Exceptions¶

http://openbookproject.net/thinkcs/python/english3e/exceptions.html

dealing with bugs is normal part of programming
debugging is a very handy programming skill

category of bugs¶

syntax errors
logical/semantic errros
runtime errors/exceptions

exceptions¶

when runtime error occurs, it creats an exception object
program halts; Python prints out the traceback with error message
https://docs.python.org/3/tutorial/errors.html

In [1]:

print(55/0)

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-1-ef255b193978> in <module>()
----> 1 print(55/0)

ZeroDivisionError: division by zero

In [2]:

alist = []
print(alist[0])

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-2-d994a3a20fe2> in <module>()
      1 alist = []
----> 2 print(alist[0])

IndexError: list index out of range

In [3]:

atup = ('a', 'b', 'c')
atup[0] = 'A'

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-8aeda75553d6> in <module>()
      1 atup = ('a', 'b', 'c')
----> 2 atup[0] = 'A'

TypeError: 'tuple' object does not support item assignment

each exception has two parts- Name: description

catching exceptions¶

use try and except blocks
try statement has several separate clauses/parts

    try:
        # statement(s) thay may potentially throw(s) exception
    except ExceptionName1:
        # catch specific exception ExceptionName1
        # statement(s) to handle the exception
    [except ExceptionName2 as err:
        # statements
    ]
    [except:
        # catch any error not caught by previous except blocks
    ]
    [else:
        # follows all except clause
        # executes if try clause does NOT raise an exception
    ]
    [finally:
        # clean-up actions that must be executed under all circumstances; 
        # exectued on the way out when try block is left via a break, continue, or return statement
    ]

[ ] optional

example 1¶

In [6]:

try:
    x = int(input("Enter dividend: "))
    y = int(input("Enter divisor: "))
    quotient = x/y
    remainder = x%y
except ZeroDivisionError as ex:
    print('Exception occured:', ex)
    print('arguments:', ex.args)
except:
    print('Some exception occured...')
else:
    print("quotient=", quotient)
    print("remainder=", remainder)
finally:
    print("executing finally clause")

Enter dividend: 10
Enter divisor: 2
quotient= 5.0
remainder= 0
executing finally clause

In [ ]:

example 2¶

input validation

In [7]:

while True:
    try:
        x = int(input("Please enter a number: "))
        break
    except ValueError:
        print("Oops! That was not a valid number. Try again...")

Please enter a number: f
Oops! That was not a valid number. Try again...
Please enter a number: dsaf
Oops! That was not a valid number. Try again...
Please enter a number: adsf
Oops! That was not a valid number. Try again...
Please enter a number: asdf
Oops! That was not a valid number. Try again...
Please enter a number: 10

raising exceptions¶

raise statement allows programer to throw their own exceptions

example 1¶

In [8]:

raise NameError("MyException")

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-8-290333e3086c> in <module>()
----> 1 raise NameError("MyException")

NameError: MyException

In [9]:

try:
    raise NameError('My Exception')
except NameError:
    print('An exception flew by...')
    raise

An exception flew by...

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-9-9b6ca7775e88> in <module>()
      1 try:
----> 2     raise NameError('My Exception')
      3 except NameError:
      4     print('An exception flew by...')
      5     raise

NameError: My Exception

user-defined exceptions¶

one can define their own exceptions and raise them as needed
should typically derive from the Exception class, either directly or indirectly

example 1¶

In [12]:

class InputError(Exception):
    """
    Exception raised for errors in the input.
    
    Attributes:
    expression -- input expression in which the error occured
    message -- explaination of the error
    """
    def __init__(self, expression, message):
        self.expression = expression
        self.message = message
    

In [13]:

help(InputError)

Help on class InputError in module __main__:

class InputError(builtins.Exception)
 |  Exception raised for errors in the input.
 |  
 |  Attributes:
 |  expression -- input expression in which the error occured
 |  message -- explaination of the error
 |  
 |  Method resolution order:
 |      InputError
 |      builtins.Exception
 |      builtins.BaseException
 |      builtins.object
 |  
 |  Methods defined here:
 |  
 |  __init__(self, expression, message)
 |      Initialize self.  See help(type(self)) for accurate signature.
 |  
 |  ----------------------------------------------------------------------
 |  Data descriptors defined here:
 |  
 |  __weakref__
 |      list of weak references to the object (if defined)
 |  
 |  ----------------------------------------------------------------------
 |  Methods inherited from builtins.Exception:
 |  
 |  __new__(*args, **kwargs) from builtins.type
 |      Create and return a new object.  See help(type) for accurate signature.
 |  
 |  ----------------------------------------------------------------------
 |  Methods inherited from builtins.BaseException:
 |  
 |  __delattr__(self, name, /)
 |      Implement delattr(self, name).
 |  
 |  __getattribute__(self, name, /)
 |      Return getattr(self, name).
 |  
 |  __reduce__(...)
 |      helper for pickle
 |  
 |  __repr__(self, /)
 |      Return repr(self).
 |  
 |  __setattr__(self, name, value, /)
 |      Implement setattr(self, name, value).
 |  
 |  __setstate__(...)
 |  
 |  __str__(self, /)
 |      Return str(self).
 |  
 |  with_traceback(...)
 |      Exception.with_traceback(tb) --
 |      set self.__traceback__ to tb and return self.
 |  
 |  ----------------------------------------------------------------------
 |  Data descriptors inherited from builtins.BaseException:
 |  
 |  __cause__
 |      exception cause
 |  
 |  __context__
 |      exception context
 |  
 |  __dict__
 |  
 |  __suppress_context__
 |  
 |  __traceback__
 |  
 |  args

In [1]:

def getInteger():
    x = input('Enter an integer number: ')
    if not x.isdigit():
        raise InputError(x, 'That is not an integer!')
    return int(x)

In [15]:

x = getInteger()
print(x)

Enter an integer number: dsaf

---------------------------------------------------------------------------
InputError                                Traceback (most recent call last)
<ipython-input-15-f90a077ee9cc> in <module>()
----> 1 x = getInteger()
      2 print(x)

<ipython-input-14-6a80b90df6da> in getInteger()
      2     x = input('Enter an integer number: ')
      3     if not x.isdigit():
----> 4         raise InputError(x, 'That is not an integer!')
      5     return int(x)

InputError: ('dsaf', 'That is not an integer!')

catch user-defined exception¶

In [2]:

try:
    x = getInteger() #may throw InputError
except InputError as ie:
    print('Exception:', ie)
    # can throw ie again
else:
    print('{}^2 = {}'.format(x, x**2))

Enter an integer number: 10
10^2 = 100

In [ ]: