interact with in the following steps
2. Use the file
• Reading data from the file, writing data to the file, or both
3. Close the file
• Once you have finished using the file, closing it severs its connection with the program and
releases the resources. It also ensures that anything you wrote to the file is actually saved
7

Opening a File
• Opening a file requires you to specify the filename
– If the file does not exist, it will be created
– If the file is not in the same folder as the program, you must specify the path to the file
• You also typically need to specify the mode to open it in
– This controls what you can do with the file once it is open
– These are the most widely-implemented modes, however modes and their
abbreviations can differ slightly between languages:
• Read (“r”) allows reading only (file must exist)
• Write (“w”) allows writing only (creates or overwrites the file, starting out empty)
• Append (“a”) allows writing only (writes to the end of the file, instead of overwriting it)
• Read/Write (“rw” / “r+” / “w+”…) allows reading and writing (slight differences in each mode)
– Some languages require different modes depending on whether it is a text or binary file
8

Opening a File
• In Python, the built-in function “open()” opens a file and returns a file object
– In some languages, a module is needed to interact with files
– These examples open a file named “file.txt”, in “read” mode, into a variable named “f”:
• If the file is in another folder, you must specify the path:
– The “r” at the start of the string makes Python treat it as a “raw” string - treating any
backslashes in the string as literal characters rather than escape characters
• This is useful when the string contains backslashes, which are likely in path names
9
# no module necessary
f = open('file.txt', 'r')
Python
import java.io.FileReader;
FileReader f = new FileReader("file.txt");
Java
// no module necessary
$f = fopen('file.txt', 'r');
PHP
#include <stdio.h>
FILE *f = fopen("file.txt", "r");
C / C++
# no module necessary
f = File.open('file.txt', 'r')
Ruby
f = open(r'D:otherstufffile.txt', 'r') Python

Writing to a File
• Once the file is open in a mode that allows writing, you just need to write data to
it using a “write” function
– Similar to using “print()”, but instead of displaying on the screen, it writes to a file
– Most of the time, you will be starting out from a new/blank file, rather than appending
• Writing to a file uses the variable/object created when the file was opened
– Either by using the “write” method (function) of the file object:
– Or by passing the variable to a “write” function as a parameter:
12
f = open('file.txt', 'w')
f.write('This text will be written to the file!')
f.close()
Python
$f = fopen('file.txt', 'w');
fwrite($f, 'This text will be written to the file!');
fclose($f);
PHP (“fwrite()” is a standalone
function that you pass the
file variable and text to)
(“write()” is a function that
you call on the file object
and pass the text to)

Writing to a File
• Note that while “print()” automatically adds a line break at the end of your line,
“.write()” does not:
• Use “n” to insert line breaks where you need them:
f = open('joke.txt', 'w')
f.write('What do you call a group of 8 hobbits?')
f.write('A hobbyte.')
f.close()
Python
f = open('joke.txt', 'w')
f.write('What do you call a group of 8 hobbits?n')
f.write('A hobbyte.')
f.close()
Python
13

Closing a File
• Once you have finished interacting with the file in your program, you must close it
– Not doing this can result in losing the data you wrote, since the data is often written to
a buffer in memory and only saved to the file on the hard drive when the file is closed
– Not closing a file can also keep it “locked”; Unable to be used by other programs
• This involves using the “close” method on the file object or calling the function to
close a file, depending on language:
• Ideally, minimise the time that a file is open for, e.g.:
– Open it in “read” mode, read the entire text into a variable, close it
– Open/Create it in “write” mode, write everything at once, close it
f.close()
Python $f = fopen('file.txt', 'r');
fclose($f);
PHP
14

“Test Generator” Case Study – Review of Version 1
• Last week we went over a case study, designing and writing a program to
generate randomised maths tests
Prompt user for + or - and store in 'op' variable.
If op is '+'
Print 'Addition Test'
Else
Print 'Subtraction Test'
Set op to '-'
Print 'Name: __________'
Loop 10 times
Generate random number between 0 and 9 and store it as 'num_one'
Generate random number between 0 and 9 and store it as 'num_two'
If op is '-' and num_one < num_two
Swap num_one and num_two
Print 'num_one op num_two = ____'
Pseudocode
15

