Python Web Flask源码解读(三)——模板渲染过程

0x00 使用模板

首先看一个来自官方文档使用模板渲染的例子
from flask import render_template
@app.route('/hello/')
@app.route('/hello/<name>')
def hello(name=None):
    return render_template('hello.html', name=name)
在项目目录下需要有一个templates目录，并创建了一个hello.html文件
/templates
    /hello.html
hello.html的内容为
<!doctype html>
<title>Hello from Flask</title>
{% if name %}
  <h1>Hello {{ name }}!</h1>
{% else %}
  <h1>Hello, World!</h1>
{% endif %}
这个模板中name是参数，通过调用render_template方法就可以根据参数实现html模板文件的渲染。
0x01 Flask.render_template
def render_template(template_name, **context):
    """Renders a template from the template folder with the given
    context.
    :param template_name: the name of the template to be rendered
    :param context: the variables that should be available in the
                    context of the template.
    current_app.update_template_context(context)
    return current_app.jinja_env.get_template(template_name).render(context)
方法的注释很清楚，从templates文件夹中找到名称为template_name的文件进行渲染。其中current_app是通过以下语句初始化
_request_ctx_stack = LocalStack()
current_app = LocalProxy(lambda: _request_ctx_stack.top.app)
LocalStack就是一个栈的实现类。而_request_ctx_stack是在Flask.request_context()方法中将当前的上下文实例push到栈里面的
def request_context(self, environ):
    """Creates a request context from the given environment and binds
    it to the current context.  This must be used in combination with
    the `with` statement because the request is only bound to the
    current context for the duration of the `with` block.
    Example usage::
        with app.request_context(environ):
            do_something_with(request)
    :params environ: a WSGI environment
    return _RequestContext(self, environ)
_RequestContext类实现了上下文管理器协议，它可以在with语句中使用
class _RequestContext(object):
    """The request context contains all request relevant information.  It is
    created at the beginning of the request and pushed to the
    `_request_ctx_stack` and removed at the end of it.  It will create the
    URL adapter and request object for the WSGI environment provided.
    def __init__(self, app, environ):
        self.app = app
        self.url_adapter = app.url_map.bind_to_environ(environ)
        self.request = app.request_class(environ)
        self.session = app.open_session(self.request)
        self.g = _RequestGlobals()
        self.flashes = None
    def __enter__(self):
        _request_ctx_stack.push(self)
    def __exit__(self, exc_type, exc_value, tb):
        # do not pop the request stack if we are in debug mode and an
        # exception happened.  This will allow the debugger to still
        # access the request object in the interactive shell.
        if tb is None or not self.app.debug:
            _request_ctx_stack.pop()
执行__enter__()时操作push，退出with语句时就执行pop操作。

回到request_context()方法，它是在wsgi_app()中被调用的
def wsgi_app(self, environ, start_response):
    """The actual WSGI application.  This is not implemented in
    `__call__` so that middlewares can be applied:
        app.wsgi_app = MyMiddleware(app.wsgi_app)
    :param environ: a WSGI environment
    :param start_response: a callable accepting a status code,
                           a list of headers and an optional
                           exception context to start the response
    with self.request_context(environ):
        rv = self.preprocess_request()
        if rv is None:
            rv = self.dispatch_request()
        response = self.make_response(rv)
        response = self.process_response(response)
        return response(environ, start_response)
从路由原理文章的分析知道，wsgi_app()在服务端接收到客户端请求时就会执行。

所以当请求来临时，就会把当前Flask实例的请求上下文实例保存到栈实例_request_ctx_stack中；请求处理后，就从栈里面弹出当前请求的上下文实例。
LocalProxy是一个代理类，它的构造函数传递了一个lambda表达式：lambda: _request_ctx_stack.top.app。

