Errors and Exception Handling

In this blog we will learn about Errors and Exception Handling in Python. You’ve definitely already encountered errors by this point in the course. For example:

Note how we get a Syntax Error, with the further description that it was an EOL (End of Line Error) while scanning the string literal. This is specific enough for us to see that we forgot a single quote at the end of the line. Understanding these various error types will help you debug your code much faster.

This type of error and description is known as an Exception. Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected during execution are called exceptions and are not unconditionally fatal.

You can check out the full list of built-in exceptions here. Now let’s learn how to handle errors and exceptions in our own code.

try and except

The basic terminology and syntax used to handle errors in Python are the try and except statements. The code which can cause an exception to occur is put in the try block and the handling of the exception is then implemented in the except block of code. The syntax follows:

try:   You do your operations here…   …except Exception I:   If there is Exception I, then execute this block.except Exception II:   If there is Exception II, then execute this block.   …else:   If there is no exception then execute this block.

We can also just check for any exception with just using except: To get a better understanding of all this let’s check out an example: We will look at some code that opens and writes a file:

Now let’s see what would happen if we did not have write permission (opening only with ‘r’):

Great! Notice how we only printed a statement! The code still ran and we were able to continue doing actions and running code blocks. This is extremely useful when you have to account for possible input errors in your code. You can be prepared for the error and keep running code, instead of your code just breaking as we saw above.

We could have also just said except: if we weren’t sure what exception would occur. For example:

Great! Now we don’t actually need to memorize that list of exception types! Now what if we kept wanting to run code after the exception occurred? This is where finally comes in.

finally

The finally: block of code will always be run regardless if there was an exception in the try code block. The syntax is:

try:

Code block here

…

Due to any exception, this code may be skipped!

finally:

This code block would always be executed.

Let’s see a new example that will take into account a user providing the wrong input:

Here if we provide non-integer value it will give error ,because of improper assignment of print on the last line .here we can use while loop to fix it. Try it by your self