DEV Community: CNavya21

Revision 11-03-2023

CNavya21 — Sat, 11 Mar 2023 09:46:05 +0000

1. What happens when you try to divide a number by 0 in JavaScript?

If we divide with positive number by zero then it will return infinity.
If we divide with zero by zero it will returns undefined.
If we divide with negative number by zero then it will returns -infinity.

2. What is the difference between a traditional for loop and "for each" loop in JavaScript?

The for loop that will use three optional expressions to implement the repeated execution of a code block.

Syntax

for (initialexpression; condition; updateexpression) {
    // for loop body
}

initialExpression : it declares variables and executes only once.
condition : it checks the condition, if the condition is false then loop will terminate and the condition is true then block of code inside of the for loop is executed.
updateexpression : updates the value of initialexpression when the condition is true.

Example

const n = 3;
for (let i = 1; i <= n; i++) {
    console.log(`I love JavaScript.`);
}
# output: I love JavaScript.
          I love JavaScript.
          I love JavaScript.

The forEach() method calls a function (a callback function) for each element in an array.It doesn't execute for an empty array.

Syntax

array.forEach(function(currentValue, index, arr), thisValue)

currentValue : It takes the current value of element.
index : It takes the index of current value of element.
arr : The array of current element.
thisValue : It takes defaultly undefined.A value passed to the function as its this value.

Example

let students = ['John', 'Sara', 'Jack'];
students.forEach(myFunction);
function myFunction(item, index, arr) {
    arr[index] = 'Hello ' + item;
}
console.log(students);
# output: ["Hello John", "Hello Sara", "Hello Jack"]

3. What is the difference between Null and undefined in JavaScript?

null is a special value that represents an empty or unknown value.

Example

var test1 = null;
console.log(test1);
# output: null

Undefined means a variable has been declared, but the value of that variable has not yet been defined.

Example

var test2;
console.log(test2);
# output : undefined

4. How to activate the strict mode and what is its use in JavaScript?

Strict mode is declared by adding "use strict"; to the beginning of a script or a function.

Advantages

Strict mode eliminates JavaScript silent errors by changing them to throw errors.
Fixes mistake that make it difficult for JavaScript engine to perform better.
Make code run faster than identical code that's not in strict mode.

5. What happens when you don't have a return statement in a function in JavaScript?

The function will automatically returns undefined.

6. What is the UNIQUE constraint in SQL and how is it different from PRIMARY KEY?

UNIQUE constraint checks for all the values in a column or set of columns are different or not.
Both the UNIQUE and PRIMARY KEY constraints provide a permission for uniqueness for a column or set of columns.
A PRIMARY KEY constraint automatically has a UNIQUE constraint.
You can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.

7. What is the difference between PRIMARY KEY and FOREIGN KEY?

Primary key:
- A primary key is used to ensure that data in the specific column is unique. A column cannot have NULL values.
- Only one primary key is allowed in a table.
- It does not allow NULL values.

2.Foreign key:

A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables.
Whereas more than one foreign key are allowed in a table.
It contains NULL values.

8. What is the unshift() method in JavaScript?

It adds one or more elements in beginning of the array and returns the new array as output.

Example

const array1 = [1, 2, 3];
console.log(array1.unshift(4, 5));
// Expected output: 5

console.log(array1);
// Expected output: Array [4, 5, 1, 2, 3]

9. In JavaScript string methods, what is the difference between charAt() and charCodeAt(), and what will happen if no arguments are provided with these methods?

charAt() method returns the character at the specified index.

Syntax

string.charAt(index)

Example

let text = "HELLO WORLD";
let letter = text.charAt(text.length-1);
#output: D

charCodeAt() returns Unicode value of the character at the specified Index.

Syntax

string.charCodeAt(index)

Example

let text = "HELLO WORLD";
let code = text.charCodeAt(0);
#output: 72

If we don't pass any argument, then it will return the character and Unicode value of first character in the string respectively.

10. What is the difference between slice() and splice() in JavaScript?

slice() : This method is used to get a new array by selecting a sub-array of a given array.

Syntax

string.slice(start, end)

Example

let text = "Hello world!"; 
let result = text.slice(3, 8);
#output: lo wo

splice() : This method is used to add/remove an item from the given array.

Syntax

splice(start, deleteCount, item1, item2, itemN)
#start is the index from where you want to modify the array
#deleteCount is an optional parameter and refers to the number of elements to remove.

Example

let fruits = ['mango', 'banana, 'apple', 'kiwi', 'orange']
fruits.splice(2)
#output: ['apple', 'kiwi', 'orange']

11. How do we ensure any package updates for any older dependencies get reflected in the package.json file?

No, It won't reflect on the package.json file when we update the npm version.It remains the same version in the dependencies.

12. How is atomicity achieved in a relational database?

The entire transaction takes place at once or doesn’t happen at all.

13. What is the difference between the "for-each" loop and map method in JavaScript?

The map() method returns a new array, whereas the forEach() method does not return a new array.
The map() method is used to transform the elements of an array, whereas the forEach() method is used to loop through the elements of an array.
The map() method can be used with other array methods, such as the filter() method, whereas the forEach() method cannot be used with other array methods.

HTML & CSS

CNavya21 — Thu, 02 Mar 2023 05:52:22 +0000

Box model

When laying out a document, the browser's rendering engine represents each element as a rectangular box according to the standard CSS basic box model.
CSS determines the size, position, and properties (color, background, border size, etc.) of these boxes. *Every box is composed of four parts defined by their respective edges: the content edge, padding edge, border edge, and margin edge.