这个操作就把当前的上下文实例通过LocalProxy进行了封装，即current_app是当前Flask实例的上下文的代理。

所以当current_app.jinja_env这个语句其实就是访问Flask的实例属性jinja_env，这个属性是在Flask的构造函数中进行初始化的。
class Flask(object):
    #: 源码太长了省略
    #: options that are passed directly to the Jinja2 environment
    jinja_options = dict(
        autoescape=True,
        extensions=['jinja2.ext.autoescape', 'jinja2.ext.with_']
    def __init__(self, package_name):
        #: 源码太长省略部分源码
        #: the Jinja2 environment.  It is created from the
        #: :attr:`jinja_options` and the loader that is returned
        #: by the :meth:`create_jinja_loader` function.
        self.jinja_env = Environment(loader=self.create_jinja_loader(),
                                     **self.jinja_options)
        self.jinja_env.globals.update(
            url_for=url_for,
            get_flashed_messages=get_flashed_messages
jinja_env是一个Environment实例。这个是jinja模板引擎提供的类，Flask框架的模板渲染就是通过jinja来实现的。

Environment需要一个loader，是通过以下方法获取的
def create_jinja_loader(self):
    """Creates the Jinja loader.  By default just a package loader for
    the configured package is returned that looks up templates in the
    `templates` folder.  To add other loaders it's possible to
    override this method.
    if pkg_resources is None:
        return FileSystemLoader(os.path.join(self.root_path, 'templates'))
    return PackageLoader(self.package_name)
默认情况下是从templates目录下构造一个FileSystemLoader的实例，这个类的作用就是从文件系统中加载模板文件的。
0x02 Environment.get_template
@internalcode
def get_template(self, name, parent=None, globals=None):
    """Load a template from the loader.  If a loader is configured this
    method ask the loader for the template and returns a :class:`Template`.
    If the `parent` parameter is not `None`, :meth:`join_path` is called
    to get the real template name before loading.
    The `globals` parameter can be used to provide template wide globals.
    These variables are available in the context at render time.
    If the template does not exist a :exc:`TemplateNotFound` exception is
    raised.
    .. versionchanged:: 2.4
       If `name` is a :class:`Template` object it is returned from the
       function unchanged.
    if isinstance(name, Template):
        return name
    if parent is not None:
        name = self.join_path(name, parent)
    return self._load_template(name, self.make_globals(globals))
get_template()方法内部调用了_load_template()方法
@internalcode
def _load_template(self, name, globals):
    if self.loader is None:
        raise TypeError('no loader for this environment specified')
    if self.cache is not None:
        template = self.cache.get(name)
        if template is not None and (not self.auto_reload or \
                                     template.is_up_to_date):
            return template
    template = self.loader.load(self, name, globals)
    if self.cache is not None:
        self.cache[name] = template
    return template
_load_template()方法首先会检查是否有缓存，如果缓存可用就使用缓存；缓存不可用就使用loader加载模板，这个loader就是前面提到的FileSystemLoader的实例（默认情况下）。
0x03 BaseLoader.load
@internalcode
def load(self, environment, name, globals=None):
    # 省略部分源码
    return environment.template_class.from_code(environment, code, globals, uptodate)
BaseLoader是FileSystemLoader的基类。这个load方法实现了模板的编译、加载等逻辑。最后是使用environment.template_class.from_code()方法。其中template_class是Template类，它代表编译后的模板对象。

from_code是Template类的静态方法，可以用来创建一个Template实例。当load方法返回时，就得到了一个Template对象。

最后回到render_template方法
def render_template(template_name, **context):
    return current_app.jinja_env.get_template(template_name).render(context)
执行了Template对象的render()方法。
0x04 Template.render
def render(self, *args, **kwargs):
    """This function accepts either a dict or some keyword arguments which
    will then be the context the template is evaluated in.  The return
    value will be the rendered template.
    :param context: the function accepts the same arguments as the
                    :class:`dict` constructor.
    :return: the rendered template as string
    ns = self.default_context.copy()
    if len(args) == 1 and isinstance(args[0], utils.MultiDict):
        ns.update(args[0].to_dict(flat=True))
    else:
        ns.update(dict(*args))
    if kwargs:
        ns.update(kwargs)
    context = Context(ns, self.charset, self.errors)
    exec self.code in context.runtime, context
    return context.get_value(self.unicode_mode)
这个方法接收一个dict类型参数，用于给模板传递参数。该方法的核心是执行exec函数。exec是Python内置函数，它可以动态的执行Python代码。
0x05 总结一下
Flask使用Jinja作为模板引擎。执行路径为
Flask.render_template => Environment.get_template => Template.render => exec
0x06 学习资料
https://palletsprojects.com/p/flask/
http://jinja.pocoo.org/docs/2.10/