Introduction To Python

11 June, 2010
-Lionel Tan Yoke Liang-
_____________________________________________________________

Objective

• To Demonstrate some of the things that can be accomplished using Python.
• To make everyone capable of writing their own scripts in easing their daily work task.
• To guide everyone to become a great learner thru self learning.

Note:-

Table Of Content:-

1. Things That You Need To Know Before Starting[link]
1. Python Blocks
2. Running Python
   1. System Call Mode
   2. Interactive Mode
3. Help


2. Data Types and Data Structures[link]
   1. Variables
      1. numbers
      2. string
   2. Collections
      1. list
      2. dict
      3. tuple
   
   
3. Conditional Loops[link]
   1. Loops
      1. if ... elif ... else
      2. for
      3. while
   2. Control
      1. break
      2. continue
      3. else
   3. List Comprehension
   
   
4. Functions[link]
   1. Defining
   2. Calling a Function
   3. Passing In Data
   4. Returning Data
   
   
5. Dealing With Files[link]
   1. Reading From Files
   2. Writing To File
   
   
6. Classes And Objects[link]
   1. Terminology
   2. Creating A Class
   3. Constructor
   4. Methods
   
   
7. Useful Links[link]
   
   
8. Google Python Class (Video)[link]
   

Things That You Need To Know Before Starting

Python Blocks

It's very important to know that python uses a very unique way of representing it's block of codes.
While most of the languages uses the curley braces ...

#!/apps/perl/bin/perl
if (1)
{
	print "Hello, World";
	return(0);
}
else
{
	print "2012 ...";
	return(1);
}

Python uses spaces ...

#!/usr/bin/env python
if True:
	print("Hello, World")
	return 0
else:
	print("2012")
	return 1

Seeing that the case for python is recognising the spaces as it's code block, it is of utmost important that the the number or spaces(or tabs) has to be equal for all the codes which falls within the same block.....
The code below doesn't work, because the number of spaces is not equal ...

#!/usr/bin/env python
if True:
	print("Hello, World")
	return 0
else:
	 print("2012")		# Error, because indentation not align
	return 1

Because of this, Here're some important points ...
• Space != Tabs.
• The same spacing seen which is created by a tab, is not equivalent to the same spacing seen which is created by the spacebar.
• The code will NOT run!
• To make life easier, it is advisible to write python code with SPACES instead of TABS.

Running Python

Before we can get started, we must first know at the very least, how to make this thing run. There're 2 ways of running python.


System Call Mode

You write your script into a text file, with a *.py extension, and run the script Example of hello.py:-

	#!/usr/bin/env python
	print("Hello, World")
	

Running from terminal:-

	%./hello.py
	

Interactive Mode

Just invoke python interatively by calling python ...

	%python
	Python 2.5.1 (r251:54863, Jul 21 2007, 15:22:34)
	[GCC 3.4.3 20060612 with Mainsoft fixes] on linux2
	Type "help", "copyright", "credits" or "license" for more information.
	>>> a = 'Hello, world'
	>>> print(a)
	Hello, world
	>>>          
	

Help

A few help that you can get.

Interactive Help

Start Interactive Python, and type help(topic)

	%python
	>>> help(str)
	>>> a = 123
	>>> help(a)		### Similar to help(int)
	




Python Official Page

http://www.python.org/doc/


Google Is Friend, Not Food ...

Searching on "Python Dictionary".







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Data Types and Data Structures

Some of Python's useful Data Types and Data Structures ...

Scalar

• type variables, which stores a single data
• string, integer, float

Collection

• type, which stores a collection of scalar type variables
• dictionary(hash), list(array), tuple(immutable list)




Scalar

Assignment

			x = 1
			y = 10
			z = x + y
			print(z)    # 11
			y += 1      # y == 11
			





Multiple Assignment

			a = b = c = 5
			d, e, f = 6, 7, 8
			





Operators

			d > 3                   # True
			d == 3                  # False
			d < e < f               # True
			d is 6                  # True
			f is not 8              # False
			a is 5 and e is not 6   # True
			





Strings

Strings in Python are identified as a contiguous set of characters in between quotation marks.

			s = 'Hello World!'

			print(s)          # Prints complete string
			print(s[0])       # Prints first character of the string
			print(s[2:5])     # Prints characters starting from 3rd to 6th
			print(s[2:])      # Prints string starting from 3rd character
			print(s * 2)      # Prints string two times
			print(s + "TEST") # Prints concatenated string
			

There are 3 ways of quoting a string in python
• Single quote

				a = 'Lionel is "handsome".'
				

