Excepciones

Introducción a la programación orientada a objetos en 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

Manejo de excepciones

Evita que el programa termine cuando se lanza una excepción
try - except - finally:

try:
    print(5 + "a")

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

# Puede haber varios bloques except
except AnotherExceptionHere:
    # Ejecuta este código si ocurre AnotherExceptionHere

# Bloque finally opcional
finally:
    print(5 * "a")

Salida del manejo de excepciones

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

Lanzar excepciones

def make_list_of_ones(length):
    if length <= 0:
        # Mensaje personalizado si ocurre ValueError
        # Detiene el programa y lanza el 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!

Las excepciones son clases

Las excepciones heredan de BaseException o Exception

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

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

Excepciones personalizadas

Hereda de Exception o de una de sus subclases
Suele ser una clase vacía

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

Excepción en el 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!

Las excepciones terminan el programa

La excepción interrumpió el constructor → objeto no creado

cust

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

Capturar excepciones personalizadas

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

¡Vamos a practicar!

Introducción a la programación orientada a objetos en Python