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Python Python Basics (2015) Logic in Python Try and Except

Trying to get past the Try/Except question

Here's my code but it says It looks like Task 1 is not passing

def add(a,b): try: return float(a) + float(b) except: ValueError return None else: return a + b

trial.py
def add(a,b):
    try:
        return float(a) + float(b)
    except: ValueError
        return None
    else:
        return a + b

4 Answers

Alex Koumparos
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.a{fill-rule:evenodd;}techdegree
Alex Koumparos
Python Development Techdegree Student 36,862 Points

Hi Yue Yue,

Glad to hear that the challenge passed for you but I should warn you that the code you just described wouldn't work the way you would expect and I think you might still be confused about how the except works.

The reason I said that your colon was in the wrong place and not the ValueError is that you are only trying to catch exceptions that are of the type ValueError. To do this you write the following:

except ValueError:

When you use this code, any bad value that causes the code in the try statement to fail will trigger a ValueError and be handled by the except clause. Importantly, any value that fails for a reason other than a ValueError (e.g., a TypeError) will not be caught by the except clause and will just crash with a stack trace.

The reason that this is important is so that when you write error handling code, you are actually handling the type of error you think you are: when you write a try statement you are doing so because you have some specific failure case in mind, which is what your except is catching. You still want the program to crash if an error that you didn't expect goes through the try statement because the code you wrote to handle the error you do expect probably isn't the right code to handle the error type you didn't expect.

In addition to this code which catches specific types of exception, there is a 'catch all' clause which will handle every type of error that wasn't specifically handled somewhere else. This is written as follows:

except:

Although this is valid Python it has narrow usage, it's generally used to pass the error off to some other part of code that will handle (or at least alert) that there is an error happening that the code can't resolve. In the real world, it's very unlikely that error handling code that can handle every possible error type can be written in that except block.

Moving the ValueError onto the next line doesn't do anything. It's like writing just 'hello world' on a line in the program, it doesn't get rendered to the screen, it doesn't alter the return value, it doesn't change any state. There is no situation where you'd just write an error name as a line of code.

I strongly recommend that you rewatch the 'Exception to the Rule' video to be sure you understand this concept as you are going to be seeing it a lot in future code you read and write. And if your error handling code doesn't behave the way you expect you are going to have a very frustrating experience (well, even more frustrating!) chasing down stubborn bugs.

Cheers

Alex

Cheers Alex - grateful for your help - worked once I moved the ValueError down to the next line :-)

Alex,

Thanks for taking the time to put me right - much appreciated.

Yue