“Test Generator” Case Study – Review of Version 1
• Last week we went over a case study, designing and writing a program to
generate randomised maths tests
import random
op = input('Addition or subtraction test? [enter "+" or "-"]: ')
if op == '+':
print('Addition Testn')
else:
print('Subtraction Testn')
op = '-'
print('Name: __________nn')
for i in range(10):
num_one = random.randint(0, 9)
num_two = random.randint(0, 9)
if op == '-' and num_one < num_two:
num_one, num_two = num_two, num_one
print(num_one, op, num_two, '= ____n')
Python
Addition Test
Name: ___________
2 + 4 = ____
6 + 5 = ____
1 + 8 = ____
9 + 2 = ____
5 + 4 = ____
6 + 4 = ____
7 + 2 = ____
5 + 3 = ____
2 + 1 = ____
9 + 7 = ____
Note the extra line breaks (“n”) to add extra blank lines in the output
(remember, print() ends with a line break automatically)
16

“Test Generator” Case Study – Writing to a File
• Now let’s change the program so that it writes its output to a file named
“output.txt” instead of showing it on the screen
Prompt user for + or - and store in 'op' variable.
Create and open 'output.txt' file in write mode
If op is '+'
Write 'Addition Test' to output file
Else
Write 'Subtraction Test ' to output file
Set op to '-'
Write 'Name: __________' to output file
Loop 10 times
Generate random number between 0 and 9 and store it as 'num_one'
Generate random number between 0 and 9 and store it as 'num_two'
If op is '-' and num_one < num_two
Swap num_one and num_two
Write 'num_one op num_two = ____' to output file
Close the output file
Print message informing user of the file
Pseudocode
17

• Now the code creates/opens “output.txt”, and instead of printing the output, writes
it to the file before closing it
import random
output = open('output.txt', 'w')
if op == '+':
output.write('Addition Testn')
else:
output.write('Subtraction Testn')
op = '-'
output.write('Name: __________nn')
for i in range(10):
output.write(str(num_one) + ' ' + op + ' ' + str(num_two) + ' = ____n')
output.close()
print('Test saved in output.txt file.')
Python
18

Comparing the Output
Addition Test
Name: ___________
2 + 4 = ____
6 + 5 = ____
1 + 8 = ____
9 + 2 = ____
5 + 4 = ____
6 + 4 = ____
7 + 2 = ____
5 + 3 = ____
2 + 1 = ____
9 + 7 = ____
Output to Screen Output to File
19

• Remember, since “.write()” doesn’t add a line break to the end like
“print()” does, we need to add them… An extra “n” at the end of each line!:
import random
output = open('output.txt', 'w')
if op == '+':
output.write('Addition Testnn')
else:
output.write('Subtraction Testnn')
op = '-'
output.write('Name: __________nnn')
for i in range(10):
output.write(str(num_one) + ' ' + op + ' ' + str(num_two) + ' = ____nn')
output.close()
print('Test saved in output.txt file.')
Python
20

Comparing the Output
Addition Test
Name: ___________
2 + 4 = ____
6 + 5 = ____
1 + 8 = ____
9 + 2 = ____
5 + 4 = ____
6 + 4 = ____
7 + 2 = ____
5 + 3 = ____
2 + 1 = ____
9 + 7 = ____
Output to Screen Output to File
21

Reading from a File
• Reading data from a file can be very useful
– Reading data from files previously created and written to by your program allows it to
“remember” things, e.g. Settings for the program, or loading previously saved data
– Programs can also read and use data from files created by other programs
• This relies on the file’s data being in a specific format, so that it can be read correctly by
different programs
– We will continue to work with text files, since they have a simple format that is easy to
understand and work with
• Reading from a file allows your program to have a new source of input;
Something other than the user typing
Be sure to go through Reading 7.1 next week – it goes over some ways of
giving structure to text files and is likely to be relevant to Assignment 2!
22

