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I have read the official Flask documentation on Blueprints and even one or two blog posts on using them.

I've even used them in my web app, but I don't completely understand what they are or how they fit into my app as a whole. How is it similar to an instance of my app but not quite? The documentation is comprehensive but I seek a layman explanation or an enlightening analogy to spark it for me. I was sufficiently perplexed when a colleague asked me to explain a Flask blueprint to them that I elected to ask here.

JoshieSimmons
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5 Answers5

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

A medallion mold with a gold medallion freshly removed from it

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 example
from flask import Blueprint

tree_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_mold

app.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.

pfabri
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Sean Vieira
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    Here is a nice article for application structure using blueprints. http://exploreflask.com/en/latest/blueprints.html – Devasish Jul 27 '17 at 05:25
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    I am still confused. Does that mean `oak/leaves` and `fir\leaves` will point tot the same code? Also, what is the purpose of the string `mold` in `Blueprint("mold", __name__)` – Codevalley Aug 07 '17 at 06:48
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    Yes, both `oak/leaves` and `fir/leaves` will be handled by `def leaves`. `"mold"` is the name of the blueprint and is used for disambiguation when referring to routes (e. g. `someBlueprint.routeName` vs. `anotherBlueprint.routeName`). – Sean Vieira Aug 08 '17 at 06:19
  • Can I use blueprints for instantiating eg: SQLAlchemy? – Bato-Bair Tsyrenov Nov 06 '17 at 14:54
  • A blueprint, no - unless you're instantiating SQLAlchemy in a `beforefirstrequest` handler (but there are probably better ways.) – Sean Vieira Nov 11 '17 at 16:53
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    How can I access the `url_prefix` in the function? Say, to check if it is oak or fir? `tree_mold.url_prefix` gives me a `None` – Arjun Feb 12 '18 at 17:49
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    @Mellkor - simply use `url_for('.relative_route_name')` (note the leading dot). For example, `url_for('.roots')` will provide a correctly scoped URL at each of the mounted points automatically. – Sean Vieira Feb 14 '18 at 04:29
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    So... Is blueprint like parent class? and Is each app like a child class? – Cristhian Boujon Feb 19 '18 at 12:38
  • Nothing at all like a class - more like a mixin, but a mixin that can be applied to multiple "fields" in your app. – Sean Vieira Feb 22 '18 at 02:29
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    How does this differ from templates (jinja)? – JobHunter69 Jun 24 '19 at 00:31
  • What said in this answer is correct but according to me it can be confusing. Indeed it points out a particular way to use flask blueprints. The main use of blueprints is not to mold an application but to split large flask applications in modules. Otherways you don't have way to split flask views.py file, that in complex applications can grow quite huge. – Marco Evasi Oct 08 '20 at 21:10
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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.

Tran Cuong
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For bigger projects, all your code shouldn't be in the same file. Instead you can segment or split bigger codes into separate files, mostly based on functionality. Like bricks forming a wall.

A simple Flask app

app = Flask(__name__)

A blueprinted Flask app

import from_any_module.part_1
import from_other_module.part_2

app = Flask(__name__)

app.register_blueprint(part_1)
app.register_blueprint(part_2)

A blueprint in the above app

from flask import Blueprint
part_1 = Blueprint(part_1)

@part_1.route('/url')
def function()
    return view
Henshal B
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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 OOP Interface Implementation style where you define some stuff but dont have to worry about it implementation details 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 instances due to files with similar names

LFMekz
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A Flask blueprint helps you to create reusable instances of your application. It does so by organizing your project in modules. Those modules are then registered the main application. They help in creating an application factory.

Ssali Jonathan
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    This does not answer the users question. He mentioned; he has used them and read about them. He needs a lower level explanation of flask blueprints. How do they work, under the hood? – Mo. Atairu Sep 27 '20 at 21:34