Ksolis at 08:38, 2 October 2017

2017-10-02T08:38:07Z

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[[Category:Python]]
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<div class="np_frontpage_header"><p>Python Primer</p></div>
<div class="np_frontpage_footer"> <p>Copyright © 2015 NobleProg™. All Rights Reserved. www.nobleprog.us</p></div>

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=Python Introduction=
*Python is an interpreted language
:*Compilation steps are not needed
::*This makes it less cumbersome
::*For example class files (i.e. java) are not needed
*Python is less complex than languages such as C++ or Java
:*Makes it easier to learn and use
:*There is a tradeoff since programs cannot be as complex

=Python Primer=
==Invoking the Interpreter==
*The Python interpreter runs the code of a program
*It is in the Python (c:\Python34 on my computer) folder
:*The folder that was used when installing python
:*It is python.exe
*The python folder should be in the path environment variable
*To run the interpreter on Windows (other systems are similar) with the path set correctly
:*Open a command prompt
:*type in the command python someFileName.py
::*This runs the program specified
:*Type in python
::*This runs python in interactive mode (see References below)

:*<big>References</big>
:::[https://www.java.com/en/download/help/path.xml Set Environment Variables Article]
:::[https://docs.python.org/2/tutorial/interpreter.html# Invoking the python interpreter in interactive mode]

==Using an IDE==
*Install an Integrated Development Environment
:*PyCharm is a good one
:*Eclipse has a plugin to allow coding in Python
*Learn how to use it
:*Create projects and files
:*Debug the projects
:*Run the projects

:<big>References</big>
:::[https://www.jetbrains.com/pycharm/ PyCharm]
:::[http://pydev.org/ PyDev plugin for Eclipse]
:::[http://www.liclipse.com/ LiClipse - Eclipse with Python and other plugins]

==Python Variables==
*Variables keep data i.e. integers, strings, floats
:*Memory locations used to store data
*Python does not declare variables
*Other languages i.e. Java do declare variables
*This makes Python simpler to code
<br/>
<br/>
'''''Variable declaration and use in Java:'''''
<pre style="color:blue;background-color:lightgrey">
int integer1 = 5;
int integer2 = 6;
float float1 = 6.75;
String myString = "This is a string";
String anotherString;

integer1 = integer2 + 7;
integer2++;
anotherString = myString + " now adding to string"; integer2++;
</pre>

<br/>

'''''Variable declaration and use in Python:'''''
<pre style="color:blue;background-color:lightgrey">
variable1 = 5
variable2 = "This is a string"

variable1 = variable2 + " now adding to string"
</pre>

<br/>
*Variable types are assigned when data is put into them
*Variables can be moved between types
*The types are:
:*numbers - integer, long, float, complex
:*String - a contiguous set of characters
:*List - items of various types, enclosed in braces [], separated by commas
:*Tuple - items of various types, enclosed in parentheses, separated by commas, read-only
:*Dictionary - contains name values pairs, a hash table

<br/>
<br/>

:<big><span style="color:#FF0000">Hands on Exercise</span></big>
:::*Start PyCharm which we installed in a prior step
:::*Using PyCharm create a project called "Python Primer"
:::*Create a new file called variables a py extension will be added
:::*Edit the file variables.py in PyCharm
:::*Create an integer, a float, and a String as variables in the file
:::*Print out the variables using print(theVariableName)
:::::Note that the parentheses are required for Python 3 and above
:::::Print is now a function it was a statement
:::*Try various operators on the integers and floats + - / * % // **
:::*Try concatenating strings

==Operators==
*The standard arithmetic operators are:
::+ addition
::- subtraction
::\* multiplication
::/ division
::% modulus - gives the remainder of division
::// integer division gives the whole number (or rounded down)
::\*\* exponents

*Comparison operators
::== example x == y -- is x equal to y (gives true or false)
::!= example x != y -- is x not equal to y (gives true or false)
::<> example x <> y -- same as above is x not equal to y
::> example x > y -- is x greater than y
::< example x < y -- is x less than y
::>= example x >= y -- is x greater than or equal to y
;:<= example x <= y -- is x less than or equal to y