Reading from a File
• Like writing, reading a file uses the variable/object created when the file was
opened. The same two approaches exist…
– Using a “read” method of the file object:
– Passing the file variable to a “read” function as a parameter:
• The “read” methods/functions will return the data they read
– Store it in a variable so that you can then use the data in your program
– Most of the time you will read an entire file and store it in a variable,
and then interact with the variable (rather than reading a file one piece at a time)
23
file_data = f.read(10)
f.close()
Python
$f = fopen('file.txt', 'r');
$file_data = fread($f, 10);
fclose($f);
PHP

Quickly Reading an Entire File
• The simplest way to read a file is to read the entire thing at once, giving you a
variable containing the entirety of the file’s content
– Some languages can open, read and close a file in a single function
– Other languages have the ability to read all of a file at once, but still require you to
open and close it in separate statements
– Some languages (e.g. C/C++) require you to specify how many bytes or characters to
read. To read the entire file in such languages, can…
1) Use other statements to determine the total size/length of the file, and read that many
2) Use a loop to continually read bytes/characters until you reach end of file
24
$file_string = file_get_contents('file.txt'); //returns a string
$file_array = file('file.txt'); //returns an array (one item per line of text)
PHP
file_data = f.read() # not giving read() a number will read the whole file
f.close()
Python

Reading Bytes and Reading Lines
• The “.read()” method in Python can be given an integer parameter to specify
how many bytes/characters to read
– Or use the “readline()” method to read a whole line
(all data until the next line break - “n” - or the end of the file)
• “.read()” and “.readline()” read data from the current “position” in the file.
When you open a file in read mode, the position is set to the start of the file
– After you read data, the position moves to the end of that data
– Hence, you can use “.read()” and/or “.readline()” to work your way through a
file from beginning to end
25
data = f.read(10) # read 10 bytes
f.close()
Python f = open('file.txt', 'r')
data = f.readline() # read a line
f.close()
Python

Reading Through a File
• A “for” loop will work its way through a file line by line:
• The “.readlines()” (note the “s”) method puts all lines of a file into a list:
26
f = open('veg.txt', 'r')
for line in f:
print(line)
f.close()
Python
f = open('veg.txt', 'r')
file_list = f.readlines()
f.close()
Python
Leek
Potato
Carrot
file_list = ['Leekn',
'Potaton',
'Carrot']

Reading and Writing Numbers
• “.read()” and “.readline()” will always return a string, even if the data is
numeric (just like the “input()” function)
– Use “int()” or “float()” to convert the data to a number if needed:
– The “.write()” method can only write strings; Use “str()” to convert numbers to
strings, and add a “n” if needed:
f = open('numbers.txt', 'r')
num_one = f.readline()
num_two = int(f.readline())
f.close()
Python
num_one set to '123n' # string
num_two set to 456 # int
f = open('numbers.txt', 'a')
num_three = 789
num_four = 123
f.write(str(num_three) + 'n' + str(num_four))
f.close()
Python
28

Files Summary
• Files allow you to store and use data external to a program
– i.e. Files allow for a new source of input and new destination for output
– Files can be either text or binary files
• To use a file, you must first open / create it
– This creates a file object/variable that is used to interact with it
– Files can be opened in various modes, read, write, append…
• Once a file is open, you can:
– Write data to it (from the beginning, or appending to the end)
– Read data from it (all at once, by byte, by line, line-by-line…)
• Once you have finished interacting with a file, close it
29

