Programs

Exception Handling in Python: Handling Exception Using Try Except

While encountering an error a program of Python gets terminated. The errors are generally problems occurring in a program that stops its execution. The errors may be due to an error in syntax or might be an exception. Whenever an incorrect statement is detected by the parser, there is an occurrence of a syntax error.

However, when the code with the correct syntax generates an error, then it’s known as an exception. Various built-in exceptions are available in python. These are raised with the occurrence of internal events where the normal flow of a program gets changed.

Therefore exceptions may be defined as certain unusual program conditions that result in an interruption of the code and hence the program flow is aborted. 

The execution of a program stops as soon as it encounters an exception. This further stops the code to execute. Hence, exceptions are errors as a result of run-time that are unable to be handled by the python script.

For exception handling in python, the scripting language python provides a solution so that the execution of the code carries on and there aren’t any interruptions. The absence of exception handling stops executing the code that exists after the code that throws an exception.

Several built-in options are available in python that allows the execution of a program without any interruption including the common exceptions. Along with this, there is a provision of python custom exceptions.  For any python program, the common exceptions that can be thrown are:

  • ZeroDivisionError: This type of exception results when zero is used to divide a number.
  • NameError: Whenever a program fails to find a name be it global or local, this type of exception occurs.
  • IndentationError: Incorrect indentation gives rise to the indentationError.
  • IOError: Failure of an Input-Output operation results in IOError.
  • EOFError: It occurs in the continuous operation of a program  even when the 

file end is reached.

Raising an exception

For throwing an exception under certain conditions the raise is used.
The execution of the program is halted and the associated exception is displayed on the screen. The display of the exceptions lets the users know what might be the underlying problem.

In python through the use of the raise clause, we can raise an exception. The application is useful in cases when the program needs to be stopped by raising an exception. 

For example: Supposedly a program needs around 1GB of memory for its execution and it tries occupying 1 GB, in that case, to stop executing the program an exception can be thrown.

Syntax for raising an exception:

Exception_class,<value>

Therefore, 

  • ‘raise’ is used for raising an exception in a program.
  • A value can be provided to an exception which can be provided to the parenthesis.
  • Accessing the value can be done with the keyword ”as”. The value given to the exception can be stored in the reference variable denoted by “e”.
  • For specifying an exception type, the value can be passed to an exception.

AssertionError

An assertion can be made in python instead of letting the program crash. An assertion is made that a specific condition is met by the program. The program will continue running if the condition is true. Else an AssertionError exception is thrown by the program when the condition turns out to be false.

Handling exceptions through the try and except block

Exceptions thrown in python are caught and then handled by the try and except blocks in python.  The code within the try block is executed normally as the program’s part.  The other block includes statements that are executed in response to the exceptions thrown by the program in the try block.

The program throws an exception whenever it encounters an error in a syntactically correct code. If the exceptions thrown are not handled properly there will be a crash in the programs. In such scenarios, the except block determines the response of the program to that exception.

The application of the try and except clause will be best understood through the following example taken from the mentioned source.

In this case, whenever there is an occurrence of an exception, the program will continue running and will inform the user that the program was not successful rather than giving a blank output.

Checkout: Python Project Ideas & Topics

The program showed the error type that was thrown through the call of the function. However, the error thrown by the function can be caught to get an idea of what actually went wrong.

Running the code in a windows machine will generate the following

The message displayed first indicates an AssertionError  through which the user is informed that the function can be executed only on a system of Linux operating system. The second message further gives the information of which function was not able to get executed.
Non existence if the file.log will generate an output with the message “Could not open file.log”. 

The program will still continue running as it is a message that relays the information to the user. A lot of built-in exceptions are available in Python docs. One exception as described is shown below.

The non-existence of the file.log in this case will generate the following output.

Various exceptions can be caught through the use of more function calls in the try clause. However, on detecting an exception, the try clause statements will stop.

The else clause 

Use of the else clause of statements can aid in instructing the program for executing a block of statements only when there are no exceptions. 

The else clause got executed only because there were no exceptions. If the code contains exceptions, then the following will result.

Python custom exceptions

Python has a number of built-in exceptions that throw an error whenever there is something wrong in the program. However, the user needs to create some customized exceptions in cases where his purpose is to be served.

A new class can be created for defining the custom exceptions. Either directly or indirectly, these classes have to be derived from the class of built-in exceptions.

The user-defined class CustomError is created that is inherited from the class Exception. Similar to the other exceptions, these exceptions too are raised through the use of ‘raise’ with an error message that is optional.

The user-defined exceptions should be placed in a separate file whenever there is a development of a large python program. It is generally a good practice to do this and is followed by most of the standard modules where the exceptions are defined separately as errors.py or exceptions.py.

The python custom exceptions are simple and as the normal classes implement everything they too follow the same. 

Syntax and Examples

Let us consider two variables which are a, and b. The input to the variables is taken from the user and the division of the numbers is performed. Now, if the denominator entered by the user is a zero.  

For handling these exceptions, the try-except blocks can be added to the program. For any type of code that leads to a suspicion of throwing exceptions. It should be placed in the try block of statements. 

Syntax of a try block

try:     

#code        

except Exception1:    

 #code        

except Exception2:    

 #code        

#code    

It shows the use of try-except statements where the code is placed in the try block and gets executed when there are no exceptions in the code. 

Syntax of else statement with try-except

try

#try statements

Except Exception1

#code

Else

#execution of code when there is no exception

A few important points:

  • The exception is not to be specified through a statement of exception in python.
  • Multiple exceptions can be declared in a code through the use of a try block as there can be many statements inside a try block that can throw exceptions of different types.
  •  An else block can be specified with a try block which gets executed when there are no exceptions thrown by the try block.
  • The else block should contain statements that are not responsible for throwing exceptions.

Also Read: Top Reasons Why Python is Popular with Developers

Conclusion 

In this article, we briefly discussed the concept of exception handling in python with some examples. Along with the built-in exceptions, the python custom exceptions are also defined briefly. Now, you can know the importance of exceptions and the handling of exceptions in python

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