What Is The Execution Context & Call Stack in Javascript?

Javascript: What Is The Execution Context? What Is The Call Stack?

What is the Execution Context in Javascript?

I bet you don’t know the answer.

What are the most basic components of a programming language?

Variables and functions right? Everybody can learn these building blocks.

But what lies beyond the basics?

What are the pillars of Javascript that you should master before calling yourself intermediate (or even senior) Javascript developer?

There are many: Scope, Closure, Callbacks, Prototype, and so on.

But before diving deeper into these concepts you should at least understand how the Javascript engine works.

In this post we’ll walk through two fundamental pieces of every Javascript engine: the Execution Context and the Call Stack.

(Don’t be scared. It’s easier than you think).

Ready?

This material is part of my Advanced Javascript class available both as remote 1 to 1 training or as on site training in Europe.

Javascript: What Is The Execution Context? What You Will Learn

In this post you’ll learn:

  • how the Javascript Engine works
  • the Execution Context in Javascript
  • what is the Call Stack
  • the difference between Global Execution Context and Local Execution Context

Javascript: What Is The Execution Context? How Does Javascript Run Your Code?

How does Javascript run your code?

If you’re a senior developer you may already know the answer.

If you’re a beginner we’ll work through together.

The truth is, Javascript internals aren’t easy.

But I guarantee you can learn them.

And by the time you learn them you’ll feel empowered and smarter.

By looking at Javascript inner functioning you’ll become a better Javascript developer, even if you can’t master every single detail.

Now, take a look at the following code:

var num = 2;

function pow(num) {
return num * num;
}

Done?

It doesn’t look hard!

Now tell me: in what order you think the browser will evaluate that code?

In other words, if YOU were the browser, how would you read that code?

It sounds like an easy one.

Most people think “yeah, the browser executes function pow and returns the result, then it assign 2 to num.”

Do you want to know my student’s answer?

Top to bottom

The browser will start at function pow, calculating num * num

JS engine will run the code line by line (kind of)

I was expecting that.

I said the same exact things  years ago.

In the next sections you’ll discover the machinery behind those apparently simple lines of code.

Javascript: What Is The Execution Context? Javascript Engines

Do you want to be an average Javascript developer?

I bet you won’t.

If you want to make a good impression in a Javascript interview you should at least know how Javascript runs your code.

(And a bunch of other stuff which I won’t cover here).

Don’t rush on these concepts.

You can’t learn everything in a day. It will take time.

The good news? I’m going to make that stuff understandable for everyone (at least I’ll try).

To understand how Javascript runs your code we should meet the first scary thing:

mister Execution Context.

What is the Execution Context in Javascript?

Every time you run Javascript in a browser (or in Node) the engine goes through a series of steps.

One of this steps involves the creation of the Global Execution Context.

Wait Valentino, what is the engine?

That is, the Javascript engine is the “engine” that runs Javascript code.

Nowadays there are two prominent Javascript engines: Google V8 and SpiderMonkey.

V8 is the Google’s open source JavaScript engine, used in Google Chrome and Node.js.

SpiderMonkey is the Mozilla’s JavaScript engine, used in Firefox.

So far we have the Javascript engine and an Execution Context.

Now it’s time to understand how they work together.

Javascript: What Is The Execution Context? How It Works?

The engine creates a Global Execution Context every time you run some Javascript code.

Execution Context is a fancy word for describing the environment in which your Javascript code runs.

It’s hard to visualize these abstract things, I feel you.

For now think of the Global Execution Context as a box:

Let’s look again at our code:

var num = 2;

function pow(num) {
return num * num;
}

How does the engine read that code?

Here is a simplified version:

Engine: Line one. There’s a variable! Cool. Let’s store it in the Global Memory.

Engine: Line three. I see a function declaration. Cool. Let’s store that in the Global Memory too!

Engine: Looks like I’m done.

If I were to ask you again: how does the browser “see” the following code, what would you say?

Yeah, it’s kind of top to bottom but …

As you can see the engine does not run the function pow!

It’s a function declaration, not a function call.

The above code will translate in some values stored in the Global Memory: a function declaration and a variable.

Global Memory?

Valentino, I’m already confused by the Execution Context and now you’re throwing the Global Memory at me?

Yes I am.

Let’s see what the Global Memory is.

Javascript: What Is The Execution Context? The Global Memory

The Javascript engine has a Global Memory too.

The Global Memory contains global variables and function declarations for later use.

If you read “Scope and Closures” by Kyle Simpson you may find that the Global Memory overlaps with the concept of Global Scope.

In fact they are the same thing.

I’m flying 10,000 feet high here, for a good reason.

Those are hard concepts.

But you shouldn’t worry for now.

I want you to understand two important pieces of our puzzle.

When the Javascript engine runs your code it creates:

  • a Global Execution context
  • a Global Memory (also called Global Scope or Global Variable Environment)

Is everything clear?

If I were you at this point I’ll:

  • write down some Javascript code
  • parse the code step by step as you were the engine
  • make a graphic representation of both the Global Execution context and the Global Memory during the execution

You can write the exercise on paper or with a prototyping tool.

For my tiny example the picture will look like so:

In the next section we’ll look at another scary thing: the Call Stack.

