key error python LEARNOVITA

What are Python KeyError Exceptions ? Expert’s Top Picks

Last updated on 27th Oct 2022, Artciles, Blog

About author

Jahir Usain (RPA with Python )

Jahir Usain is a Python developer with 4+ years of experience in Python, NLP, NLTK, and IBM Watson Assistant. His articles help the learners get insights into the domain.

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    • In this article you will get
    • Introduction
    • Exceptions versus Syntax Errors
    • Raising an Exception
    • The Assertion Error Exception
    • The try and except Block: The Handling Exceptions
    • Cleaning Up After Using finally
    • Conclusion


A Python program are terminates as soon as it encounters an error. In a Python, an error can be a syntax error or exception. In this article, will see what an exception is and how it varies from a syntax error. After that, will learn about raising an exceptions and making assertions. Then, wi,ll finish with a demonstration of a try and except block.

Example for Python Key error

Exceptions versus Syntax Errors

Syntax errors occur when a parser detects an incorrect statement. Observe a following example:

  • >>> print( 0 / 0 ))
  • File “<"stdin">“, line 1
  • print( 0 / 0 ))
  • ^
  • SyntaxError: invalid syntax

The arrow indicates where a parser ran into syntax error. In this example, there was a one bracket too many. Remove it and run a code again:

This time, ran into an exception error. This type of an error occurs whenever syntactically correct a Python code results in an error. The last line of a message indicated what type of an exception error ran into.

Instead of a showing the message exception error, Python details what type of an exception error was encountered. In this case, it was ZeroDivisionError. Python comes with a various built-in exceptions as well as possibility to create a self-defined exceptions.

Raising an Exception

Can use raise to throw exception if a condition occurs. The statement can be a complemented with custom exception.If need to throw an error when a certain condition occurs using raise, and could go about it like this:

  • x = 10
  • if x > 5:
  • raise Exception(‘x should not be exceed 5. The value of a x was: {}’.format(x))
  • When run this code, the output will be a following:
  • Traceback (most recent call last):
  • File “<"input">“, line 4, in <"module">
  • Exception: x should not be exceed 5. The value of a x was: 10

The program comes to be a halt and displays our exception to a screen, offering a clues about what went wrong.

The Assertion Error Exception

Instead of waiting for the program to crash midway, and can also start by making the assertion in Python. Assert that a certain condition is met. If this condition turns out to be a True, then that is excellent! The program can be continue. If condition turns out to be False, can have the program throw the AssertionError exception.

Have a look at a following example, where it is an asserted that a code will be executed on Linux system:

  • import sys
  • assert (‘linux’ in sys.platform), This code runs on a Linux only.

If run this code on a Linux machine, an assertion passes. If were to run this code on Windows machine, the outcome of a assertion would be False .In this example, throwing the AssertionError exception is a last thing that the program will do. The program will come to be halt and will not continue.

Python exceptions

The try and except Block: Handling Exceptions

The try and except block in a Python is used to catch and also handle exceptions. Python executes a code following the try statement as a “normal” part of a program. The code that follows an except statement is a program’s response to any exceptions in preceding try clause.

As saw earlier, when syntactically correct a code runs into an error, Python will throw the exception error.This exception are error will crash the program if it is be unhandled. The except clause a determines how a program responds to an exceptions The linux_interaction() can only run on Linux system. The assert in this function will throw the AssertionError exception if call it on an operating system other then a Linux.

The way handled the error here is by handing out the pass. If were to run this code on the Windows machine,got nothing. The good thing here is that a program did not crash. To this end, can change the pass into the something that would generate informative message, like so:

  • When an exception occurs in the program running this function, the program will be continue as well as inform about the fact that the function call was not be successful.
  • What did not get to see was a type of error that was thrown as a result of a function call. In order to see an exactly what went wrong, and would need to catch error that a function threw.
  • The first message is an AssertionError, informing that the function can only be an executed on a Linux machine. The second message are tells which function was not be executed.
  • In the previous, called a function that are wrote yourself. When executed a function, caught a AssertionError exception and printed it to screen.
  • Can have more than one function call in a try clause and anticipate catching the various exceptions. A thing to note here is that a code in the try clause will stop as soon as exception is encountered.

If a file does not exist, running this code on the Windows machine will output are following:

Function can only run on a Linux systems.

  • Linux linux_interaction() function was not be executed

Inside a try clause, you ran into an exception immediately and did not get to a part where you attempt to open file.log. Here are key takeaways:

  • A try clause is an executed up until the point where a first exception is encountered.
  • Inside a except clause, or the exception handler, find how the program responds to an exception.
  • Can anticipate the multiple exceptions and differentiate how a program should respond to them.
  • Avoid using a bare except clauses.

Cleaning up after using finally

Imagine that always had to implement a some sort of action to clean up after an executing a code. Python enables to do so using a finally clause.Everything in the finally clause will be an executed. It does not matter if can encounter an exception somewhere in a try or else clauses. Running a previous code on a Windows machine.


Raise allows to throw an exception at a any time.Assert enables to verify if a certain condition is met and throw the exception if it isn’t.In a try clause, all statements are executed until a exception is be encountered.Except is used to catch and handle exception(s) that are encountered in a try clause.Else lets code sections that are should run only when no exceptions are be encountered in a try clause.Finally enables to execute sections of code that are should always run, with or without any previously encountered exceptions.

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