Python Exception Handling

In our previous tutorial, we discussed about Python Directory. In this tutorial, we are going to learn Python Exception Handling. Python try except keywords are used for exception handling in python.

Basically, exception means something that is not expected. In real life, we are not interested to deal with exceptions. So there goes a proverb, “Exception is not an example”. But when we write programs, we have to think about exceptional cases. For example, if your user entered a string object while you were expecting an integer object as input, it will raise an exception.

Exception hampers normal program flows. If any exception happens, the programmer needs to handle that. Therefore, we are going to learn exception handling in upcoming sections.

Some Built-in Python Exceptions

List of some built-in python exceptions are given below.

Exception : This is the base class for all kind of the exceptions. All kind of exceptions should be derived from this class
ArithmeticError : This is the base class for the exception raised for any arithmetic errors.
EOFError : This exception raise when input() function read End-of-File without reading any data.
ZeroDivisionError : This exception raise when the second argument of a division or modulo operation is zero
AssertionError : This exception raise when an assert statement fails.
FloatingPointError : This exception raise when a floating point operation fails.
KeyError : This exception raise when a mapping (dictionary) key is not found in the set of existing keys.
KeyboardInterrupt : This exception raise when the user hits the interrupt key (normally Control-C or Delete). During execution, a check for interrupts is made regularly.

Besides, you can find the list of all Built-in Exception in their official site.

Python try except

While writing the code, some statements might suspicious for raising an error. Hence, those statements should be surrounded with a try-except-else block. For example, we will now raise an exception by our code. The following code will raise IndexError Exception.


name="Imtiaz Abedin"
print(name[15])
print('This will not print')

name="Imtiaz Abedin"

print(name[15])

print('This will not print')

If you try running the code, you will get below exception.


Traceback (most recent call last):
  File "/home/imtiaz/ExceptionHandling.py", line 2, in
    print(name[15])
IndexError: string index out of range

Traceback (most recent call last):

File "/home/imtiaz/ExceptionHandling.py", line 2, in

print(name[15])

IndexError: string index out of range

Because the size of the string type object ‘name’ is less than 15 and we are trying to access the index no 15. Have a look, the second print statement is not executed for that exception. So program crashes due to an exception. So, in the next code, we will handle this exception.


name="Imtiaz Abedin"
try:
   print(name[15])
except IndexError:
   print('IndexError has been found!')
print('This will be printed print.')

name="Imtiaz Abedin"

try:

print(name[15])

except IndexError:

print('IndexError has been found!')

print('This will be printed print.')

So, you can see from the above two examples that exception should be handled to avoid the program crash. In our first example, the last print statement was not executed because the program found exception before that. You can see that try except keywords are used for exception handling.

Basic Structure of Python Exception Handling

In the previous section, we demonstrate how exception raised and how to handle that. In this section, we will discuss the basic coding structure for handling exceptions. Therefore, the basic coding structure for Python Exception Handling is given below.


name="Imtiaz Abedin"
try:
   # Write the suspicious block of code
   print(name[15])
except AssertionError:  # Catch a single exception
   # This block will be executed if exception A is caught
   print('AssertionError')
except (EnvironmentError, SyntaxError, NameError) as E:  # catch multiple exception
   # This block will be executed if any of the exception B, C or D is caught
   print(E)
except :
   print('Exception')
   # This block will be executed if any other exception other than A, B, C or D is caught
else:
   # If no exception is caught, this block will be executed
   pass
finally:
   # This block will be executed and it is a must!
   pass
# this line is not related to the try-except block
print('This will be printed.')

name="Imtiaz Abedin"

try:

# Write the suspicious block of code

print(name[15])

except AssertionError: # Catch a single exception

# This block will be executed if exception A is caught

print('AssertionError')

except (EnvironmentError, SyntaxError, NameError) as E: # catch multiple exception

# This block will be executed if any of the exception B, C or D is caught

print(E)

except :

print('Exception')

# This block will be executed if any other exception other than A, B, C or D is caught

else:

# If no exception is caught, this block will be executed

pass

finally:

# This block will be executed and it is a must!

pass

# this line is not related to the try-except block

print('This will be printed.')

Here you can see that, we use except keyword in different style. The first except keyword is used to catch only one exception that is AssertionError exception.

However, the second except keyword is used to catch multiple exceptions, as you see.

If you use except keyword without mentioning any specific exception, it will catch any exception that is raised by the program.

The else block will be executed if no exception is found. Lastly, whether any exception is caught or not, the finally block will be executed.

So, if you run the above code, we will get the output:

Python Exception Handling

If you change ‘name[15]’ to ‘nameee[15]’ in the above code, you will get the following output.

Python Exception Handling