Exceptions

Introduction to Object-Oriented Programming in Python

George Boorman

Curriculum Manager, DataCamp

a = 1
a / 0

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    1/0
ZeroDivisionError: division by zero

a = [1,2,3]
a[5]

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    a[5]
IndexError: list index out of range

a = 1
a + "Hello"

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
    a + "Hello"
TypeError: unsupported operand type(s) for +: /
'int' and 'str'

a = 1
a + b

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    a + b
NameError: name 'b' is not defined

Exception handling

Prevent the program from terminating when an exception is raised
try - except - finally:

try:
    print(5 + "a")

except TypeError:
    print("You can't add an integer to a string, but you can multiply them!")

# Can have multiple except blocks
except AnotherExceptionHere:
    # Run this code if AnotherExceptionHere happens

# Optional finally block
finally:
    print(5 * "a")

Exception handling output

You can't add an integer to a string, but you can multiply them!
aaaaa

Raising exceptions

def make_list_of_ones(length):
    if length <= 0:
        # Custom message if ValueError occurs
        # Will stop the program and raise the error
       raise ValueError("Invalid length!")
    return [1]*length


make_list_of_ones(-1)

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    make_list_of_ones(-1)
  File "<stdin>", line 3, in make_list_of_ones
    raise ValueError("Invalid length!")
ValueError: Invalid length!

Exceptions are classes

Exceptions are inherited from BaseException or Exception

BaseException
+-- Exception
    +-- ArithmeticError                     
    |    +-- FloatingPointError
    |    +-- OverflowError
    |    +-- ZeroDivisionError              
    +-- TypeError
    +-- ValueError
    |    +-- UnicodeError
    |         +-- UnicodeDecodeError
    |         +-- UnicodeEncodeError
    |         +-- UnicodeTranslateError
    +-- RuntimeError
   ...
+-- SystemExit
...

¹ https://docs.python.org/3/library/exceptions.html

Custom exceptions

Inherit from Exception or one of its subclasses
Usually an empty class

class BalanceError(Exception): 
    pass


class Customer:
    def __init__(self, name, balance):
        if balance < 0 :
            raise BalanceError("Balance has to be non-negative!")
        else:
            self.name = name
            self.balance = balance

Exception in constructor

cust = Customer("Larry Torres", -100)

Traceback (most recent call last):
  File "script.py", line 11, in <module>
    cust = Customer("Larry Torres", -100)
  File "script.py", line 6, in __init__
    raise BalanceError("Balance has to be non-negative!")
BalanceError: Balance has to be non-negative!

Exceptions terminate the program

Exception interrupted the constructor → object not created

cust

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    cust
NameError: name 'cust' is not defined

Catching custom exceptions

try:
    cust = Customer("Larry Torres", -100)
except BalanceError:
    cust = Customer("Larry Torres", 0)

Let's practice!

Introduction to Object-Oriented Programming in Python