Exception Handling
• An exception is an error or condition that occurs while a program is running,
causing it to “crash” or otherwise end
– You’ve no doubt seen a few during the unit, usually related to input and data types,
e.g. trying to convert letters to integer:
• Exceptions are situations that can occur that the program cannot handle,
despite the code being syntactically valid
– Unlike syntax errors, which occur when the code is not valid and will prevent the
program from running
num = int(input('Type a number: '))
print('You typed:', num)
Python
Type a number: nope
Traceback (most recent call last):
File "H:/PythonCode/exception_example.py", line 1, in <module>
ValueError: invalid literal for int() with base 10: 'nope'
31

Exception Handling
• Exception handling is, unsurprisingly, the process of responding to exceptions
when they occur – i.e. handling them
– When an exception occurs, it is said to have been “raised”, “thrown” or “triggered”,
depending on the language
– Your response will depend on the nature of the exception:
• The program may be unable to continue, hence the response will be to display an error
message and end in a clean manner – known as “termination”
• The program may be able to continue, e.g. re-prompting for input or performing some other
action to work around the exception – known as “continuation”
– Exception handling is essentially another control structure, like selection and iteration
(loop) statements:
• It allows you to control the flow of execution in a program when an exception occurs,
transferring control to an exception handler
32

Exception Handling
• So far in the unit, we have not handled the majority of exceptions that could occur
in the code we have written
– e.g. We’ve assumed that the user enters a number when prompted to enter a number
(which can then be converted to an int/float without an exception occurring…)
– This has been to keep our code simple and understandable, avoiding added
complexity while we are learning the basics
• However we have implemented exception handling on occasion,
e.g. re-prompting for input until a number is typed
– e.g. Tasks 3 and 4 of Workshop 4 used exception handling in the “input_int()”
and “input_float()” functions we wrote
• It is important that you consider (and handle) the exceptions that could potentially
occur in the code that you write
33

Basic Exception Handling - Termination
• In this example, the exception handler ends the program:
– If a “ValueError” exception is raised when trying to convert the “num” variable to an
integer, the exception handler runs
• The exception handler prints an error message, then uses “sys.exit()” to end the
program (importing “sys” to do so)
– If the exception is not raised (i.e. the conversion to integer was successful), the
exception handler does not run
• The code continues, printing the “You typed” line…
try: # prompt for a number and try to convert it to an integer
except ValueError: # show error message and exit
print('Invalid input. Ending program.')
import sys
sys.exit()
Python
Type a number: nope
Invalid input. Ending program.
34

Basic Exception Handling - Continuation
• This exception handler continues the program:
– If a “ValueError” exception is raised when trying to convert the “num” variable to an
integer, the exception handler runs
• The exception handler just puts an integer of 7 into “num”
• It does not end the program – the code then continues to the “You typed” line after the
exception handler ends
– While this example is quite simplistic, it illustrates the concept of handling an
exception without terminating the program
try: # prompt for a number and try to convert it to an integer
except ValueError: # just use 7
num = 7
Python
Type a number: nope
You typed: 7
35

Basic Exception Handling - Continuation
• This exception handler also continues the program:
– This version places the try/except code inside an endless loop
• If a “ValueError” exception is raised, the code does not reach the “break” statement,
instead going immediately to the exception handler
• After printing the error message, the end of the loop body is reached – back to “try:”…
• If the exception is not raised, the code reaches the “break” statement
– This ends the loop, allowing the program to reach the “You typed” line…
36
while True: # endless loop
try: # prompt for a number and convert it to an integer
break # break out of the loop
except ValueError: # show error message
print("Invalid input, try again.")
Python
Type a number: nope
Invalid input, try again.
Type a number: ok, fine
Invalid input, try again.
Type a number: 6
You typed: 6

Exception Names
• Different types of exceptions can occur, and each type of exception that the
language recognises is given a name
– The exceptions that different languages recognise and the names they have differ
• Some of the common exceptions in Python are:
– NameError, raised when you try to refer to a variable or function that doesn’t exist
• e.g. spelling its name wrong
– TypeError, raised when you try to use the wrong data type
• e.g. round('this is a string', 2)
– ValueError, raised when you use the right data type, but the value is not appropriate
• e.g. int('not numeric')
– IndexError, raised when you try to refer to an index that doesn’t exist
• e.g. referring to an index of 5 in a list of 3 items
37

