CSC1018F: Object Orientation,

Exceptions and File HandlingDiving into Python Ch. 5&6

Think Like a Comp. Scientist Ch. 11-13

Lecture Outline

Recap of Intermed Python [week 2]Object Orientation:

Module importingDefining, initializing and instantiating ClassesClass attributesClass methods

ExceptionsFile Handling:

Opening, reading, writing and closing

Recap of Intermediate Python

Basics:Conditionals (if statements)Iteration (while and for loops)

Introspection:Optional and Named ArgumentsBuilt-in FunctionsFiltering Listsand & orLambda Functions

Recap Exercise: Image Thresholding

Assume that an image is represented:With greyscale pixel values in the range 0 (black) to 255 (white) As a list of lists of integersEach sublist represents a row of pixels

Write functions to perform the following operations:

Threshold(image, threshval) - return an image which has all values below threshval converted to 0 and all those equal or above to 255. Threshval should default to 100Challenge: try and compress the body of the function into a single line

def Threshold(image, threshval = 100):

return [[(pixel < threshval and [0] or [1])[0] forpixel in row] for row in image]

Importing Modules

import moduleAllows access to methods and attributes of moduleBut requires a module.method specifier

from module import methodImports method directly into the local namespaceCan be called without a specifier

from module import *Imports all aspects of module into the local namespaceCan be dangerous with namespace conflicts

Defining and Initializing Classes

No separate class interface definition requiredAllows multiple inheritance (passed as comma-separated arguments to class)

Ancestor methods must be explicitly called

self is specified as first parameter to all class methods

Provides a reference to the class instanceNot required when calling the method

__init__ acts like a class constructor

class ClassName(Ancestor):"doc for class"__init__(self, otherparam = “default”):

Ancestor.__init__(self)

Using Classes

Import appropriatelyimport ClassFile

Class instantiated by calling it like a function passing __init__ parametersc = ClassName(initparam)

Class methods and attributes can be accessed with ‘.’ notationc.__class__c.__doc__

Class Attributes

Instance variablesData specific to an instanceQualified by instance name, e.g., instance.varBy convention initialized in __init__

Class attributesData owned by the class itself, not particular to an instanceDeclared in the class itself, usually after the doc stringAvailable directly from the class, as well as its instances, e.g., classname.attrib, instance.attribMost useful as class-level constants

Class Methods

Note: method overloading is not supported in PythonSpecial Class Methods:

Methods with a different calling syntaxExample: __len__(self) called as len(instance) Other cases __getitem__, __setitem__

Private Class Methods:Determined entirely by name (start with two underscores “__”)Not accessible outside the classSimilar notation for private functions and attributes

Handling Exceptions

Use try…except block to handle exceptions Use raise to generate exceptionsDefault behaviour is to print an error and exitBut in many cases you can anticipate exceptions

For example, trying to open a file that doesn’t exist

try:fsock = open("/notthere")

except IOError:print "The file does not exist"

else:print "File opened"

Working with File Objects

file = open(name, mode, buffering)Takes a file name and optional mode and buffering parameter, returns a file objectname and mode are useful file object attributes

file.tell()Returns the current position in an open file

file.seek(pos, mode)mode = 0, move to absolute posmode = 1, move to curr + posmode = 2, move to end - pos

File.close()Closes an open file. This can be confirmed with the boolean attribute closed

Reading and Writing

data = file.read(numbytes)Returns a string containing the read data

file.write(data)If file is opened in ‘w’ mode then data overwrites any existing file contentsIf file is opened in ‘a’ mode then data is appended to the end of the file

Also read up on sys.modules and how to work with directories

Revision Exercise

Create an image class with the following methods:

__init__ - creates a white image with a user specified x, y resolutionCheckValidity() - which checks if all pixel rows have the same lengthPadTrunc(l) - which appends white pixels to rows < length l or truncates rows > length lThreshold(t) - set all pixels <= t to 0 and the rest to 255 Resize(x, y) - alters the resolution of an image by making calls to PadTrunc and/or dropping rows