Content area
- The content area, bounded by the content edge, contains the "real" content of the element, such as text, an image, or a video player.
- Its dimensions are the content width (or content-box width) and the content height (or content-box height). It often has a background color or background image.
Padding area
- The padding area, bounded by the padding edge, extends the content area to include the element's padding. Its dimensions are the padding-box width and the padding-box height.
- The thickness of the padding is determined by the padding-top, padding-right, padding-bottom, padding-left, and shorthand padding properties.
Border area
- The border area, bounded by the border edge, extends the padding area to include the element's borders. Its dimensions are the border-box width and the border-box height.
- The border area's size can be explicitly defined with the width, min-width, max-width, height, min-height, and max-height properties.
Margin area
- The margin area, bounded by the margin edge, extends the border area to include an empty area used to separate the element from its neighbors. Its dimensions are the margin-box width and the margin-box height.
- The size of the margin area is determined by the margin-top, margin-right, margin-bottom, margin-left, and shorthand margin properties.

Inline vs Block elements

In HTML elements historically were categorized as either "block-level" elements or "inline-level" elements. Since this is a presentational characteristic it is nowadays specified by CSS in the Flow Layout.

Example for inline

<div>
  The following span is an <span class="highlight">inline element</span>; its
  background has been colored to display both the beginning and end of the
  inline element's influence.
</div>

In this example, the

block-level element contains some text. Within that text is a element, which is an inline element. Because the element is inline, the paragraph correctly renders as a single, unbroken text flow.

Example for block element

<div style="border: 1px solid black;"> About Us </div>

Positioning: absolute/relative

Absolute
This is a very powerful type of positioning that allows you to literally place any page element exactly where you want it.
You use the positioning attributes top, left, bottom, and right to set the location.
Absolute positioned elements are removed from the normal flow, and can overlap elements.

Syntax

position: absolute;

Relative
The relative positioning property is used to set the element relative to its normal position.
Setting the top, right, bottom, and left properties of a relatively-positioned element will cause it to be adjusted away from its normal position.

Syntax

position: relative;

Common CSS structural classes

Structural pseudo classes allow access to the child elements present within the hierarchy of parent elements.
We can select first-child element, last-child element, alternate elements present within the hierarchy of parent elements.

The following is the list of structural classes.

:first-child - it represents the element that is prior to its siblings in a tree structure.
:nth-child(n) - it applies CSS properties to those elements that appear at the position evaluated by the resultant of an expression.
:last-child - it represents the element that is at the end of its siblings in a tree structure.
:nth-last-child(n) - :nth-last-child(Expression) is the same as :nth-child(Expression) but the positioning of elements start from the end.
:only-child - it represents the element that is a sole child of the parent element and there is no other sibling.
:first-of-type - It selects the first element of the one type of sibling.
:nth-of-type(n) - it represents those elements of the same type at the position evaluated by the Expression.
:last-of-type - it represents the last element in the list of same type of siblings.
:nth-last-of-type(n) - It is the same as :nth-of-type(n) but it starts counting of position from the end instead of start.

Common CSS styling classes

.container : This class is used to create a container that holds the content of a web page or sometimes container which will hold other elements.
.header : This class is used to style the header section of a web page, which typically contains the site logo, navigation menu, and other important information.
.nav : This class is used to style navigation menus on a web page, such as a top or side menu.
.btn : This class is used to style buttons on a web page. It's often used to add a background color, border, and padding to create a clickable button.
.card : This class is used to create a card-style layout for content, such as a blog post or product listing. It often includes a background color, border, and padding to create a contained section for the content.
.footer : This class is used to style the footer section of a web page, which typically contains copyright information, social media links.

CSS specificity

Specificity Hierarchy :Every element selector has a position in the Hierarchy.

Inline style: Inline style has highest priority.
Identifiers(ID): ID have the second highest priority.
Classes, pseudo-classes and attributes: Classes, pseudo-classes and attributes are come next.
Elements and pseudo-elements: Elements and pseudo-elements have lowest priority.

Start at 0, add 100 for each ID value, add 10 for each class value (or pseudo-class or attribute selector), add 1 for each element selector or pseudo-element.

Example

A: h1
B: h1#content
C: <h1 id="content" style="color: pink;">Heading</h1>

The specificity of A is 1 (one element selector)
The specificity of B is 101 (one ID reference + one element selector)
The specificity of C is 1000 (inline styling)

Flex box / Grid

Flex container:The flex container specifies the properties of the parent. It is declared by setting the display property of an element to either flex or inline-flex.

Flex items:The flex items specify properties of the children. There may be one or more flex items inside a flex container.

it is used to specify the direction of the flexible items inside a flex container.
It is used to align the flex items horizontally when the items do not use all available space on the main-axis.
It is used to align the flex items vertically when the items do not use all available space on the cross-axis.

The flex container properties are:

flex-direction
flex-wrap
flex-flow
justify-content
align-items
align-content

The flex item properties are:

order
flex-grow
flex-shrink
flex-basis
flex
align-self

CSS Responsive queries

The @media rule, introduced in CSS2, made it possible to define different style rules for different media types.
Media queries can be used to check many things, such as:
- width and height of the viewport
- width and height of the device
- orientation (is the tablet/phone in landscape or portrait mode?)
- resolution

Syntax

@media not|only mediatype and (expressions) {
  CSS-Code;
}

Value and description:

all - Used for all media type devices
print - Used for printers
screen - used for computer screens, tablets, smart-phones etc.
speech - Used for screenreaders that "reads" the page out loud

Example

@media screen and (min-width: 480px) {
  body {
    background-color: lightgreen;
  }
}

Common header meta tags

The tag is used to provide such additional information. This tag is an empty element and so does not have a closing tag but it carries information within its attributes.
You can add metadata to your web pages by placing tags inside the header of the document which is represented by and tags.

