Errors and Exceptions#

There are two distinguishable kinds of errors: syntax errors and exceptions.

  • Syntax errors, also known as parsing errors, are the most common.

  • Exceptions are errors caused by statement or expression syntactically corrects.

  • Exceptions are not unconditionally fatal.

Exceptions in Python documentation

import sys
try:
    10 * (1/0)
except:
    print(sys.exc_info()[0])

<class 'ZeroDivisionError'>

try:
    4 + spam*3
except:
    print(sys.exc_info()[0])

<class 'NameError'>

try:
    '2' + 2
except:
    print(sys.exc_info()[0])

<class 'TypeError'>

Handling Exceptions#

  • In example below, the user can interrupt the program with Control-C or the stop button in Jupyter Notebook.

  • Note that a user-generated interruption is signalled by raising the KeyboardInterrupt exception.

for s in ("0.1", "foo", "1000"):
   try:
     x = int(s)
     print(f' x = {x}')
     break
   except ValueError:
     print("Oops!  That was no valid number.  Try again...")

Oops!  That was no valid number.  Try again...
Oops!  That was no valid number.  Try again...
 x = 1000

  • A try statement may have more than one except clause

  • The optional else clause must follow all except clauses.

import sys

def process_file(file):
    " Read the first line of f and convert to int and check if this integer is positive"
    try:
        i = int(open(file).readline().strip()) 
        print(i)
        assert i > 0
    except OSError as err:
        print(f"OS error: {err}")
    except ValueError:
        print("Could not convert data to an integer.")
    except:
        print("Unexpected error:", sys.exc_info()[0])

# Create the file workfile.txt
with open('workfile.txt','w') as f:
    f.write("foo")
    f.write("bar")

process_file('workfile.txt') # catch exception return by int() call

Could not convert data to an integer.

# Change permission of the file, workfile.txt cannot be read
!chmod u-r workfile.txt

process_file('workfile.txt') # catch exception return by open() call

OS error: [Errno 13] Permission denied: 'workfile.txt'

# Let's delete the file workfile.txt
!rm -f workfile.txt

process_file('workfile.txt') # catch another exception return by open() call

OS error: [Errno 2] No such file or directory: 'workfile.txt'

# Insert the value -1 at the top of workfile.txt
!echo "-1" > workfile.txt
%cat workfile.txt

-1

process_file('workfile.txt') # catch exception return by assert()

-1
Unexpected error: <class 'AssertionError'>

Raising Exceptions#

The raise statement allows the programmer to force a specified exception to occur.

try:
    raise NameError('HiThere')
except:
    print(sys.exc_info()[0])

<class 'NameError'>

Defining Clean-up Actions#

  • The try statement has an optional clause which is intended to define clean-up actions that must be executed under all circumstances.

  • A finally clause is always executed before leaving the try statement

try:
     raise KeyboardInterrupt
except:
    print(sys.exc_info()[0])
finally:
     print('Goodbye, world!')

<class 'KeyboardInterrupt'>
Goodbye, world!

Wordcount Exercise#

  • Improve the function reduce to read the results of words by using the KeyError exception to fill in the dictionary.