Javascript: What Is The Execution Context? What Is The Call Stack?

Do you have a clear picture of how the Execution Context, the Global Memory and the Javascript engine fits together?

If not take your time to review the previous section.

We’re going to introduce another piece in our puzzle: the Call Stack.

Let’s first recap what happens when the Javascript engine runs your code.

It creates:

  • a Global Execution context
  • a Global Memory

Besides that in our example nothing more happened:

var num = 2;

function pow(num) {
return num * num;
}

The code is a pure allocation of values.

Let’s take a step further.

What happens if I call the function?

var num = 2;

function pow(num) {
return num * num;
}

var res = pow(num);

Interesting question.

The act of calling a function in Javascript makes the engine ask for help.

And that help comes from a friend of the Javascript engine: the Call Stack.

It might not sound obvious but the Javascript engine needs to keep track of what’s happening.

It relies on the Call Stack for that.

What is the Call Stack in Javascript?

The Call Stack is like a log of the current execution of the program.

In reality it’s a data structure: a stack.

How does exactly the Call Stack work?

Unsurprisingly it has two methods: push and pop.

Pushing is the act of putting something into the stack.

That is, when you run a function in Javascript the engine pushes that function into the Call Stack.

Every function call gets pushed into the Call Stack.

The first thing that gets pushed is main() (or global()), the main thread of execution of your Javascript program.

Now, the previous picture will look like so:

Popping on the other end is the act of removing something from the stack.

When a function ends executing it gets popped from the Call Stack.

And our Call Stack will look like the following:

And now? You’re ready to master every Javascript concept from there.

I’m not kidding.

But where not done! Go to the next section!

Javascript: What Is The Execution Context? The Local Execution Context

Everything seems clear so far.

Are we missing something?

We know that the Javascript engine creates a Global Execution context and a Global Memory.

Then, when you call a function in your code:

  • the Javascript engine asks for help
  • that help comes from a friend of the Javascript engine: the Call Stack
  • the Call Stack keep tracks of what function is being called in your code

Yet another thing has to happen when you run a function in Javascript.

First, the function appears in the Global Execution context.

Then, another mini-context appears alongside the function:

that little new box is called Local Execution Context.

What??

If you noticed, in the previous picture a new variable appears in the Global memory: var res.

The variables res has a value of undefined at first.

Then as soon as pow appears in the Global Execution Context, the function executes and res takes its return value.

During the execution phase a Local Execution Context gets created for holding up local variables.

What a powerful concept.

Keep that in mind.

Understanding both the Global and the Local Execution Context is the key for mastering Scope and Closures.

Javascript: What Is The Execution Context? What is the Call Stack? Wrapping up

Can you believe what’s behind 4 lines of code?

The Javascript engine creates an Execution Context, a Global Memory, and a Call Stack.

But once you call a function the engine creates a Local Execution Context which has a Local Memory.

By the end of this post you should be able to understand what happens when you run some Javascript code.

Often overlooked, Javascript internals are always seen as misterious things by new developers.

Yet they are the key for mastering advanced Javascript concepts.

If you learn Execution Context, Global Memory, and the Call Stack, then Scope, Closures, Callbacks and other stuff will be easy as a breeze.

In particular, understanding the Call Stack is paramount.

All the Javascript will start to make sense once you visualize it: you will finally understand why Javascript is asynchronous and why we do need Callbacks.

Link: https://web.archive.org/web/20180701233338/https://www.valentinog.com/blog/js-execution-context-call-stack/

#javascript 

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Buddha Community

 What Is The Execution Context & Call Stack in Javascript?
Grace  Lesch

Grace Lesch

1639778400

PySQL Tutorial: A Database Framework for Python

PySQL 

PySQL is database framework for Python (v3.x) Language, Which is based on Python module mysql.connector, this module can help you to make your code more short and more easier. Before using this framework you must have knowledge about list, tuple, set, dictionary because all codes are designed using it. It's totally free and open source.

Tutorial Video in English (Watch Now)

IMAGE ALT TEXT HERE

Installation

Before we said that this framework is based on mysql.connector so you have to install mysql.connector first on your system. Then you can import pysql and enjoy coding!

python -m pip install mysql-connector-python

After Install mysql.connector successfully create Python file download/install pysql on the same dir where you want to create program. You can clone is using git or npm command, and you can also downlaod manually from repository site.

PyPi Command

Go to https://pypi.org/project/pysql-framework/ or use command

pip install pysql-framework

Git Command

git clone https://github.com/rohit-chouhan/pysql

Npm Command

Go to https://www.npmjs.com/package/pysql or use command

$ npm i pysql

Snippet Extention for VS Code

Install From Here https://marketplace.visualstudio.com/items?itemName=rohit-chouhan.pysql

IMAGE ALT TEXT HERE

Table of contents

Connecting a Server


To connect a database with localhost server or phpmyadmin, use connect method to establish your python with database server.

import pysql

db = pysql.connect(
    "host",
    "username",
    "password"
 )

Create a Database in Server


Creating database in server, to use this method

import pysql

db = pysql.connect(
    "host",
    "username",
    "password"
 )
 pysql.createDb(db,"demo")
 #execute: CREATE DATABASE demo

Drop Database


To drop database use this method .

