Python Programming by Example (2015)

9. Error Handling

This chapter explains how to handle errors and exceptions that occur in Python application.

9.1 Error Handling

Basically when we write a program and occur error on running, Python will catch program error.

Try to write these scripts and run this program.

a = 18

b = 0

c = a / b

$ python3 ch09_01.py

You should get error, shown in Figure below.

Now we can catch error using try..except. You can read how to use it on https://docs.python.org/3/tutorial/errors.html .

Write these scripts.

try:

a = 18

b = 0

c = a / b

print('result:', str(c))

except ZeroDivisionError as e:

print('Error: division by zero')

print(e)

finally:

print('Done')

print('exit from program')

Save into a file, ch09_02.py. Then, run the file.

$ python3 ch09_02.py

You can see the program can handle the error.

9.2 Catching All Errors

On previous section, we catch error for "division by error". We can catch all errors using Exception object. Write these scripts for demo.

try:

a = 18

b = 0

c = a / b

print('result:', str(c))

except Exception as e:

print(e)

finally:

print('Done')

print('exit from program')

Save into a file, called ch09_03.py. Run the program.

$ python3 ch09_03.py

Program output:

9.3 Raising Exceptions

We can raise error from our program using raise.

For demo, write these scripts.

try:

a = 18

b = 0

c = a / b

print('result:', str(c))

except Exception as e:

raise

finally:

print('Done')

# this code is never called

print('exit from program')

Save into a file, called ch09_04.py. Then, run the program.

$ python3 ch09_04.py

You should the program raise the error so we don't words "exit from program".

9.4 Custom Exception

We can build own error with implementing inheritance Exception.

For instance, we create a class, MySimpleError, with inheritance from Exception.

class MySimpleError(Exception):

def __init__(self, code, message):

self.code = code

self.message = message

def __str__(self):

return repr(str(self.code) + ":" + self.message)

def save_to_database(self):

print('save this error into database..')

# how to use custom error

try:

print('demo custom error')

print('raise error now')

raise MySimpleError(100,'This is custom error')

except MySimpleError as e:

print(e)

e.save_to_database()

Save the program into a file, called ch09_05.py.

Run this program.

$ python3 ch09_05.py

Program output: