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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?
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  • Read about public source code if you want learn more about how it works. For Bootstrap : github.com/mbr/flask-bootstrap/blob/master/flask_bootstrap/… Commented Sep 10, 2016 at 15:08
  • I've already seen the question you linked to, but I also want to inspect the content of bootstrap/base.html, and that question doesn't provide its specific location. Furthermore, I'm willing to hear how can jinja2 find it. Commented Sep 10, 2016 at 15:09
  • Flask-boostrap is a blueprint - a sort of sub-app for a Flask app. When you register a blueprint, as in bootstrap = Bootstrap(app), it registers in the app some rules which say to Flask where the blueprint's own static and template folders are. Commented Sep 1, 2017 at 3:04

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  • 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 -
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As @davidism said in his comment, the bootstrap = Bootstrap(app) line "installs the extension on the app". The mechanism behind such installation is beyond the scope of this answer.

The bootstrap/base.html resides in the Flask-Bootstrap package. For example, on my machine it's absolute path is:

/Users/sunqingyao/Documents/Projects/flasky/venv/lib/python2.7/site-packages/flask_bootstrap/templates/bootstrap/base.html

Here is its content:

{% block doc -%} <!DOCTYPE html> <html{% block html_attribs %}{% endblock html_attribs %}> {%- block html %} <head> {%- block head %} <title>{% block title %}{{title|default}}{% endblock title %}</title> {%- block metas %} <meta name="viewport" content="width=device-width, initial-scale=1.0"> {%- endblock metas %} {%- block styles %} <!-- Bootstrap --> <link href="{{bootstrap_find_resource('css/bootstrap.css', cdn='bootstrap')}}" rel="stylesheet"> {%- endblock styles %} {%- endblock head %} </head> <body{% block body_attribs %}{% endblock body_attribs %}> {% block body -%} {% block navbar %} {%- endblock navbar %} {% block content -%} {%- endblock content %} {% block scripts %} <script src="{{bootstrap_find_resource('jquery.js', cdn='jquery')}}"></script> <script src="{{bootstrap_find_resource('js/bootstrap.js', cdn='bootstrap')}}"></script> {%- endblock scripts %} {%- endblock body %} </body> {%- endblock html %} </html> {% endblock doc -%}

The answer to "how can Jinja2 find base.html" would be added as soon as I find the relevant part in the document.

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