Catching exception from a called function

Question

I have a function that reads a CSV, checks the values in the rows, and if everything is okay, it writes the rows to a new CSV file. I have a few validation functions I'm calling within my main function to check the value and format of the rows.

I'm trying to implement my main function in a way so that when I call the other validation functions and something doesn't check out, I skip writing that row entirely.

#main function
for row in reader:
    try:
        row['amnt'] = divisible_by_5(row['amnt'])
        row['issue_d'] = date_to_iso(row['issue_d'])
        writer.writerow(row)
    except:
        continue

#Validation function
def divisible_by_5(value):
    try:
        float_value = float(value)
        if float_value % 5 == 0 and float_value != 0:
            return float_value
        else:
            raise ValueError
    except ValueError:
        return None

At the moment, the writer is still writing rows that should be skipped. For example, if a number is not divisible by 5, instead of skipping that row, the writer is just writing ''.

So, how can I handle the exception (ValueError) raised in divisible_by_5 in my for loop so I don't write lines that raise an exception?

Answer 1

You can re-raise an exception caught in an exception handler and have another handler up the call stack handle it by using an empty raise statement inside the except block:

except ValueError:
    raise  # re raises the previous exception

This way, the outer handler in 'main' can catch it and continue.

Note: As is, your empty except: block in the for loop will catch everything. Specifying the exception expected (except ValueError in this case) is generally considered a good idea.

---

More simply, don't put the body of your validation function divisible_by_5 in a try statement. Python automatically searches for the nearest except block defined and uses it if the current exception matches the exceptions specified:

def raiser():
    if False: 
        pass
    else:
        raise ValueError

try:
    raiser()
except ValueError:
    print "caught"

This will print caught.