[Tut] How to Catch and Print Exception Messages in Python

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How to Catch and Print Exception Messages in Python

Python comes with an extensive support of exceptions and exception handling. An exception event interrupts and, if uncaught, immediately terminates a running program. The most popular examples are the

Code:

IndexError

,

Code:

ValueError

, and

Code:

TypeError

.

An exception will immediately terminate your program. To avoid this, you can catch the exception with a

Code:

try/except

block around the code where you expect that a certain exception may occur. Here’s how you catch and print a given exception:

To catch and print an exception that occurred in a code snippet, wrap it in an indented

Code:

try

block, followed by the command

Code:

"except Exception as e"

that catches the exception and saves its error message in string variable

Code:

e

. You can now print the error message with

Code:

"print(e)"

or use it for further processing.

Code:

try: # ... YOUR CODE HERE ... # except Exception as e: # ... PRINT THE ERROR MESSAGE ... # print(e)

Example 1: Catch and Print IndexError

If you try to access the list element with index 100 but your lists consist only of three elements, Python will throw an

Code:

IndexError

telling you that the listindex is out of range.

Code:

try: lst = ['Alice', 'Bob', 'Carl'] print(lst[3]) except Exception as e: print(e) print('Am I executed?')

Your genius code attempts to access the fourth element in your list with index 3—that doesn’t exist!



Fortunately, you wrapped the code in a

Code:

try/catch

block and printed the exception. The program is not terminated. Thus, it executes the final

Code:

print()

statement after the exception has been caught and handled. This is the output of the previous code snippet.

Code:

list index out of range Am I executed?

Example 2: Catch and Print ValueError

The

Code:

ValueError

arises if you try to use wrong values in some functions. Here’s an example where the

Code:

ValueError

is raised because you tried to calculate the square root of a negative number:

Code:

import math try: a = math.sqrt(-2) except Exception as e: print(e) print('Am I executed?')

The output shows that not only the error message but also the string

Code:

'Am I executed?'

is printed.

Code:

math domain error Am I executed?

Example 3: Catch and Print TypeError

Python throws the

Code:

TypeError object is not subscriptable

if you use indexing with the square bracket notation on an object that is not indexable. This is the case if the object doesn’t define the

Code:

__getitem__()

method. Here’s how you can catch the error and print it to your shell:

Code:

try: variable = None print(variable[0]) except Exception as e: print(e) print('Am I executed?')

The output shows that not only the error message but also the string

Code:

'Am I executed?'

is printed.

Code:

'NoneType' object is not subscriptable Am I executed?

I hope you’re now able to catch and print your error messages.

Summary

To catch and print an exception that occurred in a code snippet, wrap it in an indented

Code:

try

block, followed by the command

Code:

"except Exception as e"

that catches the exception and saves its error message in string variable

Code:

e

. You can now print the error message with

Code:

"print(e)"

or use it for further processing.

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