Syntex Code -

pysql.dropDb([connect_obj,"table_name"])

Example Code -

pysql.dropDb([db,"demo"])
#execute:DROP DATABASE demo

Connecting a Database


To connect a database with localhost server or phpmyadmin, use connect method to establish your python with database server.

import pysql

db = pysql.connect(
    "host",
    "username",
    "password",
    "database"
 )

Creating Table in Database


To create table in database use this method to pass column name as key and data type as value.

Syntex Code -


pysql.createTable([db,"table_name_to_create"],{
    "column_name":"data_type", 
    "column_name":"data_type"
})

Example Code -


pysql.createTable([db,"details"],{
    "id":"int(11) primary", 
     "name":"text", 
    "email":"varchar(50)",
    "address":"varchar(500)"
})

2nd Example Code -

Use can use any Constraint with Data Value


pysql.createTable([db,"details"],{
    "id":"int NOT NULL PRIMARY KEY", 
     "name":"varchar(20) NOT NULL", 
    "email":"varchar(50)",
    "address":"varchar(500)"
})

Drop Table in Database


To drop table in database use this method .

Syntex Code -

pysql.dropTable([connect_obj,"table_name"])

Example Code -

pysql.dropTable([db,"users"])
#execute:DROP TABLE users

Selecting data from Table


For Select data from table, you have to mention the connector object with table name. pass column names in set.

Syntex For All Data (*)-

records = pysql.selectAll([db,"table_name"])
for x in records:
  print(x)

Example - -

records = pysql.selectAll([db,"details"])
for x in records:
  print(x)
#execute: SELECT * FROM details

Syntex For Specific Column-

records = pysql.select([db,"table_name"],{"column","column"})
for x in records:
  print(x)

Example - -

records = pysql.select([db,"details"],{"name","email"})
for x in records:
  print(x)
#execute: SELECT name, email FROM details

Syntex Where and Where Not-

#For Where Column=Data
records = pysql.selectWhere([db,"table_name"],{"column","column"},("column","data"))

#For Where Not Column=Data (use ! with column)
records = pysql.selectWhere([db,"table_name"],{"column","column"},("column!","data"))
for x in records:
  print(x)

Example - -

records = pysql.selectWhere([db,"details"],{"name","email"},("county","india"))
for x in records:
  print(x)
#execute: SELECT name, email FROM details WHERE country='india'

Add New Column to Table


To add column in table, use this method to pass column name as key and data type as value. Note: you can only add one column only one call

Syntex Code -


pysql.addColumn([db,"table_name"],{
    "column_name":"data_type"
})

Example Code -


pysql.addColumn([db,"details"],{
    "email":"varchar(50)"
})
#execute: ALTER TABLE details ADD email varchar(50);

Modify Column to Table


To modify data type of column table, use this method to pass column name as key and data type as value.

Syntex Code -

pysql.modifyColumn([db,"table_name"],{
    "column_name":"new_data_type"
})

Example Code -

pysql.modifyColumn([db,"details"],{
    "email":"text"
})
#execute: ALTER TABLE details MODIFY COLUMN email text;

Drop Column from Table


Note: you can only add one column only one call

Syntex Code -

pysql.dropColumn([db,"table_name"],"column_name")

Example Code -

pysql.dropColumn([db,"details"],"name")
#execute: ALTER TABLE details DROP COLUMN name

Manual Execute Query


To execute manual SQL Query to use this method.

Syntex Code -

pysql.query(connector_object,your_query)

Example Code -

pysql.query(db,"INSERT INTO users (name) VALUES ('Rohit')")

Inserting data


For Inserting data in database, you have to mention the connector object with table name, and data as sets.

Syntex -

data =     {
    "db_column":"Data for Insert",
    "db_column":"Data for Insert"
}
pysql.insert([db,"table_name"],data)

Example Code -

data =     {
    "name":"Komal Sharma",
    "contry":"India"
}
pysql.insert([db,"users"],data)

Updating data


For Update data in database, you have to mention the connector object with table name, and data as tuple.

Syntex For Updating All Data-

data = ("column","data to update")
pysql.updateAll([db,"users"],data)

Example - -

data = ("name","Rohit")
pysql.updateAll([db,"users"],data)
#execute: UPDATE users SET name='Rohit'

Syntex For Updating Data (Where and Where Not)-

data = ("column","data to update")
#For Where Column=Data
where = ("column","data")

#For Where Not Column=Data (use ! with column)
where = ("column!","data")
pysql.update([db,"users"],data,where)

Example -

data = ("name","Rohit")
where = ("id",1)
pysql.update([db,"users"],data,where)
#execute: UPDATE users SET name='Rohit' WHERE id=1

Deleting data


For Delete data in database, you have to mention the connector object with table name.

Syntex For Delete All Data-

pysql.deleteAll([db,"table_name"])

Example - -

pysql.deleteAll([db,"users"])
#execute: DELETE FROM users

Syntex For Deleting Data (Where and Where Not)-

where = ("column","data")

pysql.delete([db,"table_name"],where)

Example -

#For Where Column=Data
where = ("id",1)

#For Where Not Column=Data (use ! with column)
where = ("id!",1)
pysql.delete([db,"users"],where)
#execute: DELETE FROM users WHERE id=1

--- Finish ---

Change Logs

[19/06/2021]
 - ConnectSever() removed and merged to Connect()
 - deleteAll() [Fixed]
 - dropTable() [Added]
 - dropDb() [Added]
 
[20/06/2021]
 - Where Not Docs [Added]

The module is designed by Rohit Chouhan, contact us for any bug report, feature or business inquiry.

