Python Primer

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)

References

Set Environment Variables Article

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

References

PyCharm

PyDev plugin for Eclipse

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

Variable declaration and use in Java:

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++;

Variable declaration and use in Python:

variable1 = 5
variable2 = "This is a string"

variable1 = variable2 + " now adding to string"

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

Hands on Exercise

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

Hands on Exercise

Use the documentation of lists at 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

Python Operators

Python Syntax

Indentation

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

Identifier naming convention (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.

Multiline Statements (taken from TutorialsPoint.com)

total = item_one + \
        item_two + \
        item_three

Use of Quotes

Strings in Python can use single, double, or triple quotes

string1 = 'This is a string'
string2 = "This is a string"
string3 = '''This is a
multiline string'''
string4 = """This is also a
multiline string"""

Comments

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

#This is a single line comment
#This is a second single line comment

'''This is
a multiline comment'''

"""This is also
a multiline comment"""

Multiple Statement on a Single Line

use semicolons as the delimiter between statements

a = b+1; c = d + 2; print(str(a))

Control Structures

if statement - 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

a = 10
if (a == 10):
    print("In if statement")

a = 10
if (a == 10):
    print("In if statement")
else:
    print("In else statement")

while loop - a loop that continues until the loop condition has been satisfied

a = 10
while (a < 15):
    print("in while loop")
    a += 1

while loop with else statement

a = 10
while (a < 15):
    print("in while loop")
    a = a + 1
else:
    print("while loop has ended")

for loop

the general form of the for loop is

for iterating_var in sequence:
    statements(s)

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

animals = ['dog','cats','moose','deer','bear']

for theAnimal in animals:
    print("the animal is a " + theAnimal)

An example of a sequence and an iteration using the index in the list

animals = ['dog','cat','moose','deer','bear']

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

Hands on Exercise

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

def theFunctionName(parameters):
   "function_docstring"
   function statements
   return [expression]

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

f = open('workfile', 'w')
f.write("this is the file output")
f.close()

Standard Library functions must be imported

Example of using Standard Library Functions

from math import sqrt

a = sqrt(24)
print (a)

References

Built-in Functions

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

globalVar = "this string is global" #creates a global variable

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

Importing External Modules

The import statement

Bring the functions within a file into the program

Example of code import

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")

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

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")

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)

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.

Class Definitions

The form of a class definition is as follows

class ClassName
   classData = "this could be any data"
   def __init__(self):
    self.someData = []
   def someFunction(self, parameter1, parameter2):
       statement1
       statement2
       ...
       return returnValue

- 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 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

class Dog:
    kind = 'canine'         # class variable shared by all instances
    def __init__(self, name):
        self.name = name    # instance variable unique to each instance

Use of the class to understand variable scope

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

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:

class DerivedClassName(BaseClassName):

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

    def function1()
        statement1
        ...
        return somevalue

Note: the class will inherit the attributes of BaseClassName

References

Python main site

Phython.org Documentation

Beginners Programming Guides

TutorialsPoint.com Python Tutorial

Python Primer

Contents

Python Introduction

Python Primer

Invoking the Interpreter

Using an IDE

Python Variables

Operators

Python Syntax

Control Structures

Functions

Built-in and Standard Library Functions

PythonScope

Importing External Modules

Object Oriented Classes

Class Definitions

Class Variable Scope

Class Inheritance

References

Navigation menu

Personal tools

Namespaces

Variants

Views

Search

Opportunities

Navigation

Tools