Handling exceptions with try and except

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20 questions about 7 ideas of this lesson. Each answer is explained at once, and what you get right is kept in this browser.

1. Question 1 of 20 An exception matches two except clauses of the same try statement. How many of them run?

   Choose one answer.

   Both, from top to bottom The one that names the exception's own class None, because overlapping clauses are a SyntaxError Only the first one that matches

   Show the answer: Question 1 of 20

   Answer: Only the first one that matches

   At most one except clause runs. Python takes the first clause, from the top, whose class is the exception's own class or one of its base classes, runs it, and carries on after the statement.

2. Question 2 of 20 What does this print?

   Read the code, then choose one answer.

   try:
       print("a")
       n = int("4x")
       print("b")
   except ValueError:
       print("c")
   print("d")

   a b c d c d a c d a c

   Show the answer: Question 2 of 20

   Answer: a c d

   The try block runs until int("4x") raises ValueError. The rest of the block, print("b"), is skipped, the except clause prints c, and the line after the statement prints d.

3. Question 3 of 20 What does this print?

   Read the code, then choose one answer.

   def parse(text):
       try:
           return int(text)
       except ValueError:
           return 0
   
   try:
       print(parse(None))
   except TypeError:
       print("caller handled it")

   0 None Nothing; a traceback ends the program caller handled it

   Show the answer: Question 3 of 20

   Answer: caller handled it

   An exception that a try statement does not name travels on to the code that called the function, as if that try statement were not there. Here the caller names TypeError, so print(parse(None)) is abandoned and its handler runs.

4. Question 4 of 20 Which of these exceptions does a clause written except LookupError: handle?

   Choose every answer that is right.

   IndexError, from a list index past the end ZeroDivisionError, from 1 / 0 KeyError, from a missing dictionary key ValueError, from int("ten")

   Show the answer: Question 4 of 20

   Answer:

   • IndexError, from a list index past the end
   • KeyError, from a missing dictionary key

   A clause matches the class it names and every class derived from it. IndexError and KeyError are both subclasses of LookupError; ValueError is not, and ZeroDivisionError is an ArithmeticError.

5. Question 5 of 20 Which of these clauses is a SyntaxError in Python 3.14?

   Choose one answer.

   except (ValueError, TypeError): except ValueError, TypeError as err: except ValueError, TypeError: except (ValueError, TypeError) as err:

   Show the answer: Question 5 of 20

   Answer: except ValueError, TypeError as err:

   Python 3.14 (PEP 758) lets you leave out the brackets around several exception types only when the clause has no "as". With "as", the brackets are required, so the third line fails with "multiple exception types must be parenthesized when using 'as'".

6. Question 6 of 20 no-such-file.txt does not exist. What does this print?

   Read the code, then choose one answer.

   try:
       open("no-such-file.txt")
   except OSError:
       print("OSError")
   except FileNotFoundError:
       print("missing file")

   A FileNotFoundError traceback missing file OSError OSError, then missing file

   Show the answer: Question 6 of 20

   Answer: OSError

   Python takes the first clause that matches, and except OSError: matches every subclass of OSError. To handle a missing file in its own way, put except FileNotFoundError: above except OSError:.

7. Question 7 of 20 average_broad([70, 80, 90]) in too_broad.py returned 0.0 instead of 80.0. Why?

   Choose one answer.

   sum() returned 0 The list was treated as empty Python rounds the average down when it catches an exception except Exception also caught the NameError from the misspelt mark

   Show the answer: Question 7 of 20

   Answer: except Exception also caught the NameError from the misspelt mark

   len(mark) raises NameError, because the name is marks. except Exception: catches it like any other error and returns 0.0, a believable but wrong answer, and nothing points at the bug. Naming ZeroDivisionError, the only error an empty list causes, lets the typo reach the traceback.

8. Question 8 of 20 A loop's body is wrapped in try: and a bare except: that prints "skipped". What happens when the user presses Ctrl-C while it runs?

