Python Web Flask源码解读(三)——模板渲染过程

时间 2019-11-07

标签 python web flask 源码解读模板渲染过程栏目 Python 繁體版

原文原文链接

关于我
编程界的一名小小程序猿，目前在一个创业团队任team lead，技术栈涉及Android、Python、Java和Go，这个也是咱们团队的主要技术栈。
Github：github.com/hylinux1024
微信公众号：angrycodehtml

前面对Flask启动流程和路由原理都进行了源码走读。今天咱们看看模板渲染的过程。linux

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git

from flask import render_template

@app.route('/hello/')
@app.route('/hello/<name>')
def hello(name=None):
    return render_template('hello.html', name=name)
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在项目目录下须要有一个templates目录，并建立了一个hello.html文件github

/templates
    /hello.html
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hello.html的内容为shell

<!doctype html>
<title>Hello from Flask</title>
{% if name %}
  <h1>Hello {{ name }}!</h1>
{% else %}
  <h1>Hello, World!</h1>
{% endif %}
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这个模板中name是参数，经过调用render_template方法就能够根据参数实现html模板文件的渲染。编程

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)
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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是经过如下语句初始化flask

_request_ctx_stack = LocalStack()
current_app = LocalProxy(lambda: _request_ctx_stack.top.app)
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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到栈里面的小程序

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)
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_RequestContext类实现了上下文管理器协议，它能够在with语句中使用缓存

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()
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执行__enter__()时操做push，退出with语句时就执行pop操做。
回到request_context()方法，它是在wsgi_app()中被调用的微信

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)
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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。因此当请求来临时，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。这个操做就把当前的上下文实例经过LocalProxy进行了封装，即current_app是当前Flask实例的上下文的代理。因此当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
        )
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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经过jinja来实现的。 Environment须要一个loader，是经过如下方法获取的

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)
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默认状况下是从templates目录下构造一个FileSystemLoader的实例，这个类的做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))
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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
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_load_template()方法首先会检查是否有缓存，若是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（默认状况下）。

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)
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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，它表明编译后的模板对象。 from_code是Template类的静态方法，能够用来建立一个Template实例。当load方法返回时，就获得了一个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)
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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)
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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，它能够动态的执行Python代码。

0x05 总结一下

Flask使用Jinja做为模板引擎。执行路径为

Flask.render_template => Environment.get_template => Template.render => exec
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