• Double quote

				a = "Lionel can't jump."
				

• Triple quote

				a = """Lionel is "handsome", but he can't jump."""
				b = '''
					 Lionel likes PERL, 
					 but Python is cool too!
					'''
				

A few of the string methods ...

			a = 'Hello, World!'
			a.isalpha()		# False
			a.isdigit()		# False
			a.find('o')		# 4
			a.split(',')	# ['Hello', ' World!']
			a.startswith('He')	# True
			

For a full list of method...

			%python
			%help(str)
			

Collection

A Collection is something that can store more than one scalar. 3 of the more common collection used are ...

List

The closest representative of an array.
A List is an order list, and the position of the data will ALWAYS be the same, unless moved.


A few examples ...

			a = ['liang', 25, '012-1234567', 123.64]
			print(a[0])   # 'liang'
			print(a[1:3]) # [25, '012-1234567']
			a.append('haha')
			print(a)      # ['liang', 25, '012-1234567', 123.64, 'haha']
			a.reverse()
			print(a)      # ['haha', 123.64, '012-1234567', 25, 'liang']
			a.sort()
			print(a)      # [25, 123.64, '012-1234567', 'haha', 'liang']
			

For a full list of List object methods ==>

			%python
			%help(list)
			

Tuple

A Tuple is the same thing as the List. The only difference between them is that a List is a mutable object, while a Tuple is immutable. An immutable object is one which can not be editted, just like a protected object in C/C++.
And since a Tuple is an immutable object, it doesn't make sense to have any method for tuple objects that can change it. The only way you can change/edit the content of a tuple is to make it into a List, then only you make changes to the Tuple.


			t = ('liang', 25, '012-1234567', 123.64)
			### You can still access to elements of a tuple the same way
			### you access thru a list
			print(t[0])   # 'liang'
			print(t[1:3]) # (25, '012-1234567')
			a = list(t)   # Copy tuple t to a list object ==> a
			print(a)      # ['liang', 25, '012-1234567', 123.64, 'haha']
			a.reverse()
			print(a)      # ['haha', 123.64, '012-1234567', 25, 'liang']
			a.sort()
			print(a)      # [25, 123.64, '012-1234567', 'haha', 'liang']
			tt = tuple(a)
			print(tt)     # (25, 123.64, '012-1234567', 'haha', 'liang')
			

Dictionary

Exactly the same thing as a hash in other languages.
One important thing to note. The KEY of the dictionary has to be an immutable object.


Immutable Objects

• strings
• numbers
• tuples

Mutable Objects

• List
• Dictionary
And thus, looking at the above information, the key of a dictionary could be either a string, number or tuple.


Let's look at a few examples what a python's dictionary could do...


			a = {'name':'lionel tan', 'age':25, 'phone':6325}
			b = {'name':'yoke liang', 'age':25, 'phone':6315}
			employee = {1:a, 2:b}
			print(employee[1]['name'])    # 'lionel tan'
			employee[1]['age'] == employee[2]['age']   # True
			a['sex'] = 'male'
			print(employee[1]['sex'])     # 'male'
			

Iteration

Now that we've look thru the basics of creating Lists and Dictionaries, let's see how we can interate thru the data...
Most of the collection type of objects in python supports the iteration by using the for syntax...


Let's look at some example ...

			alist = ['liang', '25', '012-1234567', '123.64']
			for item in alist:
				print(item)
			### output ###
			# 'liang'
			# '25'
			# '012-1234567'
			# '123.64'
			#-------------------------------------------------------
			adict = {'name':'lionel tan', 'age':'25', 'phone':'6325'}
			for key in adict:
				print(key + ' - ' + adict[key])
			### output ###
			# 'name - lionel tan'
			# 'age - 25'
			# 'phone - 6325'
			




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Conditional Loops

Loops

if ... elif ... else

			a = 5
			if a < 4:
				print("a < 4")
			elif a < 10:
				print("a < 10")
			elif a < 15:
				print("a < 15")
			else:
				print("ELSE")

			### Answer: "a < 10"
			

for

			animals = ['fish', 'cat', 'dog']
			for x in animals:
				print("This is a " + x)