Attributes and descriptions

Name
- Name for the property. Can be anything. Examples include, keywords, description, author, revised, generator etc.
content
- Specifies the property's value.
scheme
- Specifies a scheme to interpret the property's value (as declared in the content attribute).
http-equiv
- Used for http response message headers. For example, http-equiv can be used to refresh the page or to set a cookie. Values include content-type, expires, refresh and set-cookie.

Example

<!DOCTYPE html>
<html>
  <head>
      <title>Meta Tags Example</title>
      <meta name = "keywords" content = "HTML, Meta Tags, Metadata" />
   </head>
   <body>
      <p>Hello HTML5!</p>
   </body>
</html>
# output: Hello HTML5!

SQL Concepts

CNavya21 — Tue, 21 Feb 2023 06:25:32 +0000

ACID

The ACID properties define SQL database key properties to ensure consistent, safe and robust database modification when saved.

1. Atomicity – Transaction acting on several pieces of information complete only if all pieces successfully save.

2. Consistency - The saved data cannot violate the integrity of the database.

3. Isolation - The separation of resource or data modifications made by different transactions.

4. Durability - Ensures that changes made to the database (transactions) that are successfully committed will survive permanently, even in the case of system failures.

CAP Theorem

It originally introduced as the CAP principle, can be used to explain some of the competing requirements in a distributed system with replication.
The three letters in CAP refer to three desirable properties: consistency(among replicated copies), availability(of the system for read and write operations) and partition tolerance (in the face of the nodes in the system being partitioned by a network fault).

Joins

SQL Join statement is used to combine data or rows from two or more tables based on a common field between them.

Different types of Joins are as follows:
1.INNER JOIN :

It selects all rows from both tables as long as the condition is satisfied.

Syntax

SELECT table1.column1,table1.column2,table2.column1,....
FROM table1
INNER JOIN table2
ON table1.matching_column = table2.matching_column;

table1: First table.
table2: Second table
matching_column: Column common to both the tables.

Example

SELECT ord.OrderID, cust.CustomerName
FROM Orders ord
INNER JOIN Customers cust ON ord.CustomerID = cust.CustomerID;

2.LEFT JOIN :

It returns all the rows of the table on the left side of the join and matches rows for the table on the right side of the join.

Syntax

SELECT table1.column1,table1.column2,table2.column1,....
FROM table1
LEFT JOIN table2
ON table1.matching_column = table2.matching_column;

Example

SELECT cust.CustomerName, ord.OrderID
FROM Customers cust
LEFT JOIN Orders ord ON cust.CustomerID = ord.CustomerID
ORDER BY cust.CustomerName;

3.RIGHT JOIN :

It returns all the rows of the table on the right side of the join and matching rows for the table on the left side of the join.

Syntax

SELECT table1.column1,table1.column2,table2.column1,....
FROM table1
RIGHT JOIN table2
ON table1.matching_column = table2.matching_column;

Example

SELECT cust.CustomerName, ord.OrderID
FROM Customers cust
RIGHT JOIN Orders ord ON cust.CustomerID = ord.CustomerID
ORDER BY cust.CustomerName;

4.FULL JOIN :

It creates the result-set by combining results of both LEFT JOIN and RIGHT JOIN.
The result-set will contain all the rows from both tables.
For the rows for which there is no matching, the result-set will contain NULL values.

Syntax

SELECT table1.column1,table1.column2,table2.column1,....
FROM table1
FULL JOIN table2
ON table1.matching_column = table2.matching_column;

Example

SELECT cust.CustomerName, ord.OrderID
FROM Customers cust
FULL JOIN Orders ord ON cust.CustomerID = ord.CustomerID
ORDER BY cust.CustomerName;

Aggregations, Filters

It performs a calculation on multiple values and returns a single value.
SQL provides many aggregate functions that include avg, count, sum, min, max, etc.
An aggregate function ignores NULL values when it performs the calculation, except for the count function.

Functions:

1. count()

It returns the total number of rows from a database table that matches the criteria in the SQL query.

Syntax

count(*) or count(column_name)

Example

select count(*) from employees;

2. sum()

It takes the name of the column as an argument and returns the sum of all the non NULL values in that column.

Syntax

sum(column_name)

Example

select sum(emp_id) from employees;

3. avg()

It uses the name of the column as an argument and returns the average of all the non NULL values in that column.

Syntax

avg(column_name)

Example

select avg(salary) from employees;

4. max()

It takes the name of the column as an argument and returns the maximum value present in the column. MAX() returns NULL when no row is selected.

Syntax

max(column_name)

Example

select max(salary) from employees;

5. min()

It takes the name of the column as an argument and returns the minimum value present in the column. MIN() returns NULL when no row is selected.

Syntax

min(column_name)

Example

select min(salary) from employees;

6. filter()

The filter clause is used to, as the name suggests, filter the input data to an aggregation function.

