Refer to https://realpython.com/python-exceptions/

Raising an Exception

Example

x = 10
if x > 5:
    raise Exception('x should not exceed 5.')

The AssertionError Exception

refer to https://dbader.org/blog/python-assert-tutorial

Python’s assertions are a powerful debugging tool that’s frequently underused by Python developers.

What is the assert

If the condition is True, it does nothing and your program just continues to execute.

But if the assert condition evaluates to False, it raises an AssertionError exception with an optional error message.

assert( x < 5 )

assert( x < 5 ), 'x should not exceed 5.'

import sys
assert ('linux' in sys.platform), "This code runs on Linux only."

When use assert statement

The proper use of assertions is to inform developers about unrecoverable errors in a program.

Another way to look at it is to say that assertions are internal self-checks for your program.

Python’s assert statement is a debugging aid, not a mechanism for handling run-time errors. The goal of using assertions is to let developers find the likely root cause of a bug more quickly. An assertion error should never be raised unless there’s a bug in your program.

Common Pitfalls With Using Asserts in python

Don’t Use Asserts for Data Validation

Asserts can be turned off globally in the Python interpreter. Don’t rely on assert expressions to be executed for data validation or data processing.

How to turn off globally in the Python interpreter? https://docs.python.org/3/using/cmdline.html#cmdoption-o

With -O: Remove assert statements and any code conditional on the value of debug.

Asserts That Never Fail

When you pass a tuple as the first argument in an assert statement, the assertion always evaluates as true and therefore never fails.

The try and except Block: Handling Exceptions

(1) try clause: Run this code

(2) except clause: Execute this code when there is an exception

(3) else clasue: No exceptions? Run this code

(4) finally clause: Always run this code.

Example

try:
    linux_interaction()
except AssertionError as error:
    print(error)
else:
    try:
        with open('file.log') as file:
            read_data = file.read()
    except FileNotFoundError as fnf_error:
        print(fnf_error)
finally:
    print('Cleaning up, irrespective of any exceptions.')

Summary

• raise allows you to throw an exception at any time.
• assert enables you to verify if a certain condition is met and throw an exception if it isn’t.
• In the try clause, all statements are executed until an exception is encountered.
• except is used to catch and handle the exception(s) that are encountered in the try clause.
• else lets you code sections that should run only when no exceptions are encountered in the try clause.
• finally enables you to execute sections of code that should always run, with or without any previously encountered exceptions.