Chapter 10. Files and Exceptions

Files operation skipped here. Directly head to Exception

Exceptions

exceptions = objects python use to manage errors that arise during a program's execution.
When python unsure what to do, exception object is created.
- If exceptions are handled correctly, program can continue run
- If don't handle exception, program will halt and show traceback

Syntax of exception handling: try-except

try tells python do sth
except tell python what to do if an exception is raised.

e.g. ZeroDivisionError Exception

Simple error, python will create ZeroDivisionError in response to do "Divide by Zero".
When you think an error may occur, you can write a try-except block to handle exception

try:
    print(5/0)
except ZeroDivisionError:
    print("You can't divide by zero!")

If more code followed try-except block, the program would continue running. e.g. as shown below

Using Exceptions to Prevent Crashes

Handling errors correctly is important when the program has more work to do after the error occurs.

print("Give me two numbers, and I'll divide them.")
print("Enter 'q' to quit.")

while True:
    first_number = input("\nFirst number: ")
    if first_number == 'q':
        break
    second_number = input("Second number: ")
    if second_number == 'q':
        break
    
    answer = int(first_number) / int(second_number)
    print(answer)

This function ask for numbers and will calculate division. To properly cover potential failure, use try-except-else block

try block: Python attempts to run code
except block: catch possible error
else block: if division operation is successful, carry on!

Hence, program can be more robust

    try:
        answer = int(first_number) / int(second_number)
    except ZeroDivisionError:
        print("You can't divide by 0!")
    else:
        print(answer)

Handling FileNotFoundError Exception

filename = 'alice.txt'

with open(filename, encoding='utf-8') as f:
    contents = f.read()

If python cannot find alice.txt, it will raise halt, and show traceback

Traceback (most recent call last):
  File "alice.py", line 6, in <module>
    with open(filename, encoding='utf-8') as f:
FileNotFoundError: [Errno 2] No such file or directory: 'alice.txt'

After changing python code to

filename = 'alice.txt'

try:
    with open(filename, encoding='utf-8') as f:
        contents = f.read()
except FileNotFoundError:
    print(f"Sorry, the file {filename} does not exist")

Python will show "Sorry, the file alice.txt does not exist"

Failing Silently

If you want the program to fail silently and carry on, use pass to tell Python to do nothing, and skip the try-except-else blocks

def count_words(filename):
    """Count the approximate number of words in a file."""
    try:
        --snip--
    except FileNotFoundError:
        pass
    else:
        --snip--

    filenames = ['alice.txt', 'siddhartha.txt', 'moby_dick.txt', 'little_women.txt']
    for filename in filenames:
        count_words(filename)

Storing Data

json.dump() & json.load

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