Author: rohit-chouhan
Source Code: https://github.com/rohit-chouhan/pysql
License: Apache-2.0 License

#python 

坂本  篤司

坂本 篤司

1652450700

Pythonグローバル変数–グローバル変数の例を定義する方法

この記事では、グローバル変数の基本を学びます。

まず、Pythonで変数を宣言する方法と、「変数スコープ」という用語が実際に何を意味するかを学習します。

次に、ローカル変数とグローバル変数の違いを学び、グローバル変数の定義方法とglobalキーワードの使用方法を理解します。

Pythonの変数とは何ですか?どのように作成しますか?初心者のための紹介

変数はストレージコンテナと考えることができます。

これらは、コンピュータのメモリに保存したいデータ、情報、および値を保持するためのストレージコンテナです。その後、プログラムの存続期間中のある時点でそれらを参照したり、操作したりすることもできます。

変数にはシンボリックがあり、その名前は、その識別子として機能するストレージコンテナのラベルと考えることができます。

変数名は、その中に格納されているデータへの参照とポインターになります。したがって、データと情報の詳細を覚えておく必要はありません。そのデータと情報を保持する変数名を参照するだけで済みます。

変数に名前を付けるときは、変数が保持するデータを説明していることを確認してください。変数名は、将来の自分自身と一緒に作業する可能性のある他の開発者の両方にとって、明確で簡単に理解できる必要があります。

それでは、Pythonで実際に変数を作成する方法を見てみましょう。

Pythonで変数を宣言するときは、データ型を指定する必要はありません。

たとえば、Cプログラミング言語では、変数が保持するデータの型を明示的に指定する必要があります。

したがって、整数またはint型である年齢を格納したい場合、これはCで行う必要があることです。

#include <stdio.h>
 
int main(void)
{
  int age = 28;
  // 'int' is the data type
  // 'age' is the name 
  // 'age' is capable of holding integer values
  // positive/negative whole numbers or 0
  // '=' is the assignment operator
  // '28' is the value
}

ただし、これはPythonで上記を記述する方法です。

age = 28

#'age' is the variable name, or identifier
# '=' is the assignment operator
#'28' is the value assigned to the variable, so '28' is the value of 'age'

変数名は常に左側にあり、代入する値は代入演算子の後に右側に配置されます。

プログラムの存続期間中、変数の値を変更できることに注意してください。

my_age = 28

print(f"My age in 2022 is {my_age}.")

my_age = 29

print(f"My age in 2023 will be {my_age}.")

#output

#My age in 2022 is 28.
#My age in 2023 will be 29.

同じ変数名を保持しますが、値をからにmy_age変更するだけです。2829

Pythonの可変スコープとはどういう意味ですか?

変数スコープとは、変数が利用可能で、アクセス可能で、表示可能なPythonプログラムの部分と境界を指します。

Python変数のスコープには4つのタイプがあり、 LEGBルールとも呼ばれます。

  • 局所
  • 囲み
  • グローバル
  • ビルトイン

この記事の残りの部分では、グローバルスコープを使用した変数の作成について学習することに焦点を当て、ローカル変数スコープとグローバル変数スコープの違いを理解します。

Pythonでローカルスコープを使用して変数を作成する方法

関数の本体内で定義された変数にはローカルスコープがあります。つまり、その特定の関数内でのみアクセスできます。言い換えれば、それらはその関数に対して「ローカル」です。

ローカル変数にアクセスするには、関数を呼び出す必要があります。

def learn_to_code():
    #create local variable
    coding_website = "freeCodeCamp"
    print(f"The best place to learn to code is with {coding_website}!")

#call function
learn_to_code()


#output

#The best place to learn to code is with freeCodeCamp!

関数の本体の外部からローカルスコープを使用してその変数にアクセスしようとするとどうなるかを見てください。

def learn_to_code():
    #create local variable
    coding_website = "freeCodeCamp"
    print(f"The best place to learn to code is with {coding_website}!")

#try to print local variable 'coding_website' from outside the function
print(coding_website)

#output

#NameError: name 'coding_website' is not defined

NameErrorプログラムの残りの部分では「表示」されないため、aが発生します。定義された関数内でのみ「表示」されます。

Pythonでグローバルスコープを使用して変数を作成する方法

ファイルの先頭など、関数の外部で変数を定義すると、その変数はグローバルスコープを持ち、グローバル変数と呼ばれます。

グローバル変数は、プログラムのどこからでもアクセスできます。

関数の本体内で使用することも、関数の外部からアクセスすることもできます。

#create a global variable
coding_website = "freeCodeCamp"

def learn_to_code():
    #access the variable 'coding_website' inside the function
    print(f"The best place to learn to code is with {coding_website}!")

