Raising an exception during for loop and continuing at next index in python

Question

I've been breaking my head all day, and I cannot seem to solve this. I'm supposed to write an Exception class then raise it during an iteration and continue where I left off.

    class OhNoNotTrueException(Exception):
    """Exception raised when False is encountered
    Attributes:
    message -- explanation of the error"""

    def __init__(self, value):
    self.value = value



   am_i_true_or_false = [True, None, False, "True", 0, "", 8, "False", "True", "0.0"]

  try:
     for i in am_i_true_or_false:
        if i is False:
           raise OhNoNotTrueException(i)
           continue                      #<--this continue does not work
     else:
        print(i, "is True")

  except OhNoNotTrueException as e:
  print(e.value, "is False")

However, I can't get the iteration back to the last index, even after putting continue. I'm not sure if this is the only way to do it, but I'm breaking my head over here. Anyone want to take a crack at it?

I'm supposed to get the following output:

True is true.

None is false

False is false

True is true.

0 is false

is false

8 is true.

False is true.

True is true.

0.0 is true.

juanpa.arrivillaga · Accepted Answer · 2016-07-25 04:25:16Z

3

Everything after the exception is raised will never be reached, and you will be taken outside the loop, as if everything in the try-block had never happend. Thus, you need to try/except inside the loop:

In [5]: for i in am_i_true_or_false:
   ...:     try:
   ...:         if i is False:
   ...:             raise OhNoNotTrueException(i)
   ...:         else:
   ...:             print(i, "is not False")
   ...:     except OhNoNotTrueException as e:
   ...:         print(e.value, "is False")
   ...:         
True is not False
None is not False
False is False
True is not False
0 is not False
 is not False
8 is not False
False is not False
True is not False
0.0 is not False

Notice what happens if your try-block contains the loop:

In [2]: try:
   ...:     for i in am_i_true_or_false:
   ...:         if i is False:
   ...:             raise Exception()
   ...:         else:
   ...:             print(i,"is not False")
   ...: except Exception as e:
   ...:     continue
   ...: 
  File "<ipython-input-2-97971e491461>", line 8
    continue
    ^
SyntaxError: 'continue' not properly in loop

edited Jul 25, 2016 at 4:25

answered Jul 25, 2016 at 4:15

juanpa.arrivillaga

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2 Comments

jotasi Over a year ago

As I understand his question, he actually does not want to have a continue statement in his exception block, but wants it to print that this value is false. You should probably include that.

rdmtinez Over a year ago

Thank you for the explanation! that makes a lot of sense now! I was under the impression that I had to enclose the for loop with the try bloc, and not the other way around.

jotasi · Accepted Answer · 2016-07-25 04:15:36Z

2

You should do the loop outside of the try/except block. Then one individual list entry will be checken by the try catch and afterwards the loop will continue with the next one:

for i in am_i_true_or_false:
    try:
       if i is False:
           raise OhNoNotTrueException(i)
       else:
           print("{} is True".format(i))
    except OhNoNotTrueException as e:
        print("{} is False".format(e.value))

The way you are doing it, the loop is executed until the first exception and then the except block is executed and the program ends. The continue is not reached in this case because you threw an exception which will be caught in the except block.

answered Jul 25, 2016 at 4:15

jotasi

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