Errors and Exceptions

Try-Except

• except OSError 

def character_frequency(filename):
  """Counts the frequency of each character in the given file."""
  # First try to open the file
  try:
    f = open(filename)
  except OSError:
    return None

  # Now process the file
  characters = {}
  for line in f:
    for char in line:
      characters[char] = characters.get(char, 0) + 1
  f.close() 
  return characters

Raise

• raise ValueError("Some custom error messages") 

def validate_user(username, minlen):
  assert type(username) == str, "username must be a string"
  if minlen < 1:
    raise ValueError("minlen must be at least 1")

  if len(username) < minlen:
    return False
  if not username.isalnum():
    return False
  return True

For unit test

.assertRaises() 

import unittest

from validations import validate_user

class TestValidateUser(unittest.TestCase):
  def test_valid(self):
    self.assertEqual(validate_user("validuser", 3), True)

  def test_too_short(self):
    self.assertEqual(validate_user("inv", 5), False)

  def test_invalid_characters(self):
    self.assertEqual(validate_user("invalid_user", 1), False)
    
  def test_invalid_minlen(self):
    self.assertRaises(ValueError, validate_user, "user", -1)


# Run the tests
unittest.main()