What are Flask Blueprints, exactly?

A blueprint is a template for generating a "section" of a web application. You can think of it as a mold:

You can take the blueprint and apply it to your application in several places. Each time you apply it the blueprint will create a new version of its structure in the plaster of your application.

# An examplefrom flask import Blueprinttree_mold = Blueprint("mold", __name__)@tree_mold.route("/leaves")def leaves():    return "This tree has leaves"@tree_mold.route("/roots")def roots():    return "And roots as well"@tree_mold.route("/rings")@tree_mold.route("/rings/<int:year>")def rings(year=None):    return "Looking at the rings for {year}".format(year=year)

This is a simple mold for working with trees - it says that any application that deals with trees should provide access to its leaves, its roots, and its rings (by year). By itself, it is a hollow shell - it cannot route, it cannot respond, until it is impressed upon an application:

from tree_workshop import tree_moldapp.register_blueprint(tree_mold, url_prefix="/oak")app.register_blueprint(tree_mold, url_prefix="/fir")app.register_blueprint(tree_mold, url_prefix="/ash")

Once it is created it may be "impressed" on the application by using the register_blueprint function - this "impresses" the mold of the blueprint on the application at the locations specified by url_prefix.

As pointed out in a comment by @Devasish, this article provides a good answer:

http://exploreflask.com/en/latest/blueprints.html

Quoting from the article:

An example of this would be Facebook. If Facebook used Flask, it might have blueprints for the static pages (i.e. signed-out home, register, about, etc.), the dashboard (i.e. the news feed), profiles (/robert/about and /robert/photos), settings (/settings/security and /settings/privacy) and many more. These components all share a general layout and styles, but each has its own layout as well

This is a very good interpretation, especially the part "if Facebook used Flask". It gives us a concrete situation to visualize how Blueprint actually works.

I too just stumbled up this myself and was confused after reading a few of the documentation sources. At first I thought it was like C#/Java Implementation style where you define some stuff but dont have to worry about it implementation dettails til later. However, I stumbled up this page which puts it in very very laymens (and quite hilarious present-day events) terms. https://hackersandslackers.com/flask-blueprints/

Essentially one benefit that is mentioned in the link and provides me a clear idea of it's real world usage is that I can effectively logically organize/divide the app into several parts that only need to be concerned with it's own affairs. So it provides some designed encapsulation.

Edit: I'm currently using it to segment out my webapps code. It was good decision too because I found the lead designer wants to make the frontend in Vue.js. Which I havent used yet but looking at it's project files would look far more messy and probably provide many naming collision prone.