#call the function
learn_to_code()

#access the variable 'coding_website' from outside the function
print(coding_website)

#output

#The best place to learn to code is with freeCodeCamp!
#freeCodeCamp

グローバル変数とローカル変数があり、両方が同じ名前の場合はどうなりますか?

#global variable
city = "Athens"

def travel_plans():
    #local variable with the same name as the global variable
    city = "London"
    print(f"I want to visit {city} next year!")

#call function - this will output the value of local variable
travel_plans()

#reference global variable - this will output the value of global variable
print(f"I want to visit {city} next year!")

#output

#I want to visit London next year!
#I want to visit Athens next year!

上記の例では、その特定の出力を期待していなかった可能性があります。

city関数内で別の値を割り当てたときに、の値が変わると思ったかもしれません。

たぶん、私が行でグローバル変数を参照したときprint(f" I want to visit {city} next year!")、出力は#I want to visit London next year!の代わりになると予想しました#I want to visit Athens next year!

ただし、関数が呼び出されると、ローカル変数の値が出力されます。

次に、関数の外部でグローバル変数を参照すると、グローバル変数に割り当てられた値が出力されました。

彼らはお互いに干渉しませんでした。

ただし、グローバル変数とローカル変数に同じ変数名を使用することは、ベストプラクティスとは見なされません。プログラムを実行すると混乱する結果が生じる可能性があるため、変数の名前が同じでないことを確認してください。

Pythonでキーワードを使用する方法global

グローバル変数があり、関数内でその値を変更したい場合はどうなりますか?

私がそれをしようとすると何が起こるか見てください:

#global variable
city = "Athens"

def travel_plans():
    #First, this is like when I tried to access the global variable defined outside the function. 
    # This works fine on its own, as you saw earlier on.
    print(f"I want to visit {city} next year!")

    #However, when I then try to re-assign a different value to the global variable 'city' from inside the function,
    #after trying to print it,
    #it will throw an error
    city = "London"
    print(f"I want to visit {city} next year!")

#call function
travel_plans()

#output

#UnboundLocalError: local variable 'city' referenced before assignment

デフォルトでは、Pythonは関数内でローカル変数を使用したいと考えています。

そのため、最初に変数の値を出力してから、アクセスしようとしている変数に値再割り当てしようとすると、Pythonが混乱します。

関数内のグローバル変数の値を変更する方法は、次のglobalキーワードを使用することです。

#global variable
city = "Athens"

#print value of global variable
print(f"I want to visit {city} next year!")

def travel_plans():
    global city
    #print initial value of global variable
    print(f"I want to visit {city} next year!")
    #assign a different value to global variable from within function
    city = "London"
    #print new value
    print(f"I want to visit {city} next year!")

#call function
travel_plans()

#print value of global variable
print(f"I want to visit {city} next year!")

global次のエラーが発生するため、関数でキーワードを参照する前にキーワードを使用してくださいSyntaxError: name 'city' is used prior to global declaration

以前、関数内で作成された変数はローカルスコープを持っているため、それらにアクセスできないことを確認しました。

globalキーワードは、関数内で宣言された変数の可視性を変更します。

def learn_to_code():
   global coding_website
   coding_website = "freeCodeCamp"
   print(f"The best place to learn to code is with {coding_website}!")

#call function
learn_to_code()

#access variable from within the function
print(coding_website)

#output

#The best place to learn to code is with freeCodeCamp!
#freeCodeCamp

結論

そして、あなたはそれを持っています!これで、Pythonのグローバル変数の基本を理解し、ローカル変数とグローバル変数の違いを理解できます。

この記事がお役に立てば幸いです。

基本から始めて、インタラクティブで初心者に優しい方法で学びます。また、最後に5つのプロジェクトを構築して実践し、学んだことを強化するのに役立てます。

読んでくれてありがとう、そして幸せなコーディング!

ソース:https ://www.freecodecamp.org/news/python-global-variables-examples/

#python 

Variables Globales De Python: Cómo Definir Un Ejemplo De Variable Glob

En este artículo, aprenderá los conceptos básicos de las variables globales.

Para empezar, aprenderá cómo declarar variables en Python y qué significa realmente el término 'ámbito de variable'.

Luego, aprenderá las diferencias entre variables locales y globales y comprenderá cómo definir variables globales y cómo usar la globalpalabra clave.

¿Qué son las variables en Python y cómo se crean? Una introducción para principiantes

Puede pensar en las variables como contenedores de almacenamiento .

Son contenedores de almacenamiento para almacenar datos, información y valores que le gustaría guardar en la memoria de la computadora. Luego puede hacer referencia a ellos o incluso manipularlos en algún momento a lo largo de la vida del programa.

Una variable tiene un nombre simbólico y puede pensar en ese nombre como la etiqueta en el contenedor de almacenamiento que actúa como su identificador.

El nombre de la variable será una referencia y un puntero a los datos almacenados en su interior. Por lo tanto, no es necesario recordar los detalles de sus datos e información; solo necesita hacer referencia al nombre de la variable que contiene esos datos e información.

Al dar un nombre a una variable, asegúrese de que sea descriptivo de los datos que contiene. Los nombres de las variables deben ser claros y fácilmente comprensibles tanto para usted en el futuro como para los otros desarrolladores con los que puede estar trabajando.

Ahora, veamos cómo crear una variable en Python.