Syntax

EXPRESSION
AGGREGATION FUNCTION 1
FILTER(WHERE CLAUSE)
AGGREGATION FUNCTION 2
FILTER(WHERE CLAUSE)
.
.
.
EXPRESSION

Example

select count(emp_id)
FILTER(WHERE ID!=2),
avg(length(Name))
FILTER(WHERE length(Name)>4)
from employees;

7. group by

These statement groups rows that have the same values into summary rows.
The GROUP BY statement is usually used along with aggregate functions such as count(), min(), max(), avg(), sum(), etc.

Syntax

select column_name(s)
from table_name
WHERE condition
group by column_name(s)
order by column_name(s);

Example

select COUNT(CustomerID), Country
from Customers
group by Country;

8. having

These clause was added to SQL because the WHERE keyword cannot be used with aggregate functions.

Syntax

expression
group by CLAUSE
having CLAUSE
condition

Example

select emp_id
from employees
group by Name, employed
having employed = 1;

Normalization

These is the process to eliminate data redundancy and enhance data integrity in the table. The database normalization process can be divided into following types:

1.First Normal Form (1NF)

Data is stored in tables with rows that can be uniquely identified by a Primary Key.
Data within each table is stored in individual columns in its most reduced form.There are no repeating groups.

2. Second Normal Form (2NF)

All the rules from 1NF must be satisfied.
Only those data that relates to a table’s primary key is stored in each table.

3. Third Normal Form (3NF)

All the rules from 2NF must be satisfied.
There should be no intra-table dependencies between the columns in each table.

4. Boyce-Codd Normal Form/Fourth Normal Form(BCNF of 4NF)

BCNF stands for Boyce-Codd Normal Form, which is stronger than 3NF.
This form which doesn’t contain any value dependency. A relation that is in 4NF also comes in BCNF.

5. Fifth Normal Form (5NF)

NF stands for Fifth Normal Form, where the relationship should be in 4NF to apply the fifth normal form. This normal form doesn’t contain any dependency.

6. Sixth Normal Form (6NF)

It stands for Sixth Normal Form, which is not a standardized form of normalization. Therefore it isn’t used nowadays and may give a clear and standardized normalization in the future.

Indexes

An index helps to speed up SELECT queries and WHERE clauses, but it slows down data input, with the UPDATE and the INSERT statements.
Indexes can be created or dropped with no effect on the data.

To create index command

CREATE INDEX index_name ON table_name;

To drop index command

DROP INDEX index_name;

Unique Index

CREATE UNIQUE INDEX index_name
on table_name (column_name);

To alter the index , we use rebuilding the existed table.

ALTER INDEX IndexName 
ON TableName REBUILD;

Transactions

The following commands used to control transaction.

commit

It is used to save the changes.

Syntax

commit;

Example

> delete from Customers
  WHERE Age = 25;
> commit;

rollback

These command used to undo transactions that have not already been saved to the database.

Syntax

rollback;

Example

> delete from Customers
  WHERE Age = 25;
> rollback;

savepoint

It creates points within the groups of transactions in which to ROLLBACK.

Syntax

SAVEPOINT SAVEPOINT_NAME;

The syntax for rolling back to a SAVEPOINT is as shown below.
ROLLBACK TO SAVEPOINT_NAME;

set transaction

Places a name on a transaction.

Syntax

SET TRANSACTION [ READ WRITE | READ ONLY ];

Locking mechanism

he lock is a mechanism associated with a table for restricting unauthorized access to the data.
We can apply a lock on row level, database level, table level, and page level.

Modes of lock

Exclusive Lock (X)
Shared Lock (S)
Update Lock (U)
Intent Lock (I)
Schema Lock (Sch)
Bulk Update Lock (BU)

Database Isolation Levels

It defines how one transaction is isolated from other transactions. Different types of isolation levels:

Read committed
- In select query it will take only committed values of table.
- If any transaction is opened and in-completed on table in others sessions then select query will wait till no transactions are pending on same table.
Read uncommitted
- It is used when we want even the non-committed values of the rows.
- Any updates and inserts that are even not committed should be reflected in over transaction.
Repeatable read
- In select query data of the table that is used under transaction of isolation level "Repeatable Read" can not be modified from any other sessions till transaction is completed.
Serializable
- This is the strongest of all the Isolation Levels and guarantees pure isolation.
- No other transaction will be able to read and write values till this type of transaction is running on the database.
Snapshot
- it is similar to the serializable.
- The difference is Snapshot does not hold lock on table during the transaction so table can be modified in other sessions.

Triggers

Data Definition Language(DDL) Trigger:
- DDL triggers are fired in response to the DDL events, such as CREATE, ALTER, and DROP statements.
- We can create these triggers at the database level or server level, depending on the type of DDL events.
Data Manipulation Language(DML) Trigger:
- DML triggers are fired in response to DML events like INSERT, UPDATE, and DELETE statements in the user's table or view.
- It can also be executed in response to DML like operations performed by system-defined stored procedures.
Logon Triggers:
- It fire stored procedures in response to a LOGON event.
- This event is raised when the user session is established with an instance of SQL Server.

