Python Built-in Exceptions and Errors

In this Python article, we will discuss some of the most common and frequently occurring exceptions that may arise in Python along with their reasons and corrections with some suitable examples.

1. What is an Exception in Python?

While writing a certain program we might make some mistakes that are thrown as an error during the program execution. These errors can be of two types

• Syntax errors
• Runtime errors/exceptions

Ok I get this, but what exactly is an exception?

Exceptions can be defined as an output event raised when there is an error during the execution of a program. In short, when the python script encounters an unmanageable condition, an exception is raised. Exception serves as an object to identify an error.

When we encounter an exception, either we handle the exception using try and except blocks(exception handling) or the program terminates abruptly.

Modern and advanced IDE can identify the syntax error while writing the code so we generally do not end up with syntax errors during the program execution. It might still happen if we are using some normal text editor to write a program.

2. Common Exceptions in Python

Here is the list of some common exception which can be encountered in Python.

Let’s implement some of them using certain examples.

2.1. IndexError

This exception is raised when an index is not found in a sequence.

list_data = [2,4,6,8]
x = list_data[5]

Output
Traceback (most recent call last):
File "xyz.py", line 2, in 
list_data[5]
IndexError: list index out of range

2.2. ModuleNotFoundError

This error occurs when we make an import for a module and the module can not be found.

import some_xyz_module
list_data=[2,4,6,8]
x=list_data[3]

Output
Traceback (most recent call last): 
File "xyz.py", line 1, in <module>              
import some_xyz_module ModuleNotFoundError: No module named 'some_xyz_module'

2.3. KeyError

This error occurs if a particular key in the dictionary is found absent.

dict_data={'2':"two", '4':"four", '6':"six"}
dict_data['8']

Output
Traceback (most recent call last): 
File "abc.py", line 1, in <module>              
dict_data['8'] KeyError: '8'

2.4. StopIteration

Raised when the next() method of an iterator does not point to any object.

list_data=iter([3,5,7,9,11])
print(next(list_data))
print(next(list_data))
print(next(list_data))
print(next(list_data))
print(next(list_data))
print(next(list_data))

Output
3
5
7
9
11
Traceback (most recent call last): 
File "abc.py", line 7, in <module>              
print(next(list_data))
StopIteration

2.5. KeyboardInterrupt

This error occurs when ctrl+c is pressed which Raised when there is a break in program execution.

name=input('Give me a string') #suppose user entered ctrl+c

Output
Give me a string^CTraceback (most recent call last):
  File "main.py", line 1, in 
    name=input('Give me a string') #suppose user entered ctrl+c
KeyboardInterrupt

3. What is the Difference Between Syntax Error and Exceptions in Python?

In python, the syntax error is referred to such errors which are identified as the wrong statement by the parser. For example, if we miss the colon(:) at the end of if statement.

>>> if x > 1
  File "<stdin>", line 1
    if x > 1
            ^
SyntaxError: invalid syntax

Whereas the exception means syntax in the code was correct but there is some other error which is indicated by the last line at run time.

For example in the code below we are dividing a number by Zero which is correct syntactically but will throw ZeroDivisionError as soon as the statement is executed.

>>> def test(x):
...   if x / 0 == 1:
...     print("Success")
...
>>> test(2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in test
ZeroDivisionError: division by zero

4. AssertionError Exception

With the help of assert keyword we can make sure that the code that we are gonna execute has the correct setup/environment/operating system etc.

In some cases where we have a specific requirement like an Operating System, we can write the assert statement at the beginning of the program to fail fast in case the requirements are not fulfilled. It also prevents the system to go into an undesired state because of unfulfilled dependencies.

In assert keyword, we mention if a particular condition is met then the program will throw an AssertionError exception.

If the condition is found true, then the code will continue. If the condition turns out to be false, the program throws an AssertionError exception.

import sys
assert ('linux' in sys.platform), "This program needs a Linux system."

Output
Traceback (most recent call last):   
File "<input>", line 2, in <module> 
AssertionError: This program needs a Linux system.

Now, the question may arise that how can we handle exceptions in different possible ways. Well, that’s the topic for the next article.

5. Conclusion

In this article we learned about the exception in python along with certain examples for some errors.

• What is an Exception in Python?
• Some Common Exceptions in Python
• Difference between exception error and syntax error
• AssertionError and asset keyword

Helpful Links

Please follow the Python tutorial series or the menu in the sidebar for the complete tutorial series.

Also for examples in Python and practice please refer to Python Examples.

Complete code samples are present on Github project.

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