Python Exception Handling: Try, Except, Else and Finally

As soon as Python encounters an error, it terminates immediately. An Exception event is raised. This affects the flow of the program.

In order to continue with the normal flow, we can use try, except, else and finally keywords to handle the Exception class.

Types of Errors in Python

Broadly, there are 2 types of errors.

1. Syntax Errors
2. Logical Errors(Exceptions)

Syntax Errors

Syntax errors are basically typos in your Python code. They are easy to fix.

Code

print("Pylenin"))

Output

File "folder-name", line 1
print("Pylenin"))
                ^
SyntaxError: unmatched ')'

Logical Errors(Exceptions)

Logical Errors or Exceptions are errors in the logic of a program.

Code

x = 1/0

Output

Traceback (most recent call last):
  File "folder-name", line 1
    x = 1/0
ZeroDivisionError: division by zero

Handling Errors with try, except, else and finally

• try - Tests the code for any error
• except - Catches the error, if any
• else - Only executed, if there is no error
• finally - Always executed

try-except block

First, let us understand how try and except work.

Code

try:
    x = 1/0
except:
    print("There was some issue")

Output

There was some issue

As you can see, the program wasn’t interrupted due to the division by zero error. The print statement in the except block was executed. So if there is no exception, only the code in the try block is executed.

Let’s try to find out more info about the exception that happened above. For this purpose, we can use the Exception class from python.

Code

try:
    x = 1/0
except Exception as e:
    print(e.__class__)
    print(f"Exception occured - {e}")

Output

<class 'ZeroDivisionError'>
Exception occured - division by zero

The e.__class__ tells us the python built in exception class that was cause of the above error, which is ZeroDivisionError.

Let’s try out another example. Now we will try to access an imaginary file that doesn’t exist in our working directory.

Code

try:
    x = open('imaginary_file.csv', 'r')
except Exception as e:
    print(e.__class__)
    print(f"Exception occured - {e}")

Output

<class 'FileNotFoundError'>
Exception occured - [Errno 2] No such file or directory: 'imaginary_file.csv'

Now we get the FileNotFoundError error. You can find more about built in python exceptions here.

Using try except else and finally

We can also include else and finally keywords into our python code to handle errors.

The code inside the else block is only executed if no exceptions are raised.

The code inside the finally block is always executed, no matter what!

Let’s look at the following code.

Code

import logging

log_format = "%(asctime)s::%(levelname)s::%(name)s::"\
             "%(filename)s::%(lineno)d::%(message)s"

logging.basicConfig(filename='mylogs.log', level='DEBUG', format=log_format)

def division(x, y):
    result = None
    try:
        result = x/y
    except Exception as e:
        print(f"Exception - {e} occured")
    else:
        print(f"The result of {x} divided by {y} is {result}")
    finally:
        status = None
        if not result:
            status = "FAIL"
        else:
            status = "PASS"
        logging.info(f"Division function called. RESULT: {status}")

division(3,0)

Output

#terminal
Exception - division by zero occured

#mylogs.log
2021-01-08 09:08:52,099::INFO::root::trial.py::22::Division function called. RESULT: FAIL

As you can see, the code inside the else block wasn’t executed. However, the code inside the finally block was executed and a log was created in our mylogs.log file. The finally block is therefore, a great place to log function or class calls.

Let’s change the function call and see what happens.

Code

import logging

log_format = "%(asctime)s::%(levelname)s::%(name)s::"\
             "%(filename)s::%(lineno)d::%(message)s"

logging.basicConfig(filename='mylogs.log', level='DEBUG', format=log_format)

def division(x, y):
    result = None
    try:
        result = x/y
    except Exception as e:
        print(f"Exception - {e} occured")
    else:
        print(f"The result of {x} divided by {y} is {result}")
    finally:
        status = None
        if not result:
            status = "FAIL"
        else:
            status = "PASS"
        logging.info(f"Division function called. RESULT: {status}")

division(3,1)

Output

#terminal
The result of 3 divided by 1 is 3.0

#mylogs.log
2021-01-08 09:13:24,391::INFO::root::trial.py::22::Division function called. RESULT: PASS

As you can see, the code inside the else block has been executed successfully as there was no error.

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