Python Basics

Python Basics

Python is a general-purpose interpreted, interactive, object-oriented, and high-level programming language.

It is used for:

  • Web development (server-side),

  • Software development,

  • Machine Learning & Mathematics,

  • System scripting.

Variables

Int 

    one = 1
    two = 2

Decimal 

    dem = 1.1

String 

    strval = "Hello"

Tip
 Variables declared in function are called Local Variable. All other variables are Global variables.

Global vars can be accessed within the function.

Functions

Funtion definition: 

    def addNum(one, two):
        return one + two

Function call: 

    addNum(4,5)

print("Hello" * 5) this code prints HelloHelloHelloHelloHello

IF Statement

Boolean values : True, False

Sample if statement 

    if click == True and control == "Button":
        print("Button Clicked")
    elif control == "Radio":
        print("Radio button selected")
    else 
        print("No interaction")

Lists

Lists are used to store the collection of elements of any type

General structure of list 

    Testlist = ["a", "b", "c"]
    scores = [60, 75, 70, 80, 90]

The indexing of the list(for ex: scores) goes in two ways.
one is 0,1,2,3,4 and another one is -5,-4,-3,-2,-1 

    print(scores[4])
    print(scores[-1]) 
    # both these statements print 90 

    print(scores[-5]) # prints: 60

 +---+---+---+---+---+---+
 | P | y | t | h | o | n |
 +---+---+---+---+---+---+
 0   1   2   3   4   5   6
-6  -5  -4  -3  -2  -1

Some more notation 

    print(scores[0:2])  # prints: 60, 75

    print(scores[1:3])  # prints: 75, 70

    print(scores[2:])   # prints: 70, 80, 90

To change the value in a list: 

    scores[1] = 35
    print(scores)  # prints: 60, 35, 70, 80, 90

To remove/update items from list 

    scores[1:3] = []
    print(scores)  # prints: 60, 80, 90

    # update: 

    scores[2] = []
    print(scores) # prints: 60, 75, [], 80, 90

    # list with in list

    scores[2] = ["A", "B"]
    print(scores) # prints: 60, 75, ["A", "B"], 80, 90

    print(scores[2][1])  # prints: "B"

Append the list 

    scores.append(82)
    print(scores) # prints: 60, 75, 70, 80, 90, 82

Loops

For Loop:

For Loop on string: 

    word = "Hello"
    for w in word:
        print(w) # prints:  H  e  l  l  o

For loop on list: 

    numbers = [1,2,3,4,5]
    for n in numbers
        print(n) # prints: 1  2  3  4  5

For loop on range: 

    numlist = []
    for n in range(10)    
        print(n)            ## prints 0...9 

        numlist.append(n)   ## adds numbers to a list

    # range(start, stop, step)

    for k in range(-1, -6, -2)
        print(k)    # prints: -1  -3  -5

range(5) - outputs 0, 1, 2, 3, 4

While loop:

While loop sample:

    c = 1
    while (c<=10):
        print(c)  
        c = c + 1
    # Results: 1 ... 10


    letters = ["a", "b", "c"]
    i = 0
    while (i<len(letters)):
        print(letters[i])  
        i += 1
    # Results: a  b  c

Breaking and Continuing in Loops

Break statement sample: 

    members = ["A", "B", "C", "D"]
    for i in range(len(members)):
        if members[i] == "C":
            break
    print(i+1) #prints: 3

Continue statement sample: 

    for k in range(10):
        print(k)
        if k%3 == 0:
            print("Divided by 3")
            continue
        print("Not divided by 3")

Tricky Question & Answer for reference

    f = 1
    A = [[1, 2, 3], [2, 3, 4], [3, 4, 5]]
    for i in range(0, 3):
        f =f*i
        for j in range(0, 3):
            A[i][j] = f
    print(A)

Answer is: [[0, 0, 0], [0, 0, 0], [0, 0, 0]]

Dictionaries and Sets

Sets

Sample sets code: Set removes the duplicates by default

    age = {"24", "35", "25", "35", "56"}
    print(age) # prints: {'24', '25', '56', '35'}

Convert from list:

nums = set([7,7,1,3,4,5,5,2])
print(len(nums)) #prints: 6

Way to check if an element exist in the set:

    if "25" in age
        print("People with age 25 attending")

Another sample:

    CountryList = []
    for i in range(5):
        Country = input("Please Enter your Country: ")
        CountryList.append(Country)
    CountrySet = set(CountryList)
    print(CountryList)
    print(CountrySet)

The result would be :

Please Enter your Country: "US"
Please Enter your Country: "UK"
Please Enter your Country: "US"
Please Enter your Country: "INDIA"
Please Enter your Country: "UK"
['US', 'UK', 'US', 'INDIA', 'UK']
set(['INDIA', 'UK', 'US'])

Dictionary

Dictionary will be in key value pair. Thats the difference with Sets

Format: 

    Dictionary = {"key":"value","key2":"value2","key3":"value3"}

Sample code: 

    CountryList = []
    for i in range(5):
        Country = input("Please Enter your Country: ")
        CountryList.append(Country)

    CountryDict = {}
    for Country in CountryList:
        if Country in CountryDict:
            CountryDict[Country] += 1
        else:
            CountryDict[Country] = 1
    print(CountryDict)

The result would be:

Please Enter your Country: "US"
Please Enter your Country: "France"
Please Enter your Country: "India"
Please Enter your Country: "France"
Please Enter your Country: "US"
{'India': 1, 'US': 2, 'France': 2}

Dictionary keys can be in different types - sample code: 

    dict = {}
    dict[1] = 2
    dict['1'] = 4
    dict[1] += 2
    count = 0

    print(dict) #prints: {'1': 4, 1: 4}

List inside Set is not allowed: 

    l = [1,2,3,[4,5]] #it is allowed

    s = {1,2,3,[4,5]} #this throws an error

Accessing an unspecified index in a ‘list’ will produce an error while doing so the same in ‘dictionary’ will produce a new key

