Whenever a program is written for solving any problem or for executing any particular task ,it may behave abnormally or unexpectedly because of some errors. An error also termed as a 'bug' is anything in the code that prevent a program from compiling and running correctly.

In Python there are 3 kinds of errors :

• Compile-time errors


• Runtime errors


• Exceptions

Compile-time errors

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As the name suggests, the errors that occurs while compilation or at the compile-time are called compile-time errors. These error occurs when you violate the rules of writing syntax.

Most frequent compile time errors are :

(i) Missing parenthesis

(ii) Printing the value of variable without declaring it.

Compile-time errors are classified into 2 categories :

• Syntax errors


• Semantic errors

Syntax errors :

Syntax error are the errors that occur due to the incorrect format of a Python statement. You can understand it as a structure of rules and conventions that you have to follow to construct that structure.

For Example :

In the above example, we can see a syntax error. We know that in Python, the syntax for if statement is:

if(condition):

In the example “:” was missing, hence the compilation stopped and there was an error. The correct code would be:

Semantic errors :

Semantics refers to the meaning of a statement. So, when a statement doesn’t make any sense and is not meaningful then we can say it is a Semantic error.

For Example :

In the above example, we can see a syntax error but actually it's a semantic error, because the statement: 'sum + n = sum' doesn’t make any sense. This is because an expression cannot come on the left side of an assignment according to Python language’s rule.

The correct statement would be: sum = sum + n

Runtime Errors

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This type of errors occurs after the compilation of the code, when execution is going on. Due to which the program suddenly stops running and displays an error dialog box.

For Example :

• Divide by zero


• Invalid function call


• Wrong value as input


• Infinite loop

The above statement is syntactically correct but won't let yield any output and will generate an error because division of any number by zero results in an error and leads to the illegal program termination.

What is an exception ?

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Errors occurred during execution are called exception. In other words, an exception is an error that occurs while a program is running. They indicate that something unusual has happened in the program.

Thus, an exception can be described as an interruption during the normal flow of the execution of a program, it is to be handled carefully, when an error occurs an exception is raised in Python.

Let us discuss few important terms related to Exception handling :

• Exception : an exception is an error that occurs while a program is running. They indicate that something unusual has happened in the program.


• Exception Handling : Python provides as a way to handle exception so that the programme doesn't terminate but execute some specified code when an exception occurs. This concept is called Exception Handling.


• Traceback : The lengthy error message that is shown when an exception occurs during the program run is called traceback.


• 'Try' block : Exceptional handling in Python is done using try statement. Try block constitutes a set of code that might have an exception thrown in it, if any code within the try statement causes an error execution of the code will stop and it will jump to the except statement.


• 'except' block or 'catch' block : Whenever some error exception is to be handled, it is done using the except block which stands for exception.


• 'throw' or 'raise' block : 'throw' is an action in response to an exception (error/unusual condition).


• Unhandled, uncaught exception : An exception which leads to abnormal termination of a program due to non-execution of exception thrown is termed as unhandled or uncaught exception.

Common Python Exceptions

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Exception	Description
NameError	Raised when an identifier is not found in the local or global namespace.
TypeError	Raised when an operation or function is attempted that is invalid for the specified data type.
ValueError	Raised when the built-in function for a data type has the valid type of arguments, but the arguments have invalid values specified.
ZeroDivisionError	Raised when division or modulo by zero takes place for all numeric types.
AttributeError	Raised in case of failure of attribute reference or assignment.
KeyError	Raised when the specified key is not found in the dictionary.
IndentationError	Raised when indentation is not specified properly.
IOError	Raised when an input/ output operation fails,
IndexError	Raised when an index is not found in a sequence, i.e. out of range or out of bound.
SyntaxError	Raised when there is an error in Python syntax.

NameError

This type of exception occurs when you are trying to use a variable that does not exist in the current environment.

For example,

Back to table

TypeError

This type of error raised when an operation or function is attempted that is invalid for the specified data type.

For example,

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ValueError

This exception is raised when we try to perform operation on incorrect type of value.

For example,

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ZeroDivisionError

This exception is raised when division or modulo by zero takes place for all numeric types.

For example,

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AttributeError

This exception is raised when an object does not find an attribute.

For example,

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KeyError

This type of error occurs when you are trying to access an element of a dictionary using a key that the dictionary does not contain.

For example,

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IndentationError

Indentation is one of the important factors that should be taken care of while writing Python programs. Each statement written in the block should have the same indentation level. Violation of this rule results in indentation error.

For example,

Back to table

IOError

This exception occurs whenever there is an error related to input/output such as opening a file which does not exist, trying to delete a file in use, removing USB while read/write operation is going on

For example,

Back to table

IndexError

This exception is raised when an index is not found in a sequence. In other words, it occurs whenever we try to access an index that is out of a valid range.

For example,

Back to table

SyntaxError

This error is raised when there is an error in Python syntax.

For example,

Back to table

Handling Exception in Python

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Python raises exceptions whenever there is an error at run-time .This is a point where your program crashes.That is the interpreter stops the execution of program and displays a traceback which includes error name,its description and a line number,point of the occurrence of the error.

And, the process of handling these errors is called exception handling



Python provides various exception handling techniques ,let us discuss these techniques summarily

Using try and except block

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When an error occurs,python stops the program abruptly and generate an exception message.So it highly recommended to handle exceptions

So to handle exceptions, try and except block in python is used.try block contains the code which python executes as a normal part of the program.

And the except block contains the response corresponding to any exceptions in the preceding try block

Syntax:



All the risky and suspicious code that can raise an exception should be placed inside the try block

Else block is also used along with the try and except block, the code in the else block is only executed when there is no exception

Syntax:



When an exception occurs in try block,the execution of try block stops and transferred down to the except block

Let us see the examples of the above concept:

Program1 :Using try except block

Output:

Program2 :Using try except and else block

Output:

Multiple except block

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We can also handle multiple exceptions raised by try block,by writing multiple except clauses with exception names to handle specific exceptions, as shown below:

At the occurance of an exception this script will work as:

• Control is transferred to the corresponding except clause to handle the exception occurred
• If that particular exception is not specified in any except clause. Then by default except will execute without any exception name
• If neither of the above two cases is there,then the program stops abruptly.

Example:

Program1 :Using Multiple except block

Output:

Using try with finally

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Python provides finally block which is used with the try block statements.The code in final block is always executed irrespective whether exception occurs or not

It Is used to write block of code that must execute, whether the try block raises an error or not

Syntax:



Example:

Program1 :Using try except and finally block

Output: