creating first flask app

 create application folder

by console:

mkdir my-first-flask-app

initializate environment

inside the folder ...by console:

virtualenv env

set terminal to listen our environment

source env/bin/activate

then, the word «(env)» will appear at the beginning of the prompt

install flask with his dependencies

pip3 install Flask

at the end of the installing we can list the dependencies installed by:

pip3 freeze

it will show something like this:

click==8.0.1
dataclasses==0.8
Flask==2.0.1
importlib-metadata==4.6.3
itsdangerous==2.0.1
Jinja2==3.0.1
MarkupSafe==2.0.1
typing-extensions==3.10.0.0
Werkzeug==2.0.1
zipp==3.5.0

creating our fist Flask app...

create a file called run.py and add this content:

from flask import Flask
app = Flask(__name__)
@app.route('/')
def hello_world():
    return 'Hello, World!'

every Flask application extends from class Flask so we must create 

a instance, in this case, called app with a parameter __name__ which is the name of the module.

route  decorator tell us what function call by url

hello_world() for "/" in this example

the function must return what browser will show

"Hello, World" 

Testing our application

we can use internal server to test our application, but before we must tell it which file it must call at the beginnig setting the environment variable FLASK_APP,

open and edit env/bin/activate file.

add at the end:

export FLASK_APP="run.py"

and save it.

To save this changes, we must going out from our environment

deactivate

and then, activate it again:

source env/bin/activate

now, we can start the internal server, like this:

flask run

 * Serving Flask app 'run.py' (lazy loading)
 * Environment: production
   WARNING: This is a development server. Do not use it in a production deployment.
   Use a production WSGI server instead.
 * Debug mode: off
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
127.0.0.1 - - [05/Aug/2021 02:01:35] "GET / HTTP/1.1" 200 -
127.0.0.1 - - [05/Aug/2021 02:01:35] "GET /favicon.ico HTTP/1.1" 404 -

open a browser and go to

http://127.0.0.1:5000

the browser will display

Hello, World!

by default, internal flask server listen to 5000 port

if we want to change it, 

there are 2 ways:

- setting variable FLASK_RUN_PORT

or

writing the port at the end of the executing command, just like this:

flask run --port 6000