--D. Thiebaut (talk) 07:14, 29 March 2015 (EDT)

Exceptions

Part 1: Preparation

Create a new program called lab9_1.py, and copy this code to the new Idle window.

# lab9_1.py
# Your name here

# getInput: returns an integer larger
# than 0.  Expected to be robust
def getInput():
   
   while  True:
      x = int( input( "Enter an integer greater than 0: " ) )
      if x <= 0:
         print( "Invalid entry.  Try again!" )
      else:   
         return x

def main():
   num = getInput()
   print( "You have entered", num )

main()

Test it with numbers such as -3, -10, 0, 5. Verify that the input function works well when you enter numbers.
Test your program again, and this time enter expressions such as "6.3", or "hello" (without the quotes).
Make a note of the Error reported by Python:

Modify your function and add the highlighted code below by hand. This code will catch the ValueError exception.

# getInput: returns an integer larger
# than 0.  Catches Value errors
def getInput():
    
   # repeat forever...
   while  True:

      # try to get an int
      try:
          x = int( input( "Enter an integer greater than 0: " ) )
      except ValueError:
          # the user must have entered something other than an int
          print( "Invalid entry.  Not an integer.  Try again!" )
          continue

      # No errors caught.  See if the number is negative
      if x <= 0:
         print( "You entered a negative number.  Try again!" )
      else:   
         # finally, we can return x as it is an int that is >0
         return x

Run your program and try different invalid inputs, such as strings or floats. You can also try just pressing the Return key, indicating that you are not providing anything to the input function. Verify that your program catches all these invalid entries and does not crash.

Review Class Example

Below is an example we saw in class, and that is taken from Zelle. It illustrates how we can guard some code against several types of errors. Review it. You will need to follow this example for the next set of exercises.

def ZelleExample():
    import math
    print( "solution to quadratic equation" )
    try:
        a, b, c = eval( input( "enter 3 coefficients (a,b,c) " ) )
        disc = math.sqrt( b*b - 4*a*c )
        root1 = (-b + disc )/ (2*a)
        root2 = (+b + disc )/ (2*a)
        print( "solutions: ", root1, root2 )
    except NameError:
        print( "You didn't enter 3 numbers" )
    except TypeError:
        print( "Your inputs were not all numbers" )
    except SyntaxError:
        print( "Forgot commas between the numbers?" )
    except ValueError:
        print( "No real roots, negative discriminant" )
    except:
        print( "Something went wrong..." )

Part 2: Exercise

Create a new program called lab9_2.py with the code below:

def example1():
    for i in range( 3 ):
        x = int( input( "enter an integer: " ) )
        y = int( input( "enter another integer: " ) )
        print( x, '/', y, '=', x/y )

def example2( L ):
    print( "\n\nExample 2" )
    sum = 0
    for i in range( len( L ) ):
        sum +=  L[i]

    print( "sum of items in ", L, "=", sum )


def printUpperFile( fileName ):
   file = open( fileName, "r" )
   for line in file:
       print( line.upper() )
   file.close()

def createTextFile( fileName ):
   file = open( fileName, "w" )
   file.write( "Welcome\nto\nCSC111\nIntroduction\nto\nComp.\nSci.\n" )
   file.close()

def main():
    # create a text file for use later...
    createTextFile( "csc111.txt" )

    # test first function
    example1()

    # test second function
    L = [ 10, 3, 5, 6, 9, 3 ]
    example2( L )
    #example2( [ 10, 3, 5, 6, "NA", 3 ] )

    # test third function 
    fileName = input( "Enter name of file to display (type csc111.txt): " )
    printUpperFile( fileName )

main()

The program above has many flaws; it is not very robust. We can easily make it crash.
Observe each function. Run your program a few times.
Figure out how to make each function crash
Go ahead and start forcing the first function to crash. Register the XXXXError that is generated. For example, if the output of the crash looks like this:

Traceback (most recent call last): File "/Users/thiebaut/Desktop/except0.py", line 29, in <module> main() File "/Users/thiebaut/Desktop/except0.py", line 27, in main example3( [ 10, 3, 5, 6 ] ) File "/Users/thiebaut/Desktop/except0.py", line 18, in example3 sum = sum + L[i] IndexError: list index out of range

what you are interested in is IndexError. This is the exception you want to guard your code against.

  try:
      ........
      ........
  except IndexError:
      .........

