Contents

 

Classes    

Inheritance    

Sort of Private Identifiers    

Overloading Standard Operators    

Iterators    

Object Dictionaries    

Doc Strings    

doctest    

 

 

Copyright ©, All rights reserved. 2005 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA. OpenContent ( http://www.opencontent.org/opl.shtml ) license defines the copyright on this document.

References

 

Python Tutorial, Guido van Rossum, http://www.python.org/doc/current/tut/tut.html

 

Python Reference Manual, Guido van Rossum, http://docs.python.org/ref/ref.html

 

Python Library Reference, Guido van Rossum, http://docs.python.org/lib/lib.html

 

Learning Python, Lutz & Ascher, O'Reilly, 1999

 

 

 

Classes

import math

 

class Point:

    print 'First class initialize statement'

    

    def __init__(self, x = 0, y = 0):

        print 'In constructor'

        self.x = x

        self.y = y

    

    print 'Second class initialize statement'

    

    def __add__(self, other):

        return Point(self.x + other.x, self.y + other.y)

    

    print '3rd class initialize statement'

    

    def distance(self):

        return math.sqrt(self.x * self.x + self.y * self.y)

        

    print '4th class initialize statement'

 

All class members are public

All member functions are virtual like Java

 

 

Sample Use

a = Point(1,2)

b = Point()

print a.distance()

print (a + a).distance()

print a

 

Output

First class initialize statement

Second class initialize statement

3rd class initialize statement

4th class initialize statement

In constructor

In constructor

2.2360679775

In constructor

4.472135955

<__main__.Point instance at 0x73238>

 

 

 

Initializing a Data member

class ClassData:

    x = 5

    print 'x= ' , x

    def printer(self):

        print self.x

 

example = ClassData()

example.printer()

example.x = 3

example.printer()

second = ClassData()

second.printer()

 

Output

x=  5

5

3

5

3

 

 

 

Adding attributes to an object

 

def increase(x):

    return x + 1

    

class Empty:

    pass

    

example = Empty()

example.x = 5

print example.x                            #prints 5

example.plus = increase

print example.plus(5)                #prints 6

different = Empty()

print different.x                            #runtime error

 

 

 

Deleting Attributes

class One:

    x = 9

    

    def printer(self):

        print 'inside' , self.x

    

example = One()

example.x = 5

print example.x

example.printer()

del example.x

print example.x

 

Output

5

inside 5

9

 

 

 

Inheritance

 

class Parent:

    x = 9

    z =1

    def printer(self):

        print 'parent' , self.x, self.z

    

class Child(Parent):

    y = 7

    z = 2

    def printer(self):

        Parent.printer(self)

        print 'child' , self.x, self.y, self.z

 

Sample Usage

example = Child()

example.printer()

 

Output

parent 9 2

child 9 7 2

 

 

 

Multiple Inheritance

class Mother:

    x = 0

    z = 1

    def printer(self):

        print 'Mother' , self.x, self.z

    

class Father:

    y = 7

    z = 2

    def printer(self):

        Parent.printer(self)

        print 'Father' , self.y, self.z

    

class Child(Mother, Father):

    pass

    

example = Child()

example.printer()

 

Output

Mother 0 1

 

 

 

Sort of Private Identifiers

class SortOfPrivate:

    __x = 0

    

    def __hiddenPrint(self):

        print 'Hidden'

    

    def printer(self):

        print self.__x,

        self.__hiddenPrint()

    

example = SortOfPrivate()

example.printer()

print example._SortOfPrivate__x                    #valid

example._SortOfPrivate__hiddenPrint()        #valid

example.__hiddenPrint()                                    #error

 

Identifiers starting with two underscores are name mangled to simulate private methods and fields

 

 

 

Overloading Standard Operators

class OverLoadExample:

    x = 0

    list = [1, 2, 3]

    def __del__(self):

        print 'Like a destructor'

