CSC1018F: Object Orientation, Exceptions and File Handling (Tutorial)

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CSC1018F Object Orientation, Exceptions and
File Handling(Tutorial)

• Diving into Python Ch. 56
• Think Like a Comp. Scientist Ch. 11-13

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Mini-Test Answers (1)

• If you want access to the contents of a module
  abra but do not want to bring it into the
  current namespace (i.e., all functions of the
  module must be qualified with module.function)
  then you would call
• from abra import
• from abra import function1, function2,
• import abra
• Which of the following statements concerning
  classes is NOT correct
• Data attributes of a class must be declared in
  the __init__ method
• __init__ methods are optional
• Class attributes can be accessed from both a
  class and instance as class.attrib or
  instance.attrib
• Python does not support multiple inheritance
• The __data__ notation indicates a private
  method or attribute

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Mini-Test Answers (2)

• The following python code
• try
• fsock open("/program.py")
• except IOError
• print "The file does not exist"
• except AttributeError
• print "open() not available"
• print "Program opened"
• will
• Always print Program opened
• Print Program opened only if no IOError occurs
• Print Program opened if no IOError and no
  AttributeError occur
• Fail to execute because it contains a syntax error

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Mini-Test Answers (3)

• Assuming that a file called f of length 256 has
  been newly opened, you could move the current
  read position to 128 by calling
• f.seek(-128, 2)
• f.seek(-128, 1)
• f.seek(128, 0)
• f.seek(128, 1)
• f.seek(-128, 0)
• Calling from module import twice in
  succession will cause a syntax error because of
  namespace conflicts
• True
• False

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Mini-Test Answers (4)

• The following python code executed on a windows
  system
• gtgtgt import os
• gtgtgt os.path.join("c\\documents", "photo.jpg")
• will output
• "c\\documentsphoto.jpg"
• "c\\documents\\photo.jpg"
• "c\documents\photo.jpg"

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Revision Exercise

• Create an image class with the following methods
• __init__ - creates a white image with a user
  specified x, y resolution
• CheckValidity() - which checks if all pixel rows
  have the same length
• PadTrunc(l) - which appends white pixels to rows
  lt length l or truncates rows gt length l
• Threshold(t) - set all pixels lt 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
  

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Revision Solution (1)
class Image """A greyscale image class
consisting of a rectangular matrix of
integer pixels where 0 represents black and 255
white """ def __init__(self, x 100, y
100) self.__image
self.__width x self.__height y
for i in range(y)
self.__image.append(255x) def
CheckValidity(self) for row in
self.__image if(len(row) !
self.__width) return False
return True def PadTrunc(self, length)
self.__image (len(row) lt length and
row.extend(255(le
ngth-len(row))) or
rowlength) for row in self.__image
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Revision Solution (2)
def Resize(self, x, y) if not
self.CheckValidity() print "Warning
Correcting Invalid Image"
self.PadTrunc(self.__width) why do we
resize y before x? if y lt self.__height
self.__image self.__imagey elif y gt
self.__height for i in
range(y-self.__height)
self.__image.append(255self.__width)
self.__height y if x ! self.__width
self.PadTrunc(x) self.__width x def
__str__(self) return "resolution ("
str(self.__width) ","
str(self.__height) ")\n" str(self.__image)