Al declarar variables en Python, no necesita especificar su tipo de datos.

Por ejemplo, en el lenguaje de programación C, debe mencionar explícitamente el tipo de datos que contendrá la variable.

Entonces, si quisiera almacenar su edad, que es un número entero, o inttipo, esto es lo que tendría que hacer en C:

#include <stdio.h>
 
int main(void)
{
  int age = 28;
  // 'int' is the data type
  // 'age' is the name 
  // 'age' is capable of holding integer values
  // positive/negative whole numbers or 0
  // '=' is the assignment operator
  // '28' is the value
}

Sin embargo, así es como escribirías lo anterior en Python:

age = 28

#'age' is the variable name, or identifier
# '=' is the assignment operator
#'28' is the value assigned to the variable, so '28' is the value of 'age'

El nombre de la variable siempre está en el lado izquierdo y el valor que desea asignar va en el lado derecho después del operador de asignación.

Tenga en cuenta que puede cambiar los valores de las variables a lo largo de la vida de un programa:

my_age = 28

print(f"My age in 2022 is {my_age}.")

my_age = 29

print(f"My age in 2023 will be {my_age}.")

#output

#My age in 2022 is 28.
#My age in 2023 will be 29.

Mantienes el mismo nombre de variable my_age, pero solo cambias el valor de 28a 29.

¿Qué significa el alcance variable en Python?

El alcance de la variable se refiere a las partes y los límites de un programa de Python donde una variable está disponible, accesible y visible.

Hay cuatro tipos de alcance para las variables de Python, que también se conocen como la regla LEGB :

  • local ,
  • Encerrando ,
  • globales ,
  • Incorporado .

En el resto de este artículo, se centrará en aprender a crear variables con alcance global y comprenderá la diferencia entre los alcances de variables locales y globales.

Cómo crear variables con alcance local en Python

Las variables definidas dentro del cuerpo de una función tienen alcance local , lo que significa que solo se puede acceder a ellas dentro de esa función en particular. En otras palabras, son 'locales' para esa función.

Solo puede acceder a una variable local llamando a la función.

def learn_to_code():
    #create local variable
    coding_website = "freeCodeCamp"
    print(f"The best place to learn to code is with {coding_website}!")

#call function
learn_to_code()


#output

#The best place to learn to code is with freeCodeCamp!

Mire lo que sucede cuando trato de acceder a esa variable con un alcance local desde fuera del cuerpo de la función:

def learn_to_code():
    #create local variable
    coding_website = "freeCodeCamp"
    print(f"The best place to learn to code is with {coding_website}!")

#try to print local variable 'coding_website' from outside the function
print(coding_website)

#output

#NameError: name 'coding_website' is not defined

Plantea un NameErrorporque no es 'visible' en el resto del programa. Solo es 'visible' dentro de la función donde se definió.

Cómo crear variables con alcance global en Python

Cuando define una variable fuera de una función, como en la parte superior del archivo, tiene un alcance global y se conoce como variable global.

Se accede a una variable global desde cualquier parte del programa.

Puede usarlo dentro del cuerpo de una función, así como acceder desde fuera de una función:

#create a global variable
coding_website = "freeCodeCamp"

def learn_to_code():
    #access the variable 'coding_website' inside the function
    print(f"The best place to learn to code is with {coding_website}!")

#call the function
learn_to_code()

#access the variable 'coding_website' from outside the function
print(coding_website)

#output

#The best place to learn to code is with freeCodeCamp!
#freeCodeCamp

¿Qué sucede cuando hay una variable global y local, y ambas tienen el mismo nombre?

#global variable
city = "Athens"

def travel_plans():
    #local variable with the same name as the global variable
    city = "London"
    print(f"I want to visit {city} next year!")

#call function - this will output the value of local variable
travel_plans()

#reference global variable - this will output the value of global variable
print(f"I want to visit {city} next year!")

#output

#I want to visit London next year!
#I want to visit Athens next year!

En el ejemplo anterior, tal vez no esperaba ese resultado específico.

Tal vez pensaste que el valor de citycambiaría cuando le asignara un valor diferente dentro de la función.

Tal vez esperabas que cuando hice referencia a la variable global con la línea print(f" I want to visit {city} next year!"), la salida sería en #I want to visit London next year!lugar de #I want to visit Athens next year!.

Sin embargo, cuando se llamó a la función, imprimió el valor de la variable local.

Luego, cuando hice referencia a la variable global fuera de la función, se imprimió el valor asignado a la variable global.

No interfirieron entre sí.

Dicho esto, usar el mismo nombre de variable para variables globales y locales no se considera una buena práctica. Asegúrese de que sus variables no tengan el mismo nombre, ya que puede obtener algunos resultados confusos cuando ejecute su programa.

Cómo usar la globalpalabra clave en Python

¿Qué sucede si tiene una variable global pero desea cambiar su valor dentro de una función?

Mira lo que sucede cuando trato de hacer eso:

#global variable
city = "Athens"

def travel_plans():
    #First, this is like when I tried to access the global variable defined outside the function. 
    # This works fine on its own, as you saw earlier on.
    print(f"I want to visit {city} next year!")

