File Exceptions

Errors in Programs

Sources of errors in programs are numerous
One common source of errors are incorrect assumptions about the environment in which the program is running

Missing Files

An attempt to open a file for reading that does not exist will raise an exception

Example

handle = open("missing.txt")

Handling Exceptions

We can use try and except to handle these errors

Example

try:
    handle = open("missing.txt")
except FileNotFoundError:
    print("File not found")

User Selected Files

File name need not be hard-coded
Names can be entered by a user
This makes proper error handling more important

Example

filename = input("Select a file to read:")

try:
    handle = open(filename)
except FileNotFoundError:
    print(f"Error opening {filename}")
    exit(1)

contents = handle.read()

print(contents)

Handling Multiple Exceptions

File reading may fail due to missing files
File reading may fail due to invalid access rights
These can be handled using separate except blocks

Example

filename = input("Select a file to read:")

try:
    handle = open(filename)
except FileNotFoundError:
    print(f"File '{filename}' does not exist")
    exit(1)
except PermissionError:
    print(f"File '{filename}' is not readable")
    exit(1)

contents = handle.read()

print(contents)

Exercise

print all capitalized words in a user-selected file.
Show an appropriate error message if the file is not found.

Solution

filename = input("Filename:")

try:
    handle = open(filename)
except FileNotFoundError:
    print("File not found")
    exit(1)

contents = handle.read()

for word in contents.split():
    if word.istitle():
        print(word)

Exercise

Count all if, elif, and else keywords in a program

Solution

filename = input("Filename:")

try:
    handle = open(filename)
except FileNotFoundError:
    print("File not found")
    exit(1)

contents = handle.read()

count_if = 0
count_elif = 0
count_else = 0

for word in contents.split():
    if word == "if":
        count_if += 1
    elif word == "elif":
        count_elif += 1
    elif word == "else":
        count_else += 1

print(f"if: {count_if}")
print(f"elif: {count_elif}")
print(f"else: {count_else}")

Key Ideas

Reading files creates new surface for errors
try and except can be used to deal with exceptions
Filenames can be entered by users or come from other dynamic sources