*Logical Operators
::and example x and y -- both x and y must be true to give true
::or example x or y -- either x or y must be true to give true
::not example not(x or y) -- reverses the condition in the parentheses
*String operators
::print (str) # Prints complete string
::print (str[0]) # Prints first character of the string
::print (str[2:5]) # Prints characters starting from 3rd to 5th
::print (str[2:]) # Prints string starting from 3rd character
::print (str * 2) # Prints string two times
::print (str + "TEST") # Prints concatenated string

*List operators

::list = [ 'abcd', 786 , 2.23, 'john', 70.2 ]
::tinylist = [123, 'john']

::print (list) # Prints complete list
::print (list[0]) # Prints first element of the list
::print (list[1:3]) # Prints elements starting from 2nd till 3rd
::print (list[2:]) # Prints elements starting from 3rd element
::print (tinylist * 2) # Prints list two times
::print (list + tinylist) # Prints concatenated lists

*Dictionary Operators

::dict = {}
::dict['one'] = "This is one"
::dict[2] = "This is two"

::tinydict = {'name': 'john','code':6734, 'dept': 'sales'}

::print (dict['one']) # Prints value for 'one' key
::print (dict[2]) # Prints value for 2 key
::print (tinydict) # Prints complete dictionary
::print (tinydict.keys()) # Prints all the keys
::print (tinydict.values()) # Prints all the values

:<big><span style="color:#FF0000">Hands on Exercise</span></big>
::*Use the documentation of lists at [http://www.tutorialspoint.com/python/python_lists.htm Python Lists] for this exercise
::*Create several Lists and use the following List operations, functions and methods
:::*List access operations list[0], list[1:5], to print list values
:::*Update a list value using list[someIntegerIndex] = value -- then print the list
:::*Delete a list value with del list[someIntegerIndex] -- then print the list
:::*Find and print the length of the list
:::*Use the built-in function min to find the minimum value in the list and print it
:::*Append a new value to the list
:::*Insert a new value into the list at location 2
:::*explain what pop does to the list and print a value that has been popped from the list

:References
:::[http://www.tutorialspoint.com/python/python_basic_operators.htm Python Operators]

==Python Syntax==
<big>'''Indentation'''</big>
*Indentation in Python indicates code blocks rather than beginning and ending delimiters
::For example in Java code blocks are delimited by braces i.e. {}
*All statements in a block must have the same indentation
::The default indentation is four spaces

<big>'''Identifier naming convention'''</big> (taken from TutorialsPoint.com):
*Class names start with an uppercase letter and all other identifiers with a lowercase letter.
*Starting an identifier with a single leading underscore indicates by convention that the identifier is meant to be private.
*Starting an identifier with two leading underscores indicates a strongly private identifier.
*If the identifier also ends with two trailing underscores, the identifier is a language-defined special name.

<big>'''Multiline Statements'''</big> (taken from TutorialsPoint.com)

<pre style="color:blue;background-color:lightgrey">
total = item_one + \
item_two + \
item_three
</pre>

<big>'''Use of Quotes'''</big>
*Strings in Python can use single, double, or triple quotes
<pre style="color:blue;background-color:lightgrey">
string1 = 'This is a string'
string2 = "This is a string"
string3 = '''This is a
multiline string'''
string4 = """This is also a
multiline string"""
</pre>

<big>'''Comments'''</big>

*the pound sign/hash mark "#" is used for single line comments
*Three single or double quotes are used for multiline comments

<pre style="color:blue;background-color:lightgrey">
#This is a single line comment
#This is a second single line comment

'''This is
a multiline comment'''

"""This is also
a multiline comment"""
</pre>

<big>'''Multiple Statement on a Single Line'''</big>
*use semicolons as the delimiter between statements
<pre style="color:blue;background-color:lightgrey">
a = b+1; c = d + 2; print(str(a))
</pre>

==Control Structures==

*<big>'''if statement'''</big> - a conditional statement that can have an else clause
:*note that the sub-statements of the if statement are indented
:*the normal default for indenting is four spaces

