Exceptions

Object-Oriented Programming in Python

Alex Yarosh

Content Quality Analyst @ 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
Object-Oriented Programming in Python

Exception handling

  • Prevent the program from terminating when an exception is raised
  • try - except - finally:
try:
  # Try running some code

except ExceptionNameHere: # Run this code if ExceptionNameHere happens
except AnotherExceptionHere: #<-- multiple except blocks # Run this code if AnotherExceptionHere happens ...
finally: #<-- optional # Run this code no matter what
Object-Oriented Programming in Python

Raising exceptions

  • raise ExceptionNameHere('Error message here')
def make_list_of_ones(length):
    if length <= 0:
       raise ValueError("Invalid length!")  # <--- Will stop the program and raise an error
    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!
Object-Oriented Programming in Python

Exceptions are classes

  • standard 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
Object-Oriented Programming in Python

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, self.balance = name, balance

Object-Oriented Programming in Python
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!
  • 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
Object-Oriented Programming in Python

Catching custom exceptions

try:
  cust = Customer("Larry Torres", -100)
except BalanceError:
  cust = Customer("Larry Torres", 0)
Object-Oriented Programming in Python

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Object-Oriented Programming in Python

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