Add the try/except statement inside the function, and verify that your function is now more robust and does not crash on the same input that made it crash before.

Repeat the same process for the other functions.

Classes and Objects

Cats

Below is the program we saw in class, where we create a Cat class, where each cat is defined by a name, a breed, whether it is vaccinated or not, and and age.

# cats1.py
# D. Thiebaut
# Program for Week #9
# Define a Cat class, and
# use it to create a collection of
# cats.


class Cat:
    """a class that implements a cat and its
    information.  Name, breed, vaccinated,
    tattooed, and age."""

    def __init__( self, na, brd, vacc, ag ):
        """constructor.  Builds a cat with all its information"""
        self.name       = na
        self.breed      = brd
        self.vaccinated = vacc
        self.age        = ag

    def isVaccinated( self ):
        """returns True if cat is vaccinated, False otherwise"""
        return self.vaccinated

    def getAge( self ):
        """returns cat's age"""
        return self.age

    def getName( self ):
        """returns name of cat"""
        return self.name

    def __str__( self ):
        """default string representation of cat"""
        vacc = "vaccinated"
        if not self.vaccinated:
            vacc = "not vaccinated"
        
        return "{0:20}:==> {1:1}, {2:1}, {3:1} yrs old".format(
            self.name, self.breed, vacc, self.age )
    
def main():
    """ main program.  Creates a list of cats and displays
    groups sharing the same property.
        Minou, 3, vac, stray
        Max, 1, not-vac, Burmese
        Gizmo, 2, vac, Bengal
        Garfield, 4, not-vac, Orange Tabby
    """
    cats = []
    cat = Cat( "Minou", "stray", True, 3 )
    cats.append( cat )
    
    cats.append( Cat( "Max", "Burmese", False, 1 ) )
    cats.append( Cat( "Gizmo", "Bengal", True, 2 ) )
    cats.append( Cat( "Garfield", "Orange Tabby", False, 4 ) )

    # print the list of all the cats
    print( "\nComplete List:" )
    for cat in cats:
        print( cat )

   


if __name__=="__main__":
    main()

Recreate this code in a program called Lab9_2.py
Run it.
Add a loop to your main program that outputs only the cats that are not vaccinated, and only those.

Challenge #0: getBreed() method

Add a new method to your Cat class that returns the breed of a cat. You may want to call it getBreed(). Add a new loop to your main program, such that it uses the new method on each cat and prints only the stray cats.
Add another loop to your main function, once more, and make it output the non-vaccinated cats that are 2 or older.

Challenge #1: Tattooed Cats

Assume that we now want to keep track of whether cats are tattooed or not. A tattoo is a good way to mark cats so that they can be easily identified if lost.

Add a new boolean field to the class for the tattooed status. True will mean tattooed, False, not tattooed.
Modify the constructor so that we can pass the tattooed status along with all the other information
Add a method to Cat, called isTattooed(), that will allow one to test if a cat is tattooed or not.
Modify your main program so that the tattoo status is included when each cat object is created. You may want to use this new list of cats:

Minou, 3, vaccinated, tattooed, stray
Max, 1, not vaccinated, tattooed, Burmese
Gizmo, 2, vaccinated, tattooed, Bengal
Garfield, 4, not vaccinated, not tattooed, Orange Tabby

Modify the __str__() method so that it includes the tattoo information in the returned string.
Run your main program again. It should output the same information as before, but this time the tattoo information will also appear when a cat is listed.
Add a new section to your main program that will output all the cats that are not tattooed.

Challenge #2: Vaccinated and Tattooed Cats

Make the main program display all the cats that are vaccinated and tattooed.

Challenge #3: Reading the Cat information from a CSV file

Create the short program below and name it createCatCSV.py.