<pre style="color:blue;background-color:lightgrey">
a = 10
if (a == 10):
print("In if statement")
</pre>

<pre style="color:blue;background-color:lightgrey">
a = 10
if (a == 10):
print("In if statement")
else:
print("In else statement")
</pre>

*<big>'''while loop'''</big> - a loop that continues until the loop condition has been satisfied
<pre style="color:blue;background-color:lightgrey">
a = 10
while (a < 15):
print("in while loop")
a += 1
</pre>

*while loop with else statement

<pre style="color:blue;background-color:lightgrey">
a = 10
while (a < 15):
print("in while loop")
a = a + 1
else:
print("while loop has ended")
</pre>

*<big>'''for loop'''</big>
::the general form of the for loop is
<pre style="color:blue;background-color:lightgrey">
for iterating_var in sequence:
statements(s)
</pre>
::Where iterating_var is the name of the variable
:::that represents the next item in the sequence to be
:::used in the statements of the for loop

*An example of a sequence and a for loop that iterates through the sequence

<pre style="color:blue;background-color:lightgrey">
animals = ['dog','cats','moose','deer','bear']

for theAnimal in animals:
print("the animal is a " + theAnimal)
</pre>

*An example of a sequence and an iteration using the index in the list
<pre style="color:blue;background-color:lightgrey">
animals = ['dog','cat','moose','deer','bear']

for index in range (len(animals)):
print("Using an index the animal is a " + animals[index])
</pre>

<big><span style="color:#FF0000">Hands on Exercise</span></big>
*Create a simple program using PyCharm that has a while loop and an if else statement
*The program will do the following
:::The while loop will loop 10 times.
:::The if else statements will be in the while loops code block
:::The if else statements will print out "In the if statement" for the first 5 iterations
:::The if else statements will print out "In the else statement" for the last 5 ieterations

==Functions==
*A function is a piece of reusable code
::Generally it performs an identifiable action that can be reused
:::For example opening, writing to, or closing a file
::It provides modularity for the code making it easier to understand
::There are standard functions provided with Python
::New functions can be defined by programmers (user-defined functions)

*How to define a function
::Functions begin with the keywork def, followed by the function name, and then a set of parentheses ()
::Input parameters are placed within the parentheses ()
::The first statement can be an optional documentation string (docstring)
::The code block starts with a colon : and is indented

*Example of a Function
<pre style="color:blue;background-color:khaki">
def theFunctionName(parameters):
"function_docstring"
function statements
return [expression]
</pre>

==Built-in and Standard Library Functions==

There are a number of functions that are built-in in Python
::See the Python Documentation on Build-in Functions in the References below
::These are part of the Standard Library
There are also a number of other modules in the standard library
::These give solutions to many everyday programming problems

*Build-in Functions can be called directly in the code
::Example of using Built-in Functions
<pre style="color:blue;background-color:lightgrey">
f = open('workfile', 'w')
f.write("this is the file output")
f.close()
</pre>

*Standard Library functions must be imported
::Example of using Standard Library Functions
<pre style="color:blue;background-color:lightgrey">
from math import sqrt

a = sqrt(24)
print (a)
</pre>

:<big>References</big>
::[https://docs.python.org/3/library/functions.html Built-in Functions]
::[https://docs.python.org/3/library/ Standard Library Including Built-in Functions]

==PythonScope==
*A Variable's scope
::Python has only function, global, and class scope.
::It does not have block scope
:::For example a variable first used in a for loop can be seen outside the for loop
:::In many other languages such as Java, C++, and C a variable declared in a block has block scope

*Function scope vs global scope

<pre style="color:blue;background-color:lightgrey">
globalVar = "this string is global" #creates a global variable

def anotherFunction ()
globalVar = "this string is local" #creates a variable local to the function
</pre>

==Importing External Modules==

'''''The import statement'''''
*Bring the functions within a file into the program

Example of code import
<pre style="color:blue;background-color:lightgrey">
Assume that the file importFunctions.py has the following code

def print_value(parameter):
print("the value is " + parameter)
return

To use this code in another file it must imported then used

#import the functions from importFunctions
import importFunctions

#use the print_value function
importFunctions.print_value("Print This String")
</pre>