References

ACID (https://www.geeksforgeeks.org/acid-properties-in-dbms/)
CAP Theorem (https://www.bmc.com/blogs/cap-theorem/)
Joins (https://www.geeksforgeeks.org/sql-join-set-1-inner-left-right-and-full-joins/)
Aggregation, filters (https://www.scaler.com/topics/sql/aggregate-function-in-sql/)
Normalization (https://www.sqlservercentral.com/articles/database-normalization-in-sql-with-examples)
Indexes (https://www.tutorialspoint.com/sql/sql-indexes.htm)
Transactions (https://www.tutorialspoint.com/sql/sql-transactions.html)
Locking mechanism (https://www.javatpoint.com/locks-in-sql-server)
Database Isolation levels (https://www.besttechtools.com/articles/article/sql-server-isolation-levels-by-example)

SOLID PRINCIPLES

CNavya21 — Mon, 20 Feb 2023 04:52:39 +0000

SOLID is a mnemonic abbreviation for a set of design principles created for software development in object-oriented languages.
According to Orner, while the practice of software development has changed in the past 20 years, SOLID principles are still the basis of good design.

The SOLID Principles:

The Single-Responsibility Principle (SRP)
The Open-Closed Principle (OCP)
The Liskov Substitution Principle (LSP)
The Interface Segregation Principle (ISP)
The Dependency inversion Principle (DIP)

1. Single-Responsibility Principle(SRP)

This states that a class should have a single responsibility, but more than that, a class should only have one reason to change.

Example

A change to one responsibility results to modification of the other responsibility.

class Animal:
    def __init__(self, name: str):
        self.name = name
    def get_name(self) -> str:
        pass
    def save(self, animal: Animal):
        pass

To make this conform to SRP, we create another class that will handle the sole responsibility of storing an animal to a database:

class Animal:
    def __init__(self, name: str):
            self.name = name
    def get_name(self):
        pass
class AnimalDB:
    def get_animal(self) -> Animal:
        pass
    def save(self, animal: Animal):
        pass

2. Open-Closed Principle (OCP)

In the open/closed principle classes should be open for extension, but closed for modification.
Essentially meaning that classes should be extended to change functionality, rather than being altered into something else.

Example

Let’s imagine you have a store, and you give a discount of 20% to your favorite customers using this class:
When you decide to offer double the 20% discount to VIP customers. You may modify the class like this:

class Discount:
    def __init__(self, customer, price):
        self.customer = customer
        self.price = price

    def give_discount(self):
            if self.customer == 'fav':
                return self.price * 0.2
            if self.customer == 'vip':
                return self.price * 0.4

To make it follow the OCP principle, we will add a new class that will extend the Discount.
In this new class, we would implement its new behavior:

class Discount:
    def __init__(self, customer, price):
        self.customer = customer
        self.price = price
    def get_discount(self):
            return self.price * 0.2
class VIPDiscount(Discount):
    def get_discount(self):
        return super().get_discount() * 2
class SuperVIPDiscount(VIPDiscount):
    def get_discount(self):
        return super().get_discount() * 2

3. Liskov Substitution Principle (LSP)

A sub-class must be substitutable for its super-class.
The aim of this principle is to ascertain that a sub-class can assume the place of its super-class without errors.
If the code finds itself checking the type of class then, it must have violated this principle.

Example

Let’s use our Animal example.

def animal_leg_count(animals: list):
    for animal in animals:
        if isinstance(animal, Lion):
            print(lion_leg_count(animal))
        elif isinstance(animal, Mouse):
            print(mouse_leg_count(animal))
        elif isinstance(animal, Pigeon):
            print(pigeon_leg_count(animal))

animal_leg_count(animals)

To make this function follow the LSP principle, we will follow this LSP requirements postulated by Steve Fenton:
If the super-class (Animal) has a method that accepts a super-class type (Animal) parameter.
Its sub-class(Pigeon) should accept as argument a super-class type (Animal type) or sub-class type(Pigeon type).
If the super-class returns a super-class type (Animal).
Its sub-class should return a super-class type (Animal type) or sub-class type(Pigeon).
Now, we can re-implement animal_leg_count function:

def animal_leg_count(animals: list):
    for animal in animals:
        print(animal.leg_count())

animal_leg_count(animals)

The animal_leg_count function cares less the type of Animal passed, it just calls the leg_count method.
All it knows is that the parameter must be of an Animal type, either the Animal class or its sub-class.
The Animal class now have to implement/define a leg_count method.
And its sub-classes have to implement the leg_count method:

class Animal:
    def leg_count(self):
        pass
class Lion(Animal):
    def leg_count(self):
        pass

4. Interface Segregation Principle(ISP)

Make fine grained interfaces that are client specific
Clients should not be forced to depend upon interfaces that they do not use.
This principle deals with the disadvantages of implementing big interfaces.

Example

Let’s look at the below IShape interface:

class IShape:
    def draw_square(self):
        raise NotImplementedError
    def draw_rectangle(self):
        raise NotImplementedError
    def draw_circle(self):
        raise NotImplementedError

Here, our IShape interface performs actions that should be handled independently by other interfaces.
To make our IShape interface conform to the ISP principle, we segregate the actions to different interfaces.
Classes (Circle, Rectangle, Square, Triangle, etc) can just inherit from the IShape interface and implement their own draw behavior.

class IShape:
    def draw(self):
        raise NotImplementedError
class Circle(IShape):
    def draw(self):
        pass
class Square(IShape):
    def draw(self):
        pass
class Rectangle(IShape):
    def draw(self):
        pass

5. Dependency Inversion Principle(DIP)

Dependency should be on abstractions not concretions
- High-level modules should not depend upon low-level modules. Both should depend upon abstractions.
- Abstractions should not depend on details. Details should depend upon abstractions.