   Choose one answer.

   The handler catches the KeyboardInterrupt, prints "skipped", and the loop carries on Python raises a SyntaxError, because a bare except is not allowed in a loop The KeyboardInterrupt passes through, because a bare except catches only Exception The program stops, because Ctrl-C always ends a program

   Show the answer: Question 8 of 20

   Answer: The handler catches the KeyboardInterrupt, prints "skipped", and the loop carries on

   A bare except: matches every exception, KeyboardInterrupt and SystemExit included, even though they are not derived from Exception. Naming the exceptions you expect, or at most Exception, lets Ctrl-C end the program.

9. Question 9 of 20 A function reads a port with int(settings["port"]). A missing key is a bug in the caller; text that is not a number should give port 80. Which handler fits?

   Choose one answer.

   except ValueError: return 80 except: return 80 except (KeyError, ValueError): return 80 except Exception: return 80

   Show the answer: Question 9 of 20

   Answer: except ValueError: return 80

   Name the one exception you can do something about. Bad text has a sensible answer, port 80; a missing key means the calling code is wrong, so it should reach a traceback where someone will see it.

10. Question 10 of 20 When does the else clause of a try statement run?

    Choose one answer.

    When no except clause matched the exception When an except clause has handled an exception When the try block ran to its end without an exception Always, like finally

    Show the answer: Question 10 of 20

    Answer: When the try block ran to its end without an exception

    else runs only when the try block finished without raising and was not left early by return, break or continue. Its job is to keep the try block down to the lines whose errors you mean to handle.

11. Question 11 of 20 settings is {"port": "8080"}. What does read_port(settings) do?

    Read the code, then choose one answer.

    def read_port(settings):
        try:
            text = settings["port"]
        except KeyError:
            return 80
        else:
            return int(settings["Port"])

    It raises ValueError It returns 80, because the except clause handles the KeyError It raises KeyError, because the except clause does not handle errors raised in else It returns 8080

    Show the answer: Question 11 of 20

    Answer: It raises KeyError, because the except clause does not handle errors raised in else

    The lookup in the try block succeeds, so else runs. Its lookup uses the wrong key, "Port", and the KeyError it raises is not handled by the except clause of the same statement, so it reaches the caller. That is the point of else: only the try block's errors go to the handlers.

12. Question 12 of 20 What does this print?

    Read the code, then choose one answer.

    def check(n):
        try:
            if n < 0:
                return "negative"
        except TypeError:
            return "not a number"
        else:
            return "fine"
        finally:
            print("checked")
    
    
    print(check(-5))

    checked negative negative negative checked checked fine

    Show the answer: Question 12 of 20

    Answer: checked negative

    The try block returns before it reaches its end, so the else clause never runs. On the way out, finally prints checked, and only then does print() in the caller show negative.

13. Question 13 of 20 What does finally_cleanup.py print?

    What does this program print? Choose one answer.

    class Connection:
        def __init__(self, name):
            self.name = name
            print(f"open {name}")
    
        def send(self, data):
            if not data:
                raise ValueError("nothing to send")
            print(f"sent {data!r} to {self.name}")
    
        def close(self):
            print(f"closed {self.name}")
    
    
    def upload(data):
        conn = Connection("server")
        try:
            conn.send(data)
            return "done"
        finally:
            conn.close()  # runs before the function returns, and also when send() raises
    
    
    print(upload("marks.csv"))
    try:
        upload("")
    except ValueError as err:
        print("upload failed:", err)

    open server sent 'marks.csv' to server done closed server open server upload failed: nothing to send open server sent 'marks.csv' to server closed server done open server upload failed: nothing to send closed server open server sent 'marks.csv' to server closed server done open server closed server upload failed: nothing to send

    Show the answer: Question 13 of 20

    Answer: it prints

    open server
    sent 'marks.csv' to server
    closed server
    done
    open server
    closed server
    upload failed: nothing to send

    The finally clause runs before the function hands back its return value, so "closed server" comes before "done". In the second upload, send() raises ValueError: finally closes the connection first, and only then does the exception reach the caller's except clause, which prints "upload failed".