'''''The from import statement'''''
*Bring the functions within a file into the program
*Allow the use of the functions without the leading filename

Example of the from import statement
<pre style="color:blue;background-color:lightgrey">
Assume that the file importFunctions.py has the following code

def print_value(parameter):
print("the value is " + parameter)
return

To use this code in another file it must imported then used

#from import the functions
from importFunctions import print_value

#use the print_value function without the leading filename
print_value("Print This String")
</pre>

==Object Oriented Classes==
*Python has classes which makes it object oriented
::Note that it also has functions so it is also a functional language
*Overview of Object Oriented Programming (OOP)
<pre style="color:blue;background-color:khaki">
Class - a user defined template for objects. Contains both variables and methods.
Classes allow libraries of well tested self contained code to be distributed.
Each of these self contained units is a class.
Class Variable - In Python it is a variable that is shared by all instances of a class
Objects are instances of classes.
Data Member - A class variable or instance variable that holds data
Functions Overloading - A set of functions of the same name with varying parameter types.
Each function can have different code
Instance Variable - a variable defined within a method of a class. Variables defined
outside of the classes methods are class variables (see above).
Inheritance - A class will inherit the methods of a class from which it is derived.
See the section below on inheritance
Instance - A object that has been created from a class template. It is an object of
that class.
Instantiation - The creation of an object from a class template
Method - A special type of function that belongs to and is coded within a class
Object - A unique instance of a class. An object has both data members and methods.
</pre>

===Class Definitions===
*The form of a class definition is as follows

<pre style="color:blue;background-color:khaki">
class ClassName
classData = "this could be any data"
def __init__(self):
self.someData = []
def someFunction(self, parameter1, parameter2):
statement1
statement2
...
return returnValue
</pre>

**self is not a keyword it is just an indication that the first argument passed into a function is a reference to the object (it is just called self for convenience).
**The function __init__ is the constructor for the class that runs when a class instance is created
**The statement self.someDate create a list that belongs to the instance object
**The statement classData = "this could be any data" creates a class variable shared across instance objects of the class

===Class Variable Scope===
*Class variables have a scope across all instance objects of the class
*Instance variables have scope within an instance object of the class
:::Instance variable are defined within class methods (functions in the class)

See Reference [https://docs.python.org/2/tutorial/classes.html Python Tutorial on Classes]
::Read the section on Namespaces for a better understanding of variable scope
::Read the section on Class and Instance Variables for a better understanding

''Example of a class''
<pre style="color:blue;background-color:lightgrey">
class Dog:
kind = 'canine' # class variable shared by all instances
def __init__(self, name):
self.name = name # instance variable unique to each instance
</pre>

''Use of the class to understand variable scope''
<pre style="color:blue;background-color:lightgrey">
dog1 = Dog('fido')
dog2 = Dog('Buddy')
print(dog1.name) # will give name of Fido which is an instance variable
print(dog1.kind) # will give a kind of canine which is a class variable
print(dog2.name) # will give a name of Buddy which is an instance variable
print(dog2.kind) # will give a kind of canine which is a class variable
</pre>

*Note that there is no concept of public and private variables
::For example java has public and private keywords, Python does not
::There is a convention that states:
:::Variables and functions that begin with an underscore (i.e. _variable1)
:::should be treated as private and not public

===Class Inheritance===
*A class can inherit methods and data from other classes
*The definition of the class looks like:

<pre style="color:blue;background-color:lightgrey">
class DerivedClassName(BaseClassName):

def __init__(self, etc.):
statement1
...

def function1()
statement1
...
return somevalue

</pre>

Note: the class will inherit the attributes of BaseClassName

=References=
:[https://www.python.org/ Python main site]
:[https://docs.python.org/3/ Phython.org Documentation]
:[https://wiki.python.org/moin/BeginnersGuide/Programmers Beginners Programming Guides]
:[http://www.tutorialspoint.com/python/index.htm TutorialsPoint.com Python Tutorial]

Python Primer - Revision history

Ksolis at 08:38, 2 October 2017