Example

class XMLHttpService(XMLHttpRequestService):
    pass

class Http:
    def __init__(self, xml_http_service: XMLHttpService):
        self.xml_http_service = xml_http_service
    def get(self, url: str, options: dict):
        self.xml_http_service.request(url, 'GET')
    def post(self, url, options: dict):
        self.xml_http_service.request(url, 'POST')

The Connection interface has a request method. With this, we pass in an argument of type Connection to our Http class:

class Http:
    def __init__(self, http_connection: Connection):
        self.http_connection = http_connection
    def get(self, url: str, options: dict):
        self.http_connection.request(url, 'GET')
    def post(self, url, options: dict):
        self.http_connection.request(url, 'POST')

We can now re-implement our XMLHttpService class to implement the Connection interface:

class XMLHttpService(Connection):
    xhr = XMLHttpRequest()

    def request(self, url: str, options:dict):
        self.xhr.open()
        self.xhr.send()

We can create many Http Connection types and pass it to our Http class without any fuss about errors.

class NodeHttpService(Connection):
    def request(self, url: str, options:dict):
        pass

class MockHttpService(Connection):
    def request(self, url: str, options:dict):
        pass

References

https://www.hashbangcode.com/article/solid-principles-python

OOP CONCEPT IN PYTHON

CNavya21 — Thu, 16 Feb 2023 06:40:47 +0000

Python is an object-oriented language.
An object-oriented is to design the program using classes and objects.
The main concept of OOP's is to bind the data and the functions that work on that together as a single unit so that no other part of the code can access this data.

Main concepts of OOP's

Class

Class is defined as a collection of objects.
It contains the blueprints of objects.
Classes are created by keyword class.
Attributes are the variables that belong to a class.
Attributes are always public and can be accessed using dot(.) operator. Eg: Myclass.Application.

Syntax

class ClassName:
    # statement-1
    .
    .
    .
    # statement-2

Example

class Dog:
    pass

Object

The object is an entity that has state and behavior.
A method is a function that belongs to an object.
It may be any real-world object like a mouse, keyboard, chair, table, pen, etc.
Everything in Python is an object, and almost everything has attributes and methods.
State : It is represented by the attributes of an object and also reflects the properties of an object.
Behavior : It is represented by the methods of an object and also reflects the response of an object to other objects.
Identity : It gives a unique name to an object and enables one object to interact with other objects.

self

It represents the instance of the class.
It allows you to access variables, attributes and methods of a defined class.
The self parameter doesn't have to be named “self,” as you can call it by any other name.

Example

class Dog:
    def __init__(self, breed, colour):
        self.breed = breed
        self.colour = colour
    def display(self):
        print(self.breed, self.colour)
d = Dog("Samoyed", "White")
d.display()
# output: Samoyed White

Inheritance

Inheritance is the capability of one class to derive or inherit the properties from another class.
It represents real-world relationships well.
It provides the reusability of a code and it allows us to add more features to a class without modifying it.

Types of Inheritance

1.Single Inheritance

It enables a derived class to inherit characteristics from a single-parent class.

2.Multilevel Inheritance

It enables a derived class to inherit properties from an immediate parent class which in turn inherits properties from his parent class.

3.Hierarchical Inheritance

It enables more than one derived class to inherit properties from a parent class.

4.Multiple Inheritance

It enables one derived class to inherit properties from more than one base class.

Syntax

class BaseClass:
    {body}
class DerivedClass(BaseClass):
    {body}

Example

class Person(object):
    def __init__(self, name):
        self.name = name
    def getName(self):
        return self.name
    def isEmp(Person):
        return False
class Employee(Person):
    def isEmployee(self):
        return True
per = Person("Abhi")
print(per.getName(), per.isEmployee())

per = Person("Visal")
print(per.getName(), per.isEmployee())
# output: Abhi False
          Visal False

Polymorphism

Polymorphism contains two words "poly" and "morphs".
It means having many forms.

Example

def add(x, y, z = 0):
    return x + y + z

print(add(2, 3))
print(add(2, 3, 4))
# output : 5
           9

Method Overriding

It allows a subclass or child class to provide a specific implementation of a method that is already provided by one of its super-classes or parent classes.
When a method in a subclass has the same name, same parameters or signature and same return type as a method in its super-class, then the method in the subclass is said to override the method in the super-class.

Example

class Parent():
    def __init__(self):
        self.value = "Inside Parent"
    def show(self):
        print(self.value)
class Child(Parent):
    def __init__(self):
        self.value = "Inside Child"
    def show(self):
        print(self.value)


obj1 = Parent()
obj2 = Child()

obj1.show()
obj2.show()
#output: Inside Parent
     Inside Child

Method Overloading

Two or more methods have the same name but different numbers of parameters or different types of parameters, or both.
The problem with method overloading in Python is that we may overload the methods but can only use the latest defined method.

Example

def product(a, b):
    p = a * b
    print(p)

def product(a, b, c):
    p = a * b * c
    print(p)

product(4, 5, 5)
#output: 100

Encapsulation

It describes the idea of wrapping data and the methods that work on data within one unit.
This puts restrictions on accessing variables and methods directly and can prevent the accidental modification of data.
Protected members(single underscore"_") are those members of the class that cannot be accessed outside the class but can be accessed from within the class and its subclasses.
Private members(double underscore"__") are similar to protected members, the difference is that the class members declared private should neither be accessed outside the class nor by any base class.

