Catch KeyError in Python

Question

If I run the code:

connection = manager.connect("I2Cx") 

The program crashes and reports a KeyError because I2Cx doesn't exist (it should be I2C).

But if I do:

try:     connection = manager.connect("I2Cx") except Exception, e:     print e 

It doesn't print anything for e. I would like to be able to print the exception that was thrown. If I try the same thing with a divide by zero operation it is caught and reported properly in both cases. What am I missing here?

Answer 1

If it's raising a KeyError with no message, then it won't print anything. If you do...

try:     connection = manager.connect("I2Cx") except Exception as e:     print repr(e) 

...you'll at least get the exception class name.

A better alternative is to use multiple except blocks, and only 'catch' the exceptions you intend to handle...

try:     connection = manager.connect("I2Cx") except KeyError as e:     print 'I got a KeyError - reason "%s"' % str(e) except IndexError as e:     print 'I got an IndexError - reason "%s"' % str(e) 

There are valid reasons to catch all exceptions, but you should almost always re-raise them if you do...

try:     connection = manager.connect("I2Cx") except KeyError as e:     print 'I got a KeyError - reason "%s"' % str(e) except:     print 'I got another exception, but I should re-raise'     raise 

...because you probably don't want to handle KeyboardInterrupt if the user presses CTRL-C, nor SystemExit if the try-block calls sys.exit().