Handling Errors in Python

Posted by martin on 25 Aug 2002.

Avoid the ugly error messages in Python by implementing custom error handlers.

When you're writing an application you cannot test all input to the program that users may supply. The default behavior when an error condition occurs in Python is to print an ugly error message and halt processing of the script.

Example:

print 1/'a'
Traceback (most recent call last):
 File "<stdin>", line 1, in ?
TypeError: unsupported operand type(s) for /: 'int' and 'str'

How do I handle the errors

If you are familiar with Java you can do thing the same way in Python - with a try: ... except: statement.

Example:

try:
	print 1/'a'
except TypeError:
	print 'Division is unsupported on this type of data'

With this little modification your program will run and you can print a nicer error message. This statement will handle only TypeError errors, if you want to handle other errors you can add another except statement or use an except statement with no error specified which will handle all errors.

Example:

def get_line(prompt) :
	"""Reads a line of user input."""

	try :
		while 1 :
			line = raw_input(prompt + ': ')
			if line :
				return line
	except EOFError :
		return ''
	except KeyboardInterrupt :
		print 'Cancelled by user'
		return ''


name = get_line('Enter your name')
print name

This is a simple script to request user input, note how the two except statements are used.

With a try ... except statement you can also use an else clause which is executed only if no exception was generated. This is not the same as moving the code from the else to the try section as an exception triggered in the else section will not be handled.