Errors or Exceptions

Syntactically correct.
But still on execution you’ll get an error.
So errors detected during execution are called exceptions.
Ain’t unconditionally fatal, i.e. we can handle them in our Python programs.

E.g. in a calculator your end-user might try to divide 1 by zero. And this is something you might wanna handle gracefully:

dividend = int(input("Enter dividend: "))
divisor = int(input("Enter divisor: "))
result = dividend / divisor
print(f"dividend ÷ divisor = {result}")

If you run this program and enter 15, and 5 it works just fine but what if you enter 15 and zero. Kaboom :boom:, it breaks:

python calc.py
Enter dividend: 12
Enter divisor: 0
Traceback (most recent call last):
  File "/home/kasir/projects/python/calc.py", line 3, in <module>
    result = dividend / divisor
             ~~~~~~~~~^~~~~~~~~
ZeroDivisionError: division by zero

Or if you try to enter a bogus value, instead of a valid number try to enter e.g. “abc”.

python calc.py
Enter dividend: abc
Traceback (most recent call last):
  File "/home/kasir/projects/python/calc.py", line 1, in <module>
    dividend = int(input("Enter dividend: "))
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: invalid literal for int() with base 10: 'abc'

The last line of the error message indicates what happened.

Types of Exceptions

ZeroDivisionError, NameError, TypeError, …
The string printed as the exception type is the name of the built-in exception that occurred.
- Not necessarily true for user defined ones.
  - You get to decide how you wanna log your error.
  - A good convention to have it.

`BaseException`

Common base class of all exceptions.

`Exception`

A subclasses of BaseException.
The base class of all the non-fatal exceptions.
- This means exceptions which are not subclasses of Exception indicate that the program should terminate. E.g. SystemExit.
Used as a wildcard that catches almost everything.
Can add notes to an exception in order to add extra information to it:
```
exception = Exception("Some error")
exception.add_note("Some note")
exception.add_note("Some other note")
raise exception
```
This is particularly useful in try ... except statements where you might want to add extra information to an exception that you’re handling in an except clause.

[!TIP]

Best practice: be as specific as possible with the types of exceptions that you intend to handle, and allow any unexpected exceptions to propagate.

Though for debugging purposes we can use this class like this:
try:
    pass
except ValueError:
    # ...
except Exception as e:
    print(f"Debugging, {e}")

Stack Traceback

Traceback

Where the exception occurred.
Called traceback since you get to see where the error occurred and who was calling that piece of code before it.

YouTube/Aparat

Ref

8. Errors and Exceptions.