    #However, when I then try to re-assign a different value to the global variable 'city' from inside the function,
    #after trying to print it,
    #it will throw an error
    city = "London"
    print(f"I want to visit {city} next year!")

#call function
travel_plans()

#output

#UnboundLocalError: local variable 'city' referenced before assignment

Por defecto, Python piensa que quieres usar una variable local dentro de una función.

Entonces, cuando intento imprimir el valor de la variable por primera vez y luego reasignar un valor a la variable a la que intento acceder, Python se confunde.

La forma de cambiar el valor de una variable global dentro de una función es usando la globalpalabra clave:

#global variable
city = "Athens"

#print value of global variable
print(f"I want to visit {city} next year!")

def travel_plans():
    global city
    #print initial value of global variable
    print(f"I want to visit {city} next year!")
    #assign a different value to global variable from within function
    city = "London"
    #print new value
    print(f"I want to visit {city} next year!")

#call function
travel_plans()

#print value of global variable
print(f"I want to visit {city} next year!")

Utilice la globalpalabra clave antes de hacer referencia a ella en la función, ya que obtendrá el siguiente error: SyntaxError: name 'city' is used prior to global declaration.

Anteriormente, vio que no podía acceder a las variables creadas dentro de las funciones ya que tienen un alcance local.

La globalpalabra clave cambia la visibilidad de las variables declaradas dentro de las funciones.

def learn_to_code():
   global coding_website
   coding_website = "freeCodeCamp"
   print(f"The best place to learn to code is with {coding_website}!")

#call function
learn_to_code()

#access variable from within the function
print(coding_website)

#output

#The best place to learn to code is with freeCodeCamp!
#freeCodeCamp

Conclusión

¡Y ahí lo tienes! Ahora conoce los conceptos básicos de las variables globales en Python y puede distinguir las diferencias entre las variables locales y globales.

Espero que hayas encontrado útil este artículo.

Comenzará desde lo básico y aprenderá de una manera interactiva y amigable para principiantes. También construirá cinco proyectos al final para poner en práctica y ayudar a reforzar lo que ha aprendido.

¡Gracias por leer y feliz codificación!

Fuente: https://www.freecodecamp.org/news/python-global-variables-examples/

#python 

Python Global Variables – How to Define a Global Variable Example

In this article, you will learn the basics of global variables.

To begin with, you will learn how to declare variables in Python and what the term 'variable scope' actually means.

Then, you will learn the differences between local and global variables and understand how to define global variables and how to use the global keyword.

What Are Variables in Python and How Do You Create Them? An Introduction for Beginners

You can think of variables as storage containers.

They are storage containers for holding data, information, and values that you would like to save in the computer's memory. You can then reference or even manipulate them at some point throughout the life of the program.

A variable has a symbolic name, and you can think of that name as the label on the storage container that acts as its identifier.

The variable name will be a reference and pointer to the data stored inside it. So, there is no need to remember the details of your data and information – you only need to reference the variable name that holds that data and information.

When giving a variable a name, make sure that it is descriptive of the data it holds. Variable names need to be clear and easily understandable both for your future self and the other developers you may be working with.

Now, let's see how to actually create a variable in Python.

When declaring variables in Python, you don't need to specify their data type.

For example, in the C programming language, you have to mention explicitly the type of data the variable will hold.

So, if you wanted to store your age which is an integer, or int type, this is what you would have to do in C:

#include <stdio.h>
 
int main(void)
{
  int age = 28;
  // 'int' is the data type
  // 'age' is the name 
  // 'age' is capable of holding integer values
  // positive/negative whole numbers or 0
  // '=' is the assignment operator
  // '28' is the value
}

However, this is how you would write the above in Python:

age = 28

#'age' is the variable name, or identifier
# '=' is the assignment operator
#'28' is the value assigned to the variable, so '28' is the value of 'age'

The variable name is always on the left-hand side, and the value you want to assign goes on the right-hand side after the assignment operator.

Keep in mind that you can change the values of variables throughout the life of a program:

my_age = 28

print(f"My age in 2022 is {my_age}.")

my_age = 29

print(f"My age in 2023 will be {my_age}.")

#output

#My age in 2022 is 28.
#My age in 2023 will be 29.

You keep the same variable name, my_age, but only change the value from 28 to 29.

What Does Variable Scope in Python Mean?

Variable scope refers to the parts and boundaries of a Python program where a variable is available, accessible, and visible.

There are four types of scope for Python variables, which are also known as the LEGB rule:

  • Local,
  • Enclosing,
  • Global,
  • Built-in.

For the rest of this article, you will focus on learning about creating variables with global scope, and you will understand the difference between the local and global variable scopes.

How to Create Variables With Local Scope in Python

Variables defined inside a function's body have local scope, which means they are accessible only within that particular function. In other words, they are 'local' to that function.

You can only access a local variable by calling the function.

def learn_to_code():
    #create local variable
    coding_website = "freeCodeCamp"
    print(f"The best place to learn to code is with {coding_website}!")

#call function
learn_to_code()


#output

#The best place to learn to code is with freeCodeCamp!

Look at what happens when I try to access that variable with a local scope from outside the function's body:

def learn_to_code():
    #create local variable
    coding_website = "freeCodeCamp"
    print(f"The best place to learn to code is with {coding_website}!")

