Exception & Errors in Python

Logical Error(Exception)

These errors occur at the runtime after the syntax test has been passed. They occur because of a logical error by the user; hence they are called logical errors or exceptions. 

For example, we may try to divide a number by zero, which is an invalid operation (ZeroDivisionError), or we may attempt to import a library that does not exist (ModuleNotFoundError). 

Python produces an exception object if these types of runtime issues occur. It outputs a traceback to that problem and some data about why that issue happened if it is not handled appropriately.

Example:

>>> import lib
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'lib'

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Check out this article - String slicing in Python and Convert String to List Python.

Built-in Exceptions

There are several built-in exceptions in Python. All the exceptions in Python must be instances of a class that derives from the BaseException class. Two exception classes that are not related by subclassing are never equivalent. 

Using the built-in local() method, we can see all of the built-in exceptions, as seen below.

print(dir(locals()['__builtins__']))

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To print a module that contains a dictionary of built-in exceptions, functions, and attributes, we can use the below command. 

locals()['__builtins__']

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Examples of Built-in Exceptions

There are many built-in exceptions present in Python. Some of them, along with their examples, are mentioned below.

ZeroDivisionError Exception

The AssertionError exception is raised when an assert exception fails.
Example:

a = 1/0

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Output:

ZeroDivisionError: integer division or modulo by zero

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ValueError Exception

The ValueError exception is raised when a function receives an argument that has the correct type but an invalid value.
Example:

a = 1/0

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Output:

    num = int("str")
ValueError: invalid literal for int() with base 10: 'str'

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AssertionError Exception

The AssertionError exception is raised when an assert exception fails.
Example:

assert 1<0, "Condition not met"

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Output:

    assert 1<0, "Condition not met"
AssertionError: Condition not met

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AttributeError Exception

When an attribute reference or assignment fails, such as when a non-existent attribute is referenced, an AttributeError is thrown.

Example:

class Human:
    pass 

person = Human()
print(person.tail)

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Output:

    print(person.tail)
AttributeError: 'Human' object has no attribute 'tail'

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ModuleNotFoundError Exception

The ModuleNotFoundError exception is raised when the user tries to import a library that does not exist. 
Example:

import module

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Output:

    import module
ModuleNotFoundError: No module named 'module'

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OverflowError Exception

The OverflowError exception is raised when the limits of the platform’s support for float are reached.
Example:

import math
  
print (math.exp(100000))

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Output:

    print (math.exp(100000))
OverflowError: math range error

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IndexError Exception

The IndexError exception is raised when a sequence is indexed with a number that is out of range. 
Example:

arr = [5,6,7]
arr[5]=1

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Output:

    arr[5]=1
IndexError: list assignment index out of range

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KeyError Exception

The KeyError exception is raised when there was an issue retrieving the key used. 
Example:

d = {}
d[1]=d[1]+1

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Output:

    d[1]=d[1]+1
KeyError: 1

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KeyboardInterrupt Exception

The KeyboardInterrupt exception is raised when the user uses the interrupt key to halt the program. 
Example:

while 1:
    pass 

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Output:

    pass
KeyboardInterrupt

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NameError Exception

The AssertionError exception is raised when an assert exception fails.
Example:

def fun():
    print(var)
fun()

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Output:

    print(var)
NameError: name 'var' is not defined

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You can practice by yourself with the help of online python compiler.

Frequently Asked Questions

What is the difference between exceptions and errors?

The problems in the program due to which the program is not able to complete its execution are called errors. In contrast, exceptions are raised due to some internal event that obstructs the normal flow of the execution. 

Is there any way to handle exceptions in Python?

Exceptions can be manually handled by the user using try-except statements. 

How to find a list of all exceptions in Python?

The built-in exceptions in Python can be viewed by using “locals()['__builtins__']” function. It returns a dictionary of built-in exceptions, attributes, and functions in Python.  

Conclusion

Congratulations on making it this far.

In this blog, we understood Python's type-conversion and typecasting. 

If you want to become proficient with Python programming, I suggest you take the Coding Ninjas Python Course, which will teach Python basics with Data Structures and Algorithms. 

Recommended Readings:

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• Multiple Exception Handling in Python
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