Errors And Exceptions In Python

Errors And Exceptions In Python

Errors

The errors cannot be handled generally by programmers. Like, syntax errors, recursion error, memory out of bounds, etc. The errors can and should not be handled by the programmer, for example, we can not handle stack overflow or the memory out of bounds errors.

Exceptions  

A program can contain errors or exceptions, for example when the parser detects an incorrect statement(Syntax Error) the program cannot be executed. Few errors in a program are not syntax errors. They are detected during execution of the program only, such errors are known as exceptions. This type of error occurs whenever a syntactically correct program results in an error. The program terminates as soon as it encounters an error. Most exceptions aren't handled by programs but, it is possible to write programs in Python that can handle selected exceptions.

Syntax Error

#Syntax error

a=12

b='python'

print(a+b)

Output:

Traceback (most recent call last):
  File "C:/Users/my/PycharmProjects/Facebook/venv/test.py", line 3, in <module>
    print(a+b)
TypeError: unsupported operand type(s) for +: 'int' and 'str'
Process finished with exit code 1


Identation Error


#identation error
for item in range(5):
print(item)

Output:
  File "C:/Users/my/PycharmProjects/Facebook/venv/test.py", line 4    print(item)
        ^
IndentationError: expected an indented block

Process finished with exit code 1


Recursion Error

#recursion error
def rec():
    return rec()
rec()

Output:
Traceback (most recent call last):
  File "C:/Users/my/PycharmProjects/Facebook/venv/test.py", line 4, in <module>
    rec()
  File "C:/Users/my/PycharmProjects/Facebook/venv/test.py", line 3, in rec
    return rec()
  File "C:/Users/my/PycharmProjects/Facebook/venv/test.py", line 3, in rec
    return rec()
  File "C:/Users/my/PycharmProjects/Facebook/venv/test.py", line 3, in rec
    return rec()
  [Previous line repeated 995 more times]
RecursionError: maximum recursion depth exceeded

Process finished with exit code 1

Built-in Exceptions

Python provides an excellent exception handling mechanism to handle the exceptions and exception logic gracefully. Python provides a rich set of built-in exceptions. All Built-in Exceptions in Python are inherited from the BaseException type.

Exception Class	Event
IOError	Due to an input/output operation is unsuccessful.
ArithmeticError	Due to numeric calculations is unsuccessful.
Floating-pointError	Due to a floating-point calculation is unsuccessful.
ZeroDivisionError	Due to division or modulo by zero takes place for all numeric types.
AssertionError	Due to the assert, the statement is unsuccessful.
OverflowError	Due to the result of an arithmetic operation is too large to be represented.
ImportError	Due to the imported module is not found.
IndexError	Due to the index of a sequence is out of range.
KeyboardInterruptError	Due to the user interrupts program execution, generally by pressing (Ctrl+C).
IndentationError	Due to there is a wrong indentation.
SyntaxError	Raised by the parser when a syntax error is encountered.
KeyError	Due to the specified key is not found in the dictionary.
NameError	Due to an identifier is not found in the local or global namespace.
TypeError	Due to a function or operation is applied to an object of an incorrect type.
ValueError	Due to a function gets an argument of the correct type but of an improper value.
RuntimeError	Due to a generated error does not fall into any category.

There are four main components of exception handling

try

The block of code is expected to have the exception.

except

This block of code is executed if the exception occurs.

else

This block of code is executed if no exception occurred in the code.

finally

This block of code will be executed in all circumstances, whether there is an exception or not.

The common syntax for exception handling

Exception handling

Handling  Common Exceptions

Arithmetic Error

There can be an arithmetic error like Zero Division Error, OverFlow, and Floating-point Error,

Zero Division Error

Zero Division Error occurs when a number is divided by zero.

#Zero Division Error

try:
    a=10

    b=0

    print(a/b)
except ZeroDivisionError:
    print("Zero division Error occured")
else:
    print('No Exception occured')

Output:

Zero division Error occured

Process finished with exit code 0

OverFlowError 

Overflow Error is raised when the result of the evaluation goes beyond the valid range.

#OverFlow Error

import math
try:
    print(math.exp(1000))
except OverflowError:
    print("Over Flow Error occured")
else:
    print('No Exception occured')

Output:

Over Flow Error occured

Assertion Error

The Assertion Error occurs when an Assert fails

#Assertion Error
try:
    a=10
    b='Python'
    assert a==b
except AssertionError:
    print("Assertion Error occured")
else:
    print('No Exception occured')

Output:
Assertion Error occured

ImortError
Import Error occurs when a non-existent module(unable to load) is tried to import.
#import Error
import nibabel
print([10,12,14])
Output:
Traceback (most recent call last):
  File "C:/Users/my/PycharmProjects/Facebook/venv/test.py", line 2, in <module>
    import nibabel
ModuleNotFoundError: No module named 'nibabel'

Lookup Errors
LookupError is the base class for the invalid key and index related errors.
Like IndexError and KeyError

IndexError
If the invalid index is accessed IndexError is raised.
#index Error
try:
    l=[10,12,14]
    print(l[3])
except IndexError:
    print('Wrong index')
else:
    print('No exception')
Output:
Wrong index

KeyError
If key is not available in dictionary but tried to access the KeyError is raised.
#Key Error
try:
    l={1:'Apple',2:'Ball',3:'Cat'}
    print(l[4])
except KeyError:
    print('KeyError occured')
else:
    print('No exception')

Output:
KeyError occured

MemoryError
MemoryError is raised when not enoough memory is available for the process to proceed further.

NameError
NameError is occured when a name whether it is global or local is not found.
#NameError
try:
    l={1:'Apple',2:'Ball',3:'Cat'}
    print(a) #a is not defined anywhere
except NameError:
    print('Name error')
else:
    print('No exception')

Output:
Name error
Disadvanatge of Python's Exception Handling is that the Python program with 
exception handling executes slower and the size of code also increases.