14. Question 14 of 20 A try block raises ValueError, and its finally clause runs return "all good". What happens?

    Choose one answer.

    The ValueError is raised after the return Python 3.14 refuses to run the function The function returns None The ValueError is discarded and the function returns "all good"

    Show the answer: Question 14 of 20

    Answer: The ValueError is discarded and the function returns "all good"

    A return, break or continue that leaves a finally block discards the exception that was on its way out, which is why finally_return.py printed all good. Since Python 3.14 (PEP 765) the compiler warns about it.

15. Question 15 of 20 In which of these cases does a finally clause run?

    Choose every answer that is right.

    Never after an except clause has run The try block runs return The try block raises an exception that no clause handles The try block ends normally

    Show the answer: Question 15 of 20

    Answer:

    • The try block runs return
    • The try block raises an exception that no clause handles
    • The try block ends normally

    finally runs whatever happens in the try block: a normal end, a return, an exception that a clause handles and one that nobody handles. That is what makes it the place to give back what the block borrowed.

16. Question 16 of 20 Put the clauses of a full try statement in the order Python requires.

    Give each item its position, from 1 (first).

    1. finally Position… 1234
    2. try Position… 1234
    3. else Position… 1234
    4. except Position… 1234

    Show the answer: Question 16 of 20

    Answer:

    1. try
    2. except
    3. else
    4. finally

    The try block comes first, then one or more except clauses, then else (allowed only after an except clause), and finally last of all.

17. Question 17 of 20 A try statement has except KeyError, else and finally clauses. The try block succeeds, and then the else block raises KeyError. What happens?

    Choose one answer.

    Python runs the try block again The KeyError goes to the caller and finally is skipped finally runs, then the KeyError goes on to the caller The except KeyError clause handles it, then finally runs

    Show the answer: Question 17 of 20

    Answer: finally runs, then the KeyError goes on to the caller

    else runs because the try block raised nothing. Its KeyError is not one the except clauses handle, so it leaves the statement, and finally runs on the way out, as it does on every path.

18. Question 18 of 20 In check_then_act.py, why did book_lbyl() raise KeyError although it had checked that 12B was free?

    Choose one answer.

    book_lbyl() misspelt the seat pop() always raises KeyError The in test of a dictionary checks values, not keys Another booking took the seat between the check and pop()

    Show the answer: Question 18 of 20

    Answer: Another booking took the seat between the check and pop()

    Look before you leap leaves a gap between the check and the action. Another booking used it to take 12B, so pop() found no key, and the check had protected nothing.

19. Question 19 of 20 A clean-up job may delete a report file at any moment. Which code reads it safely?

    Choose one answer.

    Call os.path.exists(path) twice before open(path) Open it inside try, with except Exception and pass Call os.path.exists(path) first, then open(path) Open it inside try, with except FileNotFoundError for a file that has gone

    Show the answer: Question 19 of 20

    Answer: Open it inside try, with except FileNotFoundError for a file that has gone

    When something else can change the data between a check and its use, try the operation and handle the one exception that means "it is not there", here FileNotFoundError.

20. Question 20 of 20 A sign-up form has five fields, and nothing may be saved unless all five are filled in. How do you check?

    Choose one answer.

    Save each field inside try and stop at the first error Catch KeyError for each missing field while saving Save all five, then catch Exception at the end Check every field with an if before saving any of them

    Show the answer: Question 20 of 20

    Answer: Check every field with an if before saving any of them

    Trying first suits one step that may fail. When several steps must all succeed or none should happen, check everything first; with nothing else changing the form, the checks cannot go out of date.