Flask-Bootstrap 是如何工作的？

Question

我正在学习 Flask Web 开发，我正在学习的教程介绍了一个名为 Flask-Bootstrap 的扩展。要使用这个扩展，你必须先初始化它，像这样：

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

对我来说很奇怪，变量 bootstrap 没有在我的模块的其余部分中使用。但是，如果我注释掉这一行，则会引发 jinja2.exceptions.TemplateNotFound 异常。此外，使用的模板以此行开头：

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

但是我在 /templates!

下没有名为 /bootstrap 的目录

我想知道发生了什么：

bootstrap = Bootstrap(app) 行的作用是什么？
bootstrap/base.html住在哪里？

Answer 1

正如@davidism 在他的评论中所说，bootstrap = Bootstrap(app) 行 "installs the extension on the app"。这种安装背后的机制超出了这个答案的范围。

bootstrap/base.html 位于 Flask-Bootstrap 包中。例如，在我的机器上它的绝对路径是：

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

内容如下：

{% 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 -%}

_{"how can Jinja2 find base.html"的答案会在文档中找到相关部分后立即添加。}

Answer 2

其他答案已经告诉你了

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

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

要理解这一点，您必须了解“Flask 的模板搜索路径”

应用程序的模板文件夹
蓝图的模板文件夹

所以Flask-Bootstrap实际上注册了一个蓝图到你的应用：

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)

设置一下就可以看清楚了 EXPLAIN_TEMPLATE_LOADING:

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

然后

export FLASK_ENV=development
flask run

当您访问页面时：

[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 -

新增，Blueprint模板文件夹被添加到模板的搜索路径中，但优先级低于实际应用程序的模板文件夹，参见http://flask.pocoo.org/docs/1.0/blueprints/#templates

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