#try to print local variable 'coding_website' from outside the function
print(coding_website)

#output

#NameError: name 'coding_website' is not defined

It raises a NameError because it is not 'visible' in the rest of the program. It is only 'visible' within the function where it was defined.

How to Create Variables With Global Scope in Python

When you define a variable outside a function, like at the top of the file, it has a global scope and it is known as a global variable.

A global variable is accessed from anywhere in the program.

You can use it inside a function's body, as well as access it from outside a function:

#create a global variable
coding_website = "freeCodeCamp"

def learn_to_code():
    #access the variable 'coding_website' inside the function
    print(f"The best place to learn to code is with {coding_website}!")

#call the function
learn_to_code()

#access the variable 'coding_website' from outside the function
print(coding_website)

#output

#The best place to learn to code is with freeCodeCamp!
#freeCodeCamp

What happens when there is a global and local variable, and they both have the same name?

#global variable
city = "Athens"

def travel_plans():
    #local variable with the same name as the global variable
    city = "London"
    print(f"I want to visit {city} next year!")

#call function - this will output the value of local variable
travel_plans()

#reference global variable - this will output the value of global variable
print(f"I want to visit {city} next year!")

#output

#I want to visit London next year!
#I want to visit Athens next year!

In the example above, maybe you were not expecting that specific output.

Maybe you thought that the value of city would change when I assigned it a different value inside the function.

Maybe you expected that when I referenced the global variable with the line print(f" I want to visit {city} next year!"), the output would be #I want to visit London next year! instead of #I want to visit Athens next year!.

However, when the function was called, it printed the value of the local variable.

Then, when I referenced the global variable outside the function, the value assigned to the global variable was printed.

They didn't interfere with one another.

That said, using the same variable name for global and local variables is not considered a best practice. Make sure that your variables don't have the same name, as you may get some confusing results when you run your program.

How to Use the global Keyword in Python

What if you have a global variable but want to change its value inside a function?

Look at what happens when I try to do that:

#global variable
city = "Athens"

def travel_plans():
    #First, this is like when I tried to access the global variable defined outside the function. 
    # This works fine on its own, as you saw earlier on.
    print(f"I want to visit {city} next year!")

    #However, when I then try to re-assign a different value to the global variable 'city' from inside the function,
    #after trying to print it,
    #it will throw an error
    city = "London"
    print(f"I want to visit {city} next year!")

#call function
travel_plans()

#output

#UnboundLocalError: local variable 'city' referenced before assignment

By default Python thinks you want to use a local variable inside a function.

So, when I first try to print the value of the variable and then re-assign a value to the variable I am trying to access, Python gets confused.

The way to change the value of a global variable inside a function is by using the global keyword:

#global variable
city = "Athens"

#print value of global variable
print(f"I want to visit {city} next year!")

def travel_plans():
    global city
    #print initial value of global variable
    print(f"I want to visit {city} next year!")
    #assign a different value to global variable from within function
    city = "London"
    #print new value
    print(f"I want to visit {city} next year!")

#call function
travel_plans()

#print value of global variable
print(f"I want to visit {city} next year!")

Use the global keyword before referencing it in the function, as you will get the following error: SyntaxError: name 'city' is used prior to global declaration.

Earlier, you saw that you couldn't access variables created inside functions since they have local scope.

The global keyword changes the visibility of variables declared inside functions.

def learn_to_code():
   global coding_website
   coding_website = "freeCodeCamp"
   print(f"The best place to learn to code is with {coding_website}!")

#call function
learn_to_code()

#access variable from within the function
print(coding_website)

#output

#The best place to learn to code is with freeCodeCamp!
#freeCodeCamp

Conclusion

And there you have it! You now know the basics of global variables in Python and can tell the differences between local and global variables.

I hope you found this article useful.

You'll start from the basics and learn in an interactive and beginner-friendly way. You'll also build five projects at the end to put into practice and help reinforce what you've learned.

Thanks for reading and happy coding!

Source: https://www.freecodecamp.org/news/python-global-variables-examples/

#python 

Hertha  Mayer

Hertha Mayer

1595334123

Authentication In MEAN Stack - A Quick Guide

I consider myself an active StackOverflow user, despite my activity tends to vary depending on my daily workload. I enjoy answering questions with angular tag and I always try to create some working example to prove correctness of my answers.

To create angular demo I usually use either plunker or stackblitz or even jsfiddle. I like all of them but when I run into some errors I want to have a little bit more usable tool to undestand what’s going on.

Many people who ask questions on stackoverflow don’t want to isolate the problem and prepare minimal reproduction so they usually post all code to their questions on SO. They also tend to be not accurate and make a lot of mistakes in template syntax. To not waste a lot of time investigating where the error comes from I tried to create a tool that will help me to quickly find what causes the problem.

Angular demo runner
Online angular editor for building demo.
ng-run.com
<>

Let me show what I mean…

Template parser errors#

There are template parser errors that can be easy catched by stackblitz

It gives me some information but I want the error to be highlighted

#mean stack #angular 6 passport authentication #authentication in mean stack #full stack authentication #mean stack example application #mean stack login and registration angular 8 #mean stack login and registration angular 9 #mean stack tutorial #mean stack tutorial 2019 #passport.js