flask项目大名鼎鼎，不需要多做介绍。我把它称之为python服务开发的TOP2项目，另外一个就是django。这两个项目各有千秋，各自有不同的应用场景，都需要深入理解，熟练掌握。本次源码选择的版本是 1.1.2，我会采用慢读法，尽自己最大努力把它讲透。本篇是第3篇-自助餐，主要包括:
flask一个简单的监听函数如下:
@app.route('/login', methods=['GET', 'POST'])
def login():
if request.method == 'POST':
return do_the_login()
else:
return show_the_login_form()如果URL多了，就需要实现多个监听函数，代码会比较乱。同时对于一个URL，在监听函数中区分http的method，进行不同的业务逻辑处理。一个函数中处理两种逻辑，也不太符合单一职责，会让代码难以维护。
这种情况下，就需要使用视图。下面是一个视图示例:
class CounterAPI(MethodView):
def get(self):
return session.get('counter', 0)
def post(self):
session['counter'] = session.get('counter', 0) + 1
return 'OK'
app.add_url_rule('/counter', view_func=CounterAPI.as_view('counter'))CounterAPI可以把一个类的实例方法注册到一个URL上，自动将get和post方法区分开。我们一起看看View的实现，先是所有View的基类:
class View(object):
@classmethod
def as_view(cls, name, *class_args, **class_kwargs):
def view(*args, **kwargs):
self = view.view_class(*class_args, **class_kwargs)
return self.dispatch_request(*args, **kwargs)
...
view.view_class = cls
view.__name__ = name
...
return viewas_view函数返回一个视图函数，在视图函数里可以派发和处理request。
MethodViewType是一个元类，定义了视图支持的所有HTTP方法的集合:
http_method_funcs = frozenset(
["get", "post", "head", "options", "delete", "put", "trace", "patch"]
)
class MethodViewType(type):
def __init__(cls, name, bases, d):
super(MethodViewType, cls).__init__(name, bases, d)
if "methods" not in d:
methods = set()
...
for key in http_method_funcs:
if hasattr(cls, key):
methods.add(key.upper())
if methods:
cls.methods = methodsMethodView是使用MethodViewType和View创建的新类：
class MethodView(with_metaclass(MethodViewType, View)):
def dispatch_request(self, *args, **kwargs):
meth = getattr(self, request.method.lower(), None)
...
return meth(*args, **kwargs)with_metaclass 是为了兼容python2的语法，可以简单的理解为继承自MethodViewType和View
dispatch_request中根据请求的http-method找到对应的方法，进行执行。
view的处理函数还可以增加装饰器，示例如下:
# 使用示例
class SecretView(View):
methods = ['GET']
decorators = [login_required]
class View(object):
@classmethod
def as_view(cls, name, *class_args, **class_kwargs):
...
if cls.decorators:
view.__name__ = name
view.__module__ = cls.__module__
# 包装上装饰器
for decorator in cls.decorators:
view = decorator(view)
...
return view
# 装饰器
def login_required(view):
@functools.wraps(view)
def wrapped_view(**kwargs):
if g