# createCatCSV.py
# D. Thiebaut
# save several cat definitions into a CSV file.

def createCatCSV( fileName ):
    file = open( fileName, "w" )
    file.write( """Minou, 3, vaccinated, tattooed, stray
Max, 1, not vaccinated, tattooed, Burmese
Gizmo, 2, vaccinated, tattooed, Bengal
Garfield, 4, not vaccinated, not tattooed, Orange Tabby
Silky, 3, vaccinated, tattooed, Siemese
Winston, 1, not vaccinated, not tattooed, stray\n""" )
    file.close()


def main():
    createCatCSV( "cats.csv" )

main()

Run the program once. Verify that it will have created a new file in the same folder/directory where your program resides. Verify (using Notepad or TextEdit) that it contains a collection of lines, each representing a cat.

Now, play with the program below:

# Lab9CatsCSV.py
# D. Thiebaut
# Seed for a program that reads cat data from
# a CSV file.


def main():
    # set the name of the csv file containing the list of cats
    fileName = "cats.csv"

    # open a csv file
    file = open( fileName, "r" )

    # process each line of the file
    for line in file:

        # split the line at the commas
        words = line.strip().split( "," )
        
        # skip lines that do not have 5 fields
        if len( words ) != 5:
            continue

        # print the fields
        for i in range( len( words ) ):
            print( "words[",i,"]=", words[i].strip(), end=", " )
        print()

if __name__=="__main__":
    main()

Merge this program and the one you wrote for Challenge #2, and make your program read the cats from a CSV file. It means your program must

extract each cat from a line of the file,
transform the string "vaccinated" in True, and "not vaccinated" in False (it's simpler than you think!)
transform the string "tattooed" in True, and "not tattooed" in False (same comment),
transform the string containing the age into an int,
and store all this information into a Cat object. All the cats should be added to a list.

Verify that the program outputs the correct lists of cats (vaccinated, 2 and older, vaccinated and tattooed, etc.)

Solution Programs

Part 1

# lab9 programs
# D. Thiebaut

# getInput: returns an integer larger
# than 0.  Expected to be robust
def getInput():
   
   while  True:
      x = int( input( "Enter an integer greater than 0: " ) )
      if x <= 0:
         print( "Invalid entry.  Try again!" )
      else:   
         return x

# betterGetInput: returns an integer larger
# than 0.  Expected to be robust
def betterGetInput():
    
   # repeat forever...
   while  True:

      # try to get an int
      try:
          x = int( input( "Enter an integer greater than 0: " ) )
      except ValueError:
          # the user must have entered something other than an int
          print( "Invalid entry.  Not an integer.  Try again!" )
          continue

      # There was no errors.  See if the number is negative
      if x <= 0:
         print( "You entered a negative number.  Try again!" )
      else:   
         return x

def main1():
   num = betterGetInput()
   print( "You have entered", num )

# =======================================================================

def example1():
    print( "You will need to enter 3 pairs of ints..." )
    while True:
       try:
            x = int( input( "enter a number: " ) )
            y = int( input( "enter another number: " ) )
            print( x, '/', y, '=', x/y )
            break
       except ZeroDivisionError:
            print( "Can't divide by 0!" )
       except ValueError:
            print( "That doesn't look like a number!" )
       except:
            print( "something unexpected happend!" )

def example2( L ):
    print( "\n\nExample 2" )
    sum = 0
    for i in range( len( L ) ):
        try:
            sum +=  L[i]
        except TypeError:
            continue

    print( "sum of items in ", L, "=", sum )


def printUpperFile( fileName ):
    try:
       file = open( fileName, "r" )
    except FileNotFoundError:
       print( "***Error*** File", fileName, "not found!" )
       return

    # if we're here, the file is found and open
    for line in file:
       print( line.upper() )
    file.close()

def createTextFile( fileName ):
   file = open( fileName, "w" )
   file.write( "Welcome\nto\nCSC111\nIntroduction\nto\nComp.\nSci.\n" )
   file.close()

def main2():
    createTextFile( "csc111.txt" )

    example1()

    L = [ 10, 3, 5, 6, 9, 3 ]
    example2( L )
    #example2( [ 10, 3, 5, 6, "NA", 3 ] )

    printUpperFile( "csc111.txt" )
    #printUpperFile( "csc1111.txt" )

main2()

