Exceptions and traceback

Education objectives

• as a user: don’t panic, read

• block try with keywords except and finally

• keyword raise

Exceptions and try, except syntax

Exceptions and errors are common in all codes. There is a good system to handle them in Python. Let’s first see the following code, which gives an error despite being syntactically correct:

letters = "abc"
i = 3
print(letters[i])

When these lines are executed, Python stops its execution and print a traceback:

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-30-8df9cec1a0ec> in <module>()
      1 letters = "abc"
      2 i = 3
----> 3 print(letters[i])

IndexError: string index out of range

Handling exception:

letters = "abc"
i = 3
try:
    print(letters[i])
except IndexError as e:
    print(f"An IndexError has been raised and caught (message: '{e}')")

An IndexError has been raised and caught (message: 'string index out of range')

Warning

Never use just

except:

It means except BaseException, i.e. “except all errors and exceptions”. A user Control-C is an exception (KeyboardInterrupt) so it would be caught and have no effect.

if you want to catch “all possible exceptions (but not KeyboardInterrupt)”, use instead

except Exception:

issubclass(KeyboardInterrupt, BaseException) is true but issubclass(KeyboardInterrupt, Exception) is false so except Exception: does not catch KeyboardInterrupt.

Yes, this might be the first time you encounter the builtin function issubclass and the concept of hierarchy of exceptions.

We computed the average value of a list of numbers in a previous exercise. The following code seems innocuous

numbers = [1, 2]
avg0 = sum(numbers) / len(numbers)

Except in the edge case when you apply it to an empty list. try and except is a good way to avoid errors in your code

numbers = []
try:
    avg0 = sum(numbers) / len(numbers)
except ZeroDivisionError:
    print("Cannot compute average of empty list")

Cannot compute average of empty list

Full syntax

try:
    ...
except <exception1> as e1:
    ...
except <exception2> as e2:
    ...
else:
    ...
finally:
    ...

Non exhaustive error list:

• ArithmeticError

• ZeroDivisionError

• IndexError

• KeyError

• AttributeError

• IOError

• ImportError

• NameError

• SyntaxError

• TypeError

Exercise 13

Place each element of this string in a list while trying to convert elements to floats (i.e. “hello” should raise an error that it cannot be represented as a float, 2 and 1.5 should become floats). Do it by catching errors.

str_variables = "hello 1.5 2"

the_list_you_should_get = ["hello", 1.5, 2.0]

Hints:

• float("a") raise ValueError,

• the str str_variable should be split.

Solution to Exercise 13

split_list = []
for value in str_variables.split():
    try:
        value = float(value)
    except ValueError:
        print(value, "is not a number")
    split_list.append(value)


print(split_list)

hello is not a number
['hello', 1.5, 2.0]