Handling Exceptions

Raising Exceptions

You can create errors with your own messages that stop the program:

raise ValueError("expected a valid Pokemon name")

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Catching Exceptions

The try.. except clause allows your program to catch errors and act on them instead of terminating the program.

cards = "234567890JQKA"

for card in cards:
    try:
        number = int(card)
        print(f"dealt {card}")
    except ValueError:
        print(f"{card} could not be dealt")

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How not to catch Exceptions

Exceptions should always be caught with an explicit error type. Using a generic except makes debugging difficult:

cards = "234567890JQKA"

for card in cards:
    try:
        number = int(card)
        print(f"dealt {car}")
    except:
        print(f"{card} could not be dealt")

In this example, car causes a NameError for every card. You don’t get any clues what exactly went wrong.

This is called the “diaper pattern” and considered a very bad habit.

What Exceptions to catch

• File operations

• web operations

• big function calls

• database operations

• NEVER CATCH everything

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Creating your own Exceptions

You can define your own types of Exceptions:

class WrongInputError(Exception): pass

and raise them:

text = input("please enter a number between 1-4: ")
if not text in "1234":
    raise WrongInputError("{} is not a number between 1-4.".format(text))

The try..except works for your own Exception types as well.

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Detailed traceback

Sometimes you may want to output very detailed information in an except block:

"""
Example:
Print the state of all variables when an Exception occurs. 
"""

import sys, traceback

def print_exc():
    tb = sys.exc_info()[2]
    while tb.tb_next: 
        # jump through execution frames
        tb = tb.tb_next
    frame = tb.tb_frame
    code = frame.f_code
    print("Frame %s in %s at line %s"%\
              (code.co_name,
               code.co_filename,
               frame.f_lineno))
    # print local variables
    for k, v in frame.f_locals.items():
        print("%20s = %s"%(k,str(v)))

    
try:
    a = 3
    b = "Carrots"
    c = a / 0
except ZeroDivisionError:
    print_exc()