How Flask-Bootstrap works?

Question

I'm learning Flask web development, and the tutorial I'm following introduces an extension called Flask-Bootstrap. To use this extension, you must initialize it first, like this:

from flask_bootstrap import Bootstrap
# ...
bootstrap = Bootstrap(app)

Weirdly enough to me, the variable bootstrap is not used in the rest of my module. However, if I comment out this line, a jinja2.exceptions.TemplateNotFound exception will be raised. Also, the templates used start with this line:

{% extends "bootstrap/base.html" %}

But I don't have a directory named /bootstrap under /templates!

I want to know what's going on:

1. What does the bootstrap = Bootstrap(app) line do?
2. Where does bootstrap/base.html reside?

Answer 1

• Other answers have told you

the bootstrap = Bootstrap(app) line "init the extension on the app".

The bootstrap/base.html resides in the Flask-Bootstrap package.

To understand this, you must know something about "Flask's template searchpath"

1. application's template folder
2. Blueprint's template folder

So the Flask-Bootstrap actually register a blueprint to your app：

class Bootstrap(object):
    def __init__(self, app=None):
        if app is not None:
            self.init_app(app)

    def init_app(self, app):
        blueprint = Blueprint(
            'bootstrap',
            __name__,
            template_folder='templates',
            static_folder='static',
            static_url_path=app.static_url_path + '/bootstrap',
            subdomain=app.config['BOOTSTRAP_LOCAL_SUBDOMAIN'])

        app.register_blueprint(blueprint)

You can see it clearly by set EXPLAIN_TEMPLATE_LOADING:

app = Flask(__name__)
app.config['EXPLAIN_TEMPLATE_LOADING'] = True

then

export FLASK_ENV=development
flask run

when you access the page:

[2018-07-12 15:28:58,659] INFO in debughelpers: Locating template "user.html":
1: trying loader of application "hello"
   class: jinja2.loaders.FileSystemLoader
   encoding: 'utf-8'
   followlinks: False
   searchpath:
     - /root/learn/python-lab/Flask/flasky/templates
   -> found ('/root/learn/python-lab/Flask/flasky/templates/user.html')
2: trying loader of blueprint "bootstrap" (flask_bootstrap)
   class: jinja2.loaders.FileSystemLoader
   encoding: 'utf-8'
   followlinks: False
   searchpath:
     - /root/learn/python-lab/Flask/flasky/venv/lib/python3.6/site-packages/flask_bootstrap/templates
   -> no match
################################################################# Note here #######
[2018-07-12 15:28:58,677] INFO in debughelpers: Locating template "bootstrap/base.html":
1: trying loader of application "hello"
   class: jinja2.loaders.FileSystemLoader
   encoding: 'utf-8'
   followlinks: False
   searchpath:
     - /root/learn/python-lab/Flask/flasky/templates
   -> no match  ### in app path not found
2: trying loader of blueprint "bootstrap" (flask_bootstrap)
   class: jinja2.loaders.FileSystemLoader
   encoding: 'utf-8'
   followlinks: False
   searchpath:
     - /root/learn/python-lab/Flask/flasky/venv/lib/python3.6/site-packages/flask_bootstrap/templates
   ## in blueprint path found the bootstrap/base.html
   -> found ('/root/learn/python-lab/Flask/flasky/venv/lib/python3.6/site-packages/flask_bootstrap/templates/bootstrap/base.html')
127.0.0.1 - - [12/Jul/2018 15:28:58] "GET /user/Yao HTTP/1.1" 200 -

• Addition，The Blueprint template folder is added to the search path of templates but with a lower priority than the actual application’s template folder, see http://flask.pocoo.org/docs/1.0/blueprints/#templates