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Python Python Basics Functions and Looping Raising Exceptions

Christopher Caron
Christopher Caron
1,270 Points
Posted by Christopher Caron
Christopher Caron
1,270 Points

I can't understand ValueError as err

Okay so I think I am confused -- sorry if this was answered elsewhere but I couldn't find it.

except ValueError as err:
  print("Oh no! That's not a valid value. Try again.")
  print("({})".format(err))

In that code, as soon as we get a ValueError it is going to be printing "Oh no! That's not a valid value. Try again." Right?

And then on the next line, it knows to print what we wrote in the function. How does it know to print what we wrote in the function? Is it because we assigned ValueError to err (sorry if "assigned" isn't the right term there) so when we use .format(err) it knows to reference ValueError in our function? If that's the case, why can't we just say

except ValueError
  print("Oh no! That's not a valid value. Try again.")
  print("({})".format(ValueError))

If that's NOT the case, what am I missing?

1 Answer

Alexander Davison
Alexander Davison
65,469 Points
Alexander Davison
Alexander Davison
65,469 Points

ValueError is not a value itself, it is actually a class. So except ValueError as err catches errors that are instances of the ValueError class and err holds the error string itself

Using as in this case isn't "assignment". In other cases it isn't assignment either, but is holding an alias of a value as an identifier.