In the previous article, we learnt about what is an exception and brief idea about it. In this article, we will learn how to handle the exceptions in python. When these exceptions occur, it causes the current process to stop and passes it to the calling process until it is handled. If not handled, our program will crash. If the exception never handled then our program will be stopped unexpectedly. 

How to catch exceptions in Python?

We can handle exception using try statement. An operation which can raise exception written in a try block and the code which we want to execute once we encounter the exception is written in except block.

Syntax:

try:

    code which can raise exception

except [exception]:

   code to handle exception 

Example:

try:
    print 1/(5-5)
except:
    print "You made a mistake. You can't divide by zero"

Output:

You made a mistake. You can't divide by zero

In the above example, we have divided the 1 by zero which raised an exception and the code written in except block is got executed.

Catching Specific Exceptions:

In our previous example, we did not mention any specific exception in the except block. But this does not imply a good programming practice. As the good programming practice is to handle each possible exception meaningfully. The try block can have multiple except block to handle each exception differently and only one of the matching except block will be executed when an exception occurs. 

We can use a tuple of values to specify multiple exceptions in an except clause. Here is an example pseudo code.

try:
   # Have your actual business logic
   pass

except ValueError:
   # handle ValueError exception
   pass

except (TypeError, ZeroDivisionError):
   # This handles multiple exceptions TypeError and ZeroDivisionError
   pass

except:
   # handle all other exceptions
   pass

Raising Exceptions:

You can also raise your exception forcefully using the keyword Raise. We can also pass the exception message along with it to have the cause of the exception.

Syntax:

raise NameOfTheException("Exception message")

Example:

try:
    a= -2
    if a < 0:
    	raise ValueError("a is not a positive number!")
except ValueError as ve:
	print(ve)

Output:

a is not a positive number!​

Try … except … else clause:

The else clause in a try , except statement, must follow all except clauses and is useful for code that must be executed if the try clause does not raise an exception.

Syntax:

try:

     code for try block

except:

     code to handle exception goes here…

else:

     code you want to execute if no error raised from try block

Try With finally Clause:

We can also use optional finally clause along with the try block. The block of code in finally clause gets executed regardless of error occurred or not. The main purpose of this is to release external resources or remove locks from objects in use.

Syntax:

try:

     code for try block

except:

     code to handle exception goes here…

finally:

     any tasks you want to execure regard less of error occurred or not. ex: Releasing db connection

Example:

try:
   f = open("test.txt","a")
   # Some more code for file manipulation...
finally:
   f.close()

In the above example, we want to close the file if any error occurs during I/O operations.

That's all for handling exceptions for now. It is always good practice to handle the exceptions in a way that user gets meaningful error messages and can act based on that. Even a good error handling will prevent your code from breaking but one should remember that putting every bit of code in try…except block is also not a good practice.