CS2304: Object Oriented Programming - Virginia...

CS2304: Python for Java Programmers

Monti 2014

CS2304: Object Oriented Programming


Monti 2014

Object Oriented Python

•  Python is a multi-paradigm language so you can program in several different styles.

•  We’ve focused procedural code so far, but objects are everywhere.

•  Strings, numbers, sequences are all objects in Python.


Monti 2014

General Form

•  Note the self everywhere. It’s required and similar to this.

•  Remember indentation is very important.

class ClassName(Parent1, Parent2): # constructor def __init__(self): self.member1 = ...

# member methods def classFunction(self, var1): statements1


Monti 2014

A Relevant Example

•  My CurrentFile class from Project 2, note the indentation:

class CurrentFile: # constructor, initialize the state of file def __init__(self): self.is_open = False self.is_saved = True self.data = "" self.name = "" self.cursor = 0 ...


Monti 2014

A Relevant Example: Part II •  __str__ is like toString() in Java and I use it

print the file: def __str__(self): rv = ””; pos = 0 lines = self.data.split("\n") for line in lines: length = len(line) if length != 0: if (self.data[len(self.data) - 1] != "\n" and x == len(lines) - 1): length -= 1 # format from the project spec rv += “...".format(line, pos, pos + length)


Monti 2014

A Relevant Example: Part III •  __str__ is like toString() in Java and I use it

print the file: ... # inside of the last if # print the cursor in the right place if pos <= self.cursor <= pos + length: rv += " " * (self.cursor - pos) + "^\n" # inside of the for loop pos += length + 1 # last line in the function return rv


Monti 2014

Creating And Using an Instance •  Now we can stop using self:

>>> cf = CurrentFile() >>> cf.data ‘’ >>> cf.cursor 0 >>> cf.__str__() # will return the string >>> print(cf) # will print the file using __str__()


Monti 2014

Class Methods

•  Writing your own methods is basically the same. •  Lets add change_cursor to CurrentFile. •  Here’s a very simple definition:

•  And this how you would use it:

def change_cursor(self, cursor_value): self.cursor = cursor_value

>>> cf = CurrentFile() >>> cf.cursor 0 >>> cf.change_cursor(100000) >>> cf.cursor 100000


Monti 2014

Static Members

•  Python supports static members that exist outside of an instance:

•  Note: I’m using the class name, not an instance

name.

class hello: HELLO = “hello” def __init__(self): self.i = 1 j = 0

>>> hello.HELLO ‘hello’


Monti 2014

Other Built-in Methods

•  We’ve seen __init__ is the constructor. •  __str__ converts the object into a human

readable string. •  Here are some other built-in methods:

•  __lt__, __gt__, __eq__, etc •  __repr__ •  __bool__

•  You can override all of these if you want to use your class with Python operators, etc.

•  If you bought the book Chapter 6 has some good examples.


Monti 2014

Adding New Members

•  Classes in Python are dynamic and it’s possible to add new members after creation:

class hello: HELLO = “hello” def __init__(self): self.i = 1 j = 0

>>> h = hello() >>> h.i 1 >>> h.k = ‘o’ >>> h.k


Monti 2014

Public And Private Members •  There really aren’t private members. •  The convention is that variables and functions

that start with a _ or __ shouldn’t be touched. •  If you use the double underscores Python will

make it harder to obtain by mangling the name:

class hello: def __init__(self): self.__i = 1

>>> h = hello() >>> h.i AttributeError: 'hello' object has no attribute 'i' >>> h._hello__i 1

CS2304: Object Oriented Programming - Virginia...

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