Exceptions

A Python exception is a runtime error signal that indicates a event.

Compared to the assert functionality, exceptions can be caught and dealt with in a programmatic way.
This allows for a piece of code to be robust, and not completely self-destruct should something forseeably bad happen.

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file = open("secrets.txt", r")
# FileNotFoundError: [Errno 2] No such file or directory: 'secrets.txt'

You can throw an exception by raise-ing it

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raise FileNotFoundError

Try .. Except .. Finally

Most languages have a try..catch block - In Python this is called a try..except block

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try:
  # do these stuff here
  # they will execute as usual
  # if something bad happens
  # like this:
  print(1/0)
except ZeroDivisionError:
  # instead of the programming crashing
  # we can programmatically respond to the error
  print("Cannot divide by zero")
else:
  # if no exceptions occur
  # we could do some stuff here
  # though you might as well just put it after the usual statements in the original try block
  pass
finally:
  # At the end of either case (general case, or exception)
  # we can run the same final bit of code
  print("Finishing loop")

Custom Exceptions

A custom exception can be created by extending the Exception base class.
This exception can then be caught in the except clause of a try..except block

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class NoSuchPerson(Exception):
  pass


def getPerson(id):
  people = {}
  if id not in people:
    raise NoSuchPerson(id)
  return people[id]


try:
  getPerson("andrew1532")
except NoSuchPerson as e:
  print(f"No user with ID `{e}` exists")