Example

class Base:
    def __init__(self):
        self._a= 2
class Derived(Base):
    def __init__(self):
        Base.__init__(self)
        print("Base class: ", self._a)

        self._a = 3
        print("Outside class: ", self._a)
obj1 = Derived()
obj2 = Base()

print("Accessing obj1: ", obj1._a)
print("Accessing obj2: ", obj2._a)
#output: Base class:  2
    Outside class:  3
    Accessing obj1:  3
    Accessing obj2:  2

Data Abstraction

It hides the unnecessary code details from the user.
It can be achieved through creating abstract classes.
data hiding: We use double underscore("__") before the attributes names and those attributes will not be directly visible outside.
Printing objects :Printing objects give us information about objects we are working with.

Example

class MyClass:
     __hiddenVariable = 0
     def add(self, increment):
        self.__hiddenVariable += increment
        print (self.__hiddenVariable)

myObject = MyClass()     
myObject.add(2)
myObject.add(5)

print (myObject.__hiddenVariable)
#output: 2
         7
Traceback (most recent call last):
  File "filename.py", line 13, in 
    print (myObject.__hiddenVariable)
AttributeError: MyClass instance has 
no attribute '__hiddenVariable'

Conclusion

In Python, Object-Oriented programming(OOP's) is a programming paradigm that uses objects and classes in programming. It aims to implement real-world entities like inheritance, polymorphism, encapsulation, etc. in the programming.

References

Reference links for OOP's concepts.

1.(https://www.geeksforgeeks.org/python-oops-concepts/)

2.(https://www.javatpoint.com/python-oops-concepts)

PYTHON : STRING METHODS

CNavya21 — Thu, 16 Feb 2023 04:10:14 +0000

capitalize()

It returns the string in the form of first character is upper case and the rest in lower case.

Syntax

string.capitalize()

Example

txt = "hi, welcome!"
a = txt.capitalize()
print(a)
#output:Hi, welcome!

casefold()

It returns the string where all the characters are lower case.

Syntax

string.casefold()

Example

txt = "This Is My Example"
x = txt.casefold()
print(x)
#output: this is my example

center()

It is a string class function that is used to position a string by padding it with a specified character.

Syntax

string.center(length, character)
#length - length of the required string.
#character - to fill the missing space on each side and default is " "(space).

Example

txt = "banana"
x = txt.center(20, "x")
print(x)
#output: xxxxxxxbananaxxxxxxx

count()

This method returns the number of times a specified value appears in the string.

Syntax

string.count(value, start, end)
#value - value to search for a string
#start - position to start the search and default is 0
#end - position to end the search and default is end of the string

Example

txt = "I love apples, apple are my favorite fruit"
x = txt.count("apple", 10, 24)
print(x)
#output: 1

endswith()

This method method returns True if the string ends with the specified value, otherwise False.

Syntax

string.endswith(value, start, end)

Example

txt = "Hello, welcome to my world."
x = txt.endswith("!")
print(x)
#output:False

expandtabs()

This method sets the tab size to the specified number of whitespaces.

Syntax

string.expandtabs(tabsize)
#tabsize - a number specifying the tabsize and default is 8.

Example

txt = "W\te\tl\tc\to\tm\te"
print(txt.expandtabs(1)) 
#output: W e l c o m e

find()

It finds the first occurrence of the specified value.
It will returns -1 if the value not found.
This method is almost same as index() method, the only difference is that the index() method rasie an exception if the value is not found.

Syntax

string.find(value, start, end)

Example

txt = "Hello, welcome to my world."
x = txt.find("n")
print(x)
#output: -1

format()

This method formats the specified value or values and insert them inside the string placeholder.
the placeholder is defined using curly brackets {}.

Syntax

string.format(value1, value2,....)
# values can required one or more values .

Example

txt = "For only {price:.2f} dollars!"
print(txt.format(price = 49))
#output: For only 49.00 dollars!

index()

This method finds the first occurrence of the specified value.
It raises an exception if the value is not found.

Syntax

string.index(value, start, end)

Example

txt = "Hello, welcome to my world."
x = txt.index("m")
print(x)
#ouput: 12

isalnum()

This method returns true if all the characters are alphanumeric ,means letter(a-z) and numbers (0-9).

Syntax

string.isalnum()

Example

txt = "company123"
x = txt.isalnum()
print(x)
#output: True

isalpha()

It returns true if all the characters are alphabet letters(a-z).

Syntax

string.isalpha()

Example

txt = "company123"
x = txt.isalpha()
print(x)
#output: False

isascii()

It returns True if all the characters are ascii characters(a-z).

Syntax

string.isascii()

Example

txt = "company123"
x = txt.isascii()
print(x)
#output: true

isdecimal()

It returns True if all the characters are decimanls(0-9).

Syntax

string.isdecimal()

Example

a = "\u0030"
print(a.isdecimal())
#ouput: True

swapcase()

It returns a string where all the upper case letters to lower case and vise versa.

Syntax

string.swapcase()

Example

txt = "Hello My Name Is PETER"
x = txt.swapcase()
print(x) 
#output:hELLO mY nAME iS peter

join()

It takes all items in an iterable and joins them into one string.

Syntax

string.join(iterable)

Example

myDict = {"name": "John", "country": "Norway"}
mySeparator = "TEST"
x = mySeparator.join(myDict)
print(x)
#ouput: nameTESTcountry

split()

It splits into a list.