			### Answer:-
			### This is a fish
			### This is a cat
			### This is a dog
			




			stockprice = { 'ALTR':'$42', 'XLNX':'$12', 'INTC':'$15' }
			# example #1
			for sym in stockprice:
				print( sym + " is currently " + stockprice[sym] )

			# example #2
			for sym,price in stockprice.items():
				print( sym + " is curerntly " + price )

			### Answer:-
			### ALTR is currently $42
			### XLNX is currently $12
			### INTC is currently $15
			

while

			a = 3
				while a > 0:
					print(a)
					a -= 1

			### Answer:-
			### 3
			### 2
			### 1
			





Controls

break

Use break to come out from a loop prematurely.

			a = 10
			while a > 0:
				print(a)
				if a == 8:
					break
				a = a - 1

			### Answer:-
			### 10
			### 9
			### 8
			

continue

Use continue to skip executing the rest of this loop, and proceed directly to the next loop.

			a = 4
			while a > 0:
				if a == 2:
					continue
				print(a)
				a = a - 1

			### Answer:-
			### 4
			### 3
			### 1
			

else

else can be use on while and for loop in python.
This concept is a bit new, but once you get the hang of it, you'll like it ;)
The else block will executed if and only if the original while or for loops completely iterated all of it's elements.



			a = 4
			while a > 0:
				if a == 2:
					break
				print(a)
				a = a - 1
			else:
				print("-Loop Ends-")
			print("code continue running ...")

			### Answer:-
			### 4
			### 3
			### code continue running
			




... comparing the above code with this one below ...

			a = 4
			while a > 0:
				print(a)
				a = a - 1
			else:
				print("-Loop Ends-")
			print("code continue running ...")

			### Answer:-
			### 4
			### 3
			### 2
			### 1
			### -Loop Ends-
			### code continue running
			


... Do you see the difference ? :)





List Comprehension

List comprehension is a very powerful feature in python.

		animalList =  ["bear","bee","cat","fish","horse","llama"]
		newlist = []
		for animal in animalList:
			if 'a' in animal:
				newlist.append(animal)
		### newlist ==> ['bear', 'cat', 'llama']
		




... comparing the above code with this one which is equivalent below...


		animalList =  ["bear","bee","cat","fish","horse","llama"]
		newlist = [ animal for animal in animalList if 'a' in animal ]
		### newlist ==> ['bear', 'cat', 'llama']
		


... Do you see how neat it is? :) ...



You can even do this ...


		animalList =  ["bear","bee","cat","fish","horse","llama"]
		newlist = [ animal.upper() for animal in animalList if 'a' in animal ]
		### newlist ==> ['BEAR', 'CAT', 'LLAMA']
		




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Functions

Defining A Function

		def printMyName():
			print("My name is Lionel")

		def printMyAge():
			print("Every year 25")
		





Calling A Function

		printMyName()
		# My name is Lionel
		



You can even assign your function as a reference to another variable, Like this ...

		a = printMyName
		a()
		# My name is Lionel
		

From the above example, have you noticed something?

By calling the function name WITH the brackets, - printMyName() -,
we are actually executing the function itself. But, when calling the function name WITHOUT the brackets, - printMyName -,
it is actually returning a reference to that function,
which we can eventually assign it to any other variables.



And with that in mind, this opens up a whole new world of creative opportunities ..... like this ...

		my = {'name':printMyName, 'age':printMyAge}

		print("What's your name?")
		my['name']()

		print("How old are you?")
		my['age']()

		### Answer:-
		### What's your name?
		### My name is Lionel
		### How old are you?
		### Every year 25
		



... and like this ... :)

		def correct():
			print("Awesome !!! Fantastic !!!")

		def wrong():
			print("Stupid ...")

		answer = [wrong, correct]

		x = 5
		answer[ True if x is 5 else False ]()
		### Awesome !!! Fantastic !!!
		





Passing In Data

		def printMyName(name):
			print("My name is " + name)

		printMyName("Rain!")
		### My name is Rain!
		

		def printMyData(name="Liang, age=25):
			print("My name is " + name)
			print("I am " + str(age) + " years old.")

		printMyData(age=18)
		### My name is Liang
		### I am 18 years old.

		printMyData("YeeChin", 31)
		### My name is YeeChin
		### I am 31 years old.
		





