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In front of the Flaskstartup process and routing principles have been the source commuting. Today we take a look at the template rendering process.
0x00 using a template
First look at an example using a template to render official document from
from flask import render_template
@app.route('/hello/')
@app.route('/hello/<name>')
def hello(name=None):
return render_template('hello.html', name=name)Under the project directory needs a templatesdirectory and create a hello.htmlfile
/templates
/hello.htmlhello.htmlContent for
<!doctype html>
<title>Hello from Flask</title>
{% if name %}
<h1>Hello {{ name }}!</h1>
{% else %}
<h1>Hello, World!</h1>
{% endif %}This template nameis a parameter by calling render_templatecan be achieved according to the parameter method htmlrenders the template file.
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)Notes approach is clear, from templatesfind a name for the folder in template_namea file for rendering. Which current_appis initialized by the following statement
_request_ctx_stack = LocalStack()
current_app = LocalProxy(lambda: _request_ctx_stack.top.app)LocalStackIt is a realization of class stack . And _request_ctx_stackin the Flask.request_context()current context in the method of example pushto the inside of the stack
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)_RequestContextClass implements protocol context manager, it can withuse statement
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()To perform __enter__()the operation push, exit withon the implementation of the statement when the popoperation.
Back request_context()method, which is wsgi_app()to be called in
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)From routing principle known analytical articles, and wsgi_app()will be executed when the server receives a client request.
Therefore, when a request comes, the current will be Flaskrequested instance context instance is saved to stack instance _request_ctx_stack; and the request treatment, to pop the current context instance request from the stack inside.
LocalProxyIs a proxy class, its constructor is passed a lambdaexpression: lambda: _request_ctx_stack.top.app.
This operation is put through the current context instance LocalProxyis encapsulated, i.e., current_appthe current Flaskcontext of the proxy instance.
So when current_app.jinja_envthe statement actually accesses the Flaskinstance attribute jinja_env, this property is in Flaskthe constructor to initialize.
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_envIt is an Environmentexample. This is the jinjatemplate engine provides a class Flasktemplate frame rendering is through jinjato achieve.
EnvironmentA need loader, is obtained by the following method
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)By default, from templatesconstructing a directory FileSystemLoaderinstance, the role of this class is to load the template file from the file system.
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()Internal method calls the _load_template()method
@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()The method first checks whether there is a cache, if the cache is available on the use of cache; the cache is not available to use loaderto load a template, this loaderis the previously mentioned FileSystemLoaderinstance (by default).
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)BaseLoaderIt is the FileSystemLoaderbase class. This loadmethod enables compilation of templates, such as load logic. Finally, using the environment.template_class.from_code()method. Which template_classis a Templateclass that represents the template objects compiled.
from_codeIs a Templatekind of static method can be used to create an Templateinstance. When the loadmethod returns, there was obtained a Templatetarget.
And finally back render_templatemethod
def render_template(template_name, **context):
...
return current_app.jinja_env.get_template(template_name).render(context)Implementation of Templateobject render()methods.
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)This method receives a dicttype parameter, used to pass parameters to the template. The method of the core is to performexec a function. execA Pythonbuilt-in function, it can perform dynamic Pythoncode.
0x05 summarize
FlaskUse Jinjaas a template engine. Execution path
Flask.render_template => Environment.get_template => Template.render => exec0x06 learning materials
- https://palletsprojects.com/p/flask/
- http://jinja.pocoo.org/docs/2.10/