Syntax

string.split(separator, maxsplit)
#separator - use when splitting the string and by default any whitespace is a separator
# maxsplit - how many splits to do and default value is -1, which is "all occurrences"

Example

txt = "apple#banana#cherry#orange"
x = txt.split("#")
print(x)
#output: ['apple', 'banana', 'cherry', 'orange']

zfill()

It adds zeros at the beginning of the string, until it reaches the specified length.

Syntax

string.zfill(len)

Example

txt = "50"
x = txt.zfill(10)
print(x)
#output: 0000000050

upper()

It returns a string where all characters are in upper case.

Syntax

string.upper()

Example

txt = "hello"
a = txt.upper()
print(a)
# output: HELLO

PYTHON : ARRAY METHODS

CNavya21 — Wed, 15 Feb 2023 11:29:37 +0000

Arrays is a collections of one or more items at the same time.
We can access the elements using index values of array.

append()

It is used to add or append the element to existing array at the end of array list.

Syntax

list.append(element)

Example

a = [1, 2, 3]
b = [7, 8, 9]
b.append(2)
print(b)
# output: [7, 8, 9, [1, 2, 3]]

clear()

It is used to clear or remove all the elements in the array list.

Syntax

list.clear()

Example

a = ["Hi", "Welcome", "Bye"]
a.clear()
print(a)
# output: []

copy()

It is used to copy the specific list of data.

Syntax of copy

list.copy()

Example

a = ["ball", "bat", "cat", "dog"]
x = a.copy()
print(x)
# output: ['ball', 'bat', 'cat', 'dog']

count()

It gives the number of elements for specified value in the list.

Syntax

list.count(value)

Example

n = [1,2,3,4,1,3,6,4,8]
m = n.count(3)
print(m)
# output: 2

extend()

It adds the specific list of elements at the end of the current array list.

Syntax

list.extend(iterable)
# iterable - it can be a list, set, tuple etc.

Example

n = ['This is a list']
m = ['List of elements.']
m.extend(n)
print(m)
# output: ['This is a list', 'List of elements.']

index()

It returns the index value of a specific element.
if we have duplicates in the list, the index will the take the first occurs of the element in the list.

Syntax

list.index(elemnt)
#elemnt - it can be string, number, list etc.

Example

n = [1,2,3,4,1,3,6,4,8]
x = n.index(4)
print(x)
# output: 3

insert()

It used to insert the element at specific index value in the array. #### Syntax

list.insert(pos, elemnt)
# pos - at which position of index we want to insert.
# elemnt - it can be string, number, object etc.

Example

a = ['ball', 'bat', 'cat', 'dog']
a.insert(1, 'lion']
print(a)
# output: ['ball', 'lion', 'bat', 'cat', 'dog']

pop()

*It removes the element at a specific value.

Syntax

list.pop(pos)
#pos - at which position of index we want.

Example

a = ['ball', 'bat', 'cat', 'dog']
a.pop(1)
print(a)
# output: ['ball', 'cat', 'dog']

remove()

It removes the first occurrence of the element with specified value. #### Syntax

list.remove(elemnt)
#elemnt - it can be string, number, list etc.

Example

n = [1,2,3,4,1,3,6,4,8]
n.remove(1)
print(n)
# output: [2, 3, 4, 1, 3, 6, 4, 8]

reverse()

It create a new string containing the original data in reverse order.

Syntax

list.reverse()

Example

a = ['ball', 'bat', 'cat', 'dog']
a.reverse()
print(a)
# output: ['dog', 'cat', 'bat', 'ball']

sort()

It will sort the list in ascending order by default.

Syntax

list.sort(reverse = true|false, key=myfun)
#reverse -  reverse=True will sort the list descending. Default is reverse=False
#key - function to specify the sorting criteria.

Example

n = [1,2,3,4,1,3,6,4,8]
n.sort()
print(n)
# output: [1, 1, 2, 3, 3, 4, 4, 6, 8]

Basic Cli commands(sudo, apt, touch, cat, less)

CNavya21 — Thu, 02 Feb 2023 04:41:19 +0000

Sudo

It allow a user with proper permissions to execute a command as another user, such as the superuser.

Flags of sudo

-V : used to check the version.
-l : used to list which we have to print from current host.
-v : sudo will update the user’s timestamp, prompting for the user’s password if necessary.
-h : used to help.
-k : it used to sudo invalidates the user’s timestamp.
-K : it used to remove the user’s timestamp entirely.
-b : it tells sudo to run the given command in the background.
-p : allows you to override the default password prompt and use a custom one.
-u : user can run the specified command as a user other than root.
-s : runs the shell specified by the SHELL environment variable if it is set or the shell as specified in the file password.
- : indicates that sudo should stop processing command line arguments.

Syntax of sudo

sudo [command]

apt (Advanced Packaging Tool)

It perform as installation of new software packages, upgrade of existing software packages, updating of the package list index, and even upgrading the entire Ubuntu system.
Install package

sudo apt install <package_name>

Update package in index

sudo apt update

Remove a package

sudo apt remove <package_name>

Upgrade package

sudo apt upgrade

touch

It is used to create a file, update the modification and access time of each file with the help of touch command.

Syntax of touch

touch <file_name>

Flags in touch

-a : used to change the access time of a file.
-m : help you to change only the modification time of a file.
-r : it update time with reference to the other mentioned command.
-t : we can change the access time of a file by determining a specified time to it.

cat

Create a file bye using '>' symbol.
It can read the data inside the file.

Syntax of cat

create a file

cat > 'file_name'

to read a file

cat 'file_name'

less

It automatically adjust with the width and height of the terminal window.

less 'file_name'