        

    def __str__(self):            #convert to a string

        return `self.x`

    

    def __getitem__(self, key):    #indexing operation

        return self.list[key]

    

    def __setitem__(self, key, value):    #indexing operation

        self.list[key] = value

    

a = OverLoadExample()

print a

a[1] = 5

print a[1]

 

Output

0

5

Like a destructor

 

Overloading Details

 

 

 

Iterators

Used by the for statement

 

Iterator methods

1. next()

2. __iter__()

 

Example

>>> a = 'cat'

>>> iterator = iter(a)

>>> iterator

<iterator object at 0x6ff10>

>>> iterator.next()

'c'

>>> iterator.next()

'a'

>>> iterator.next()

't'

>>> iterator.next()

Traceback (most recent call last):

  File "<stdin>", line 1, in ?

StopIteration

 

 

 

Iterator Class

Example (from Python Tutorial)

class Reverse:

    "Iterator for looping over a sequence backwards"

    def __init__(self, data):

        self.data = data

        self.index = len(data)

    def __iter__(self):

        return self

    def next(self):

        if self.index == 0:

            raise StopIteration

        self.index = self.index - 1

        return self.data[self.index]

 

Sample Usage

for c in Reverse('cat'):

    print c

 

Output

t

a

c

 

(Example from Python tutorial)

 

 

Generators

Used to create iterators

Resumable functions

 

Example (from Python tutorial)

def reverse(data):

    for index in range(len(data)-1, -1, -1):

        yield data[index]

    

for c in reverse('cat'):

    print c

 

 

 

Object Iteration Example

 

class MixedType:

    x = 0

    list = [1, 2, 3]

    

    def __iter__(self):

        return self.__mixedTypeGenerator()

    

    def __mixedTypeGenerator(type):

        yield type.x

        for a in type.list:

            yield a

    

a = MixedType()

for x in a:

    print x

 

Output

0

1

2

3

 

 

 

Object Dictionaries

 

class ObjectDictionary:

 

    def createX(self):

        self.x = 5

        

    def createY(self):

        self.y = 10

    

a = ObjectDictionary()

print a.__dict__

a.createX()

print a.__dict__

a.createY()

print a.__dict__

a.__dict__['sam'] = 20

print a.__dict__

a.pete = 30

print a.__dict__

 

Output

{}

{'x': 5}

{'y': 10, 'x': 5}

{'y': 10, 'x': 5, 'sam': 20}

{'y': 10, 'x': 5, 'pete': 30, 'sam': 20}

 

 

 

Doc Strings

Example (from Python manual )

def function():

    "function doc string"

    return 5

    

class DocStringExample:

    "This is the class doc string"

    

    def method(self):

        "method doc string"

        print 'hi'

        

print function.__doc__

print DocStringExample.__doc__

print DocStringExample.method.__doc__

 

Output

function doc string

This is the class doc string

method doc string

 

 

 

doctest

"""

This is the "example" module.

>>> factorial(5)

120

"""

 

def factorial(n):

    """Return the factorial of n, an exact integer >= 0.

 

    >>> [factorial(n) for n in range(6)]

    [1, 1, 2, 6, 24, 120]

    >>> factorial(30)

    265252859812191058636308480000000L

    """

    result = 1

    factor = 2

    while factor <= n:

        result *= factor

        factor += 1

    return result

 

def _test():

    import doctest

    doctest.testmod()

 

if __name__ == "__main__":

    _test()

 

 

 

Running the Tests in the Comments

 

Place the code in a file - examples.py

Run the module

Al 21->python examples.py

 

If all test pass, no output

If a test fails list of failures

Example of No failures

Al 21->python examples.py

Al 22->

Example of a Failure

Al 24->python examples.py

*****************************************************************

Failure in example: factorial(5)

from line #2 of __main__

Expected: 121

Got: 120

*****************************************************************

1 items had failures:

   1 of   1 in __main__

***Test Failed*** 1 failures.

Al 25->

 

More details on doc test