Returning Data

		def myName():
			return("Lionel")
		
		def myData():
			return( ['YeeChin', '25'] )

		print( myName() )
		### Lionel

		[name, age] = myData()
		print( name + " is " + age + " years old already lor ~~~")
		### YeeChin is 25 years old already lor ~~~
		







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Dealing With Files

Reading From Files

		filename = "./a.txt"
		fh = open(filename, 'r')
		for line in fh:
			print(line)
		fh.close()
		





Writing To Files

		filename = "./a.txt"
		fh = open(filename, 'w')
		fh.write("writing this first line into a file...")
		fh.write("This is the 2nd line.")
		fh.close()
		







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Classes And Objects

Terminology

Class: A user-defined prototype for an object that defines a set of attributes that characterize any object of the class. The attributes are data members (class variables and instance variables) and methods, accessed via dot notation.

Class variable: A variable that is shared by all instances of a class. Class variables are defined within a class but outside any of the class's methods. Class variables aren't used as frequently as instance variables are.

Data member: A class variable or instance variable that holds data associated with a class and its objects.

Function overloading: The assignment of more than one behavior to a particular function. The operation performed varies by the types of objects (arguments) involved.

Instance variable: A variable that is defined inside a method and belongs only to the current instance of a class.

Inheritance : The transfer of the characteristics of a class to other classes that are derived from it.

Instance: An individual object of a certain class. An object obj that belongs to a class Circle, for example, is an instance of the class Circle.

Instantiation : The creation of an instance of a class.

Method : A special kind of function that is defined in a class definition.

Object : A unique instance of a data structure that's defined by its class. An object comprises both data members (class variables and instance variables) and methods.

Operator overloading: The assignment of more than one function to a particular operator.





Creating a Class

		class People():
			"""
			This is a documentation. 
			A string that follows right after a block code is always a documentation.
			It is a good practice to document all your specs/algorithm/documentation here.
			"""

			def __init__(self, n, a):
				"""
				This is a constructor
				"""
				self.name = n
				self.age = a

			def getName(self):
				"""
				This is a method.
				A Method is just like a normal function.
				It's just another way of naming it ;)
				"""
				return(self.name)

			def setStatus(self, status):
				"""
				This is another method, which reads in 1 input parameter.
				"""
				self.status = status




			def getStatus(self):
				return(self.status)

			def getAge(self)
				return(self.age)
		





Constructor

Every Class needs one, and only one constructor.
A constructor is a method which binds an object to it's own class.
Ok. I know this might sounds very confusing to some of you.
Let's take it one step at a time.


Whenever a user does this ...

		yltan = People("Lionel", 25)
		

The above action is call Instantiating a new instance of the class(People).
yltan is the newly created instance, or object.

Now, when the above action takes place, the class will automatically call it's constructor method,
which is the __init__.
Now ....... do you see some light? ;)


Let's take a look and see what the constructor method __init__ does:-

		def __init__(self, n, a):
			self.name = n
			self.age  = a
		

• It 'binds' a reference(pointer) self to the class.
• 2 parameters is being passed into the constructor, n and a.
• n is being assigned to the self.name instance variable
• a is being assigned to the self.age instance variable


The self.name and self.age instance variable is now being 'binded' to the instance yltan. Let's say another instance is being created:-

		kyleong = People("Eric", 38)
		

The class will create another new set of self.name and self.age instance variable,
and bind them to the newly created instance, kyleong.




Methods

A method is just another name for a function within a class.
Let's take a look at the previous example:-

			def getAge(self)
				return(self.age)
			def getName(self)
				return(self.name)
		

Since the created instance is always having a binding with the class, we can therefore
access to all the members of the class thru a reference.
And that is why, all the members in a class has to be passed with a first parameter of a reference
to it's own instance, and by convention, everyone names it as self, although it can be any variable name.

So, now, if we want to know the age of the instances that we've created just now, all we need to do is just to
access to the instance's respective member, and it will automatically grep the data for us:-

		print( kyleong.getName() + " is " + str(kyleong.getAge()) + " years old .... already!" )
		print( yltan.getName() + " is " + str(yltan.getAge()) + " years old .... only!" )
		### Eric is 38 years old .... already!
		### Lionel is 25 years old .... only!
		







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Useful Links

1. Python Official Website
2. Python Tutorial For Beginners
3. Another Python For Beginners
4. Dive Into Python
5. A Collection Of Python Bloggers' Feeds
6. Data Structure & Algorithm with Object Oriented Design Pattern in Python
7. A Byte Of Python
   
   



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Google Python Class (Video)



http://www.youtube.com/watch?v=tKTZoB2Vjuk


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