Input and Output

Input 

    age = int(input("Enter age: "))
    print(age)

    # another sample 

    Scores = []
    for i in range(5):
        curscore = float(input("Enter the score" + str(i) + " : "))
        Scores.append(curscore)
    print(Scores) 
    # Result: Printing 5 decimal scores entered by the user

Output

Both the statements below output the same text 

    print("The score you entered: "+str(curscore)+" good") 
    print("The score you entered:",curscore,"good")

Note the usage of , in print statement. It also adds an extra space

\n in print statement adds new line 

    print("The score is:\n" + str(curscore))

File I/O

Syntax:

    file = open("filename", "r") # "r", "w", "a", "r+" -> Read and Write
    # file operations
    file.close()

Read / Write

Example code for file read/write: 

    vacationspots = ["Singapore", "Bali", "Europe", "USA", "Srilanka"]

    vfile = open("VacationPlaces", "w")
    for spot in vacationspots:
        vfile.write(spot + "\n")
    vfile.close

    vfile = open("VacationPlaces", "r")
    print(vfile.read()) # to print whole file content

    # To print file content line by line

    for line in vfile:
        print(line, end="")

    vfile.close

To read single line from file, use this code 

    firstline = vfile.readline()
    print(firstline)

This will change the position of the pointer. For ex: if you execute read() code after this line, then it will print from the last position only. 

    print(vfile.readline())
    print(vfile.read())

    # Output

    Singapore

    Bali
    Europe
    USA
    Srilanka

Append

    finalspot = "Tailand"
    vfile = open("VacationPlaces", "a")
    vfile.write(finalspot)
    vfile.close

With keyword :

With block would automatically close the file once the end of block is reached.

    with open("VacationPlaces", "r") as vfile:
        for line in vfile:
            print(line)

to color terminal texts use termcolor package - https://pypi.org/project/termcolor/

Classes

Introductions to classes

Syntax: 

    class className:
        def __init__(self):
            self.attribute1 = 0
        
        def anotherfunc(self):
            actions(a)

  • self has to be added in each function of the class. that refers the class itself

  • init function refers the constructor of the class

Sample class code: 

    class Team:
        def __init__(self):
            self.TeamName = "NaN"
            self.TeamOrigin = "NaN"

        def DefineTeamName(self, name):
            self.TeamName = name

    TeamA = Team()
    print(TeamA.TeamName)   # prints "NaN"

    TeamA.DefineTeamName("Tiger")
    print(TeamA.TeamName)   # prints "Tiger"

Default value:

    class Team:
        def __init__(self, Name="Name", Origin="Origin"):
            self.TeamName = Name
            self.TeamOrigin = Origin


    TeamA = Team()
    print(TeamA.TeamName)   # prints "Name"


    TeamB = Team("Cheetah")
    print(TeamB.TeamName)   # prints "Cheetah"

Class Inheritance / Concrete class

Syntax : 

    class InheritanceClass(ParentClass):
        def __init__(self, input1, input2):
            ParentClass.__init__(self)
            self.attribute1 = input1
            self.attribute2 = input2

Sample code : 

    class Team:
        def __init__(self, Name="CSK", Origin="TN"):
            self.TeamName = Name
            self.TeamOrigin = Origin

    class Player(Team):
        def __init__(self):
            Team.__init__(self)
            self.PlayerName = "NaN"
            self.Score = 0

        def setName(self, name):
            self.PlayerName = name

        def __str__(self):
            return "Team - " + self.TeamName + \
                " has player '" + self.PlayerName + "'"

    PlayerA = Player()
    PlayerA.setName("Dhoni")
    print(PlayerA.PlayerName) 
    print(PlayerA.Score)     
    print(PlayerA.TeamName)  
    print(PlayerA.TeamOrigin)
    print(PlayerA)  # Prints the string from '__str__' function

OUTPUT:

    Dhoni
    0
    CSK
    TN
    Team - CSK has player 'Dhoni'
    __str__ function returns something when the object is printed

Practical scenario of using classes

  • Pets = Base class

    • name, age, hunger

  • Dog = Concrete class inherits Pets

    • base attributes + breed, favoriteToy

  • Cat = Concrete class inherits Pets

    • base attributes + place

  • Humans = Base class

    • name, pets

    • dog and cat objects are created and passed to human class as list.

    class Human:
        def __init__(self, name, pets):
            self.name = name
            self.pets = pets

    PetLover = Human("Alice", [husky_dog, typical_cat])
    print(PetLover.pets[1].name + " - " + str(PetLover.pets[1].age))

Importing

SYNTAX:

import package_name

SAMPLE: 

    import random

    randInt = random.randint(0, 10) 
    print(randInt)      # Integer : 0 >= N <= 10

    randFloat = random.random() 
    print(randFloat)    # Float : 0.0 >= N < 1.0

    randUniform = random.uniform(1, 50) 
    print(randUniform)  # Float : 1.0 >= N < 50.0

OUTPUT:

2
0.1554929844739973
5.860057875357238

User of Nick-name:

    import random as r

    randInt = r.randint(0, 10) 
    print(randInt)

We could import everything from random package so that all the functions can be used without any prefix. However we will have to be careful about the name conflict with other packages.

    from random import *

    randInt = randint(0, 10) 
    print(randInt)

random package can work well with lists.. check out..

    from random import *

    simplelist = [1,5,3,8,2,11]
    pickElement = r.choice(simplelist) 
    print(pickElement) # prints random pick from the list


    r.suffle(simplelist)
    print(simplelist) # now the element order is changed