Try and Catch/Except
• Most languages implement exception handling as follows:
– Code that might cause an exception is placed in a “try” block
– This is followed by a “catch” block, containing the code that will be run if (and only if)
the exception occurs
• In Python, the word “except” is used instead of “catch”
• If the exception occurs, the program immediately goes to the start of the “catch” block
(skipping later lines in the “try” block)
try:
<statement(s) that can cause exceptions>
catch <exception name>:
<statements(s) to run if exception occurs>
This is known as an
“exception handler”
38

• It is optional to specify an exception name after “except”
– If you just write “except:”, that exception handler will catch all exceptions – useful if
various things could go wrong and it is appropriate to handle them all in the same way
• You can specify multiple “except” blocks after a “try” block
– You can specify a different exception name in each one, allowing you specify different
exception handlers for each different exception that can occur in the “try” block…
try:
<statement(s) that can cause exceptions>
except <exception name>:
<exception handler for that exception>
except:
<exception handler for any other exception>
39

• This example demonstrates multiple exceptions/handlers:
40
Try to:
Prompt for a filename and store in 'filename' variable
Open the file named in 'filename' in read mode
Read all lines of the file into 'line_list' list variable
Print the number of lines
Prompt for which line to view and store in 'line_num' int variable
Print line of the file, using 'line_num' - 1 as index of 'line_list'
Except if the file cannot be found:
Print appropriate error message
Except if the user’s input cannot be converted to an integer:
Except if an invalid line number is specified:
Except if any other exception occurs:
Print generic error message
Pseudocode

• This example demonstrates multiple exceptions/handlers:
try:
filename = input('File name to open: ')
f = open(filename, 'r') # FileNotFoundError will occur if the file does not exist
line_list = f.readlines()
f.close()
print('File contains', len(line_list), 'lines.')
line_num = int(input('View which line?: ')) # ValueError will occur if input is not an int
print(line_list[line_num - 1]) # IndexError will occur if the number is not a valid index number
except FileNotFoundError:
print('File not found. Check spelling.')
except ValueError:
print('Invalid input. Int required.')
except IndexError:
print('Invalid line number.')
except:
print('Something went wrong.')
Python
41

• Let’s look at the flow of execution through the program:
①
②
③
④
⑤
try:
filename = input('File name to open: ')
f = open(filename, 'r') # potential FileNotFoundError
line_list = f.readlines()
f.close()
print('File contains', len(line_list), 'lines.')
line_num = int(input('View which line?: ')) # potential ValueError
print(line_list[line_num - 1]) # potential IndexError
except FileNotFoundError:
print('File not found. Check spelling.')
except ValueError:
print('Invalid input. Int required.')
except IndexError:
print('Invalid line number.')
except:
print('Something went wrong.')
Python
File name to open: file.txt
File contains 4 lines.
View which line?: nope
Invalid input. Int required.
42

Else and Finally
• After all the “except” blocks, you can optionally include:
– An “else” block, which will only run if no exceptions occurred
– A “finally” block, which will always run
• Runs after the “try” block if no exceptions occur, or after the exception handler if they do
• This is often used for “cleanup” (e.g. closing a file/connection), since it is sure to always run
try:
try statement 1
try statement 2
try statement 3
handler statements
else:
else statements
try:
try statement 1
try statement 2
try statement 3
handler statements
finally:
finally statements
43

Exception Handling Summary
• Exception handling allows you to catch and handle errors or conditions that
would otherwise cause a program to crash
• Statements that can raise an exception are placed in a “try”
– If an exception occurs when run the statements, the program immediately goes to the
exception handler for it
• Exception handlers are “catch”/”except” blocks after a “try” that are triggered
when the exception they name occurs
– Catch all exception handlers are also possible
– Exception handlers may terminate the program or continue it
• “else” and “finally” blocks can expand the capabilities of exception handling
44

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