Part 2

# lab9_2sol.py
# D. Thiebaut
# Program for Week #9
# Define a Cat class, and
# use it to create a collection of
# cats.


class Cat:
    """a class that implements a cat and its
    information.  Name, breed, vaccinated,
    tattooed, and age."""

    def __init__( self, na, brd, vacc, tat, ag ):
        """constructor.  Builds a cat with all its information"""
        self.name       = na
        self.breed      = brd
        self.vaccinated = vacc
        self.tattooed   = tat
        self.age        = ag

    def isTattooed( self ):
        """returns whether the cat is tattooed or not"""
        return self.tattooed
    
    def isVaccinated( self ):
        """returns True if cat is vaccinated, False otherwise"""
        return self.vaccinated

    def getAge( self ):
        """returns cat's age"""
        return self.age

    def getName( self ):
        """returns name of cat"""
        return self.name

    def getBreed( self ):
        """returns breed of cat"""
        return self.breed

    def __str__( self ):
        """default string representation of cat"""
        vacc = "vaccinated"
        if not self.vaccinated:
            vacc = "not vaccinated"
        tat = "tattooed"
        if not self.tattooed:
            tat = "not tattooed"
        
        return "{0:20}:==> {1:1}, {2:1}, {3:1}, {4:1} yrs old".format(
            self.name, self.breed, vacc, tat, self.age )

def createCatCSV( fileName ):
    file = open( fileName, "w" )
    file.write( """Minou, 3, vaccinated, tattooed, stray
Max, 1, not vaccinated, tattooed, Burmese
Gizmo, 2, vaccinated, tattooed, Bengal
Garfield, 4, not vaccinated, not tattooed, Orange Tabby
Silky, 3, vaccinated, tattooed, Siemese
Winston, 1, not vaccinated, not tattooed, stray
Bob, 2, not vaccinated, not tattooed, Burmese\n""" )
    
    file.close()

def readCatCSV( fileName ):
    """ reads the CSV file and puts all the cats
    in a list of Cat objects"""

    # get the lines from the CSV file
    file = open( fileName, "r" )
    lines = file.readlines()
    file.close()

    # create an empty list of cats
    cats = []
    
    # get one line at a time
    for line in lines:
        # split a line in words
        fields = line.split( "," )

        # skip lines that do not have 5 fields
        if len( fields ) != 5:
            continue

        # get 5 fields into 5 different variables
        name, age, vac, tat, breed = fields
        breed = breed.strip()
        
        # transform "vaccinated" or "not vaccinated" in True or False
        if vac.lower().find( "not " )==-1:
            vac = True
        else:
            vac = False
            
        # transform "tattooed" or "not tattooed" in True or False
        if tat.lower().find( "not " )==-1:
            tat = True
        else:
            tat = False

        # add a new cat to the list
        cats.append( Cat( name, breed, vac, tat, int(age) ) )

    # done with the lines.  Return the cat list
    return cats
    
def main():
    """ main program.  Creates a list of cats and displays
    groups sharing the same property.
    """

    createCatCSV( "cats.csv" )
    
    cats = readCatCSV( "cats.csv" )

    # print the list of all the cats
    print( "\nComplete List:" )
    for cat in cats:
        print( cat )

    # print the stray cats
    print( "\nStray Cats:" )
    for cat in cats:
        if cat.getBreed().lower().strip()=="stray":
            print( cat )
    

    # print the non-vaccinated cats 2 and older
    print( "\nNon-vaccinated cats 2 or older:" )
    for cat in cats:
        if cat.getAge() >= 2 and cat.isVaccinated()==False:
            print( cat )

    # print the non-tattooed cats  
    print( "\nNon-tattooed cats" )
    for cat in cats:
        if cat.isTattooed()==False:
            print( cat )
    
if __name__=="__main__":
    main()

</showafterdate>

...

CSC111 Lab 9 2015

Contents

Exceptions

Part 1: Preparation

Review Class Example

Part 2: Exercise

Classes and Objects

Cats

Challenge #0: getBreed() method

Challenge #1: Tattooed Cats

Challenge #2: Vaccinated and Tattooed Cats

Challenge #3: Reading the Cat information from a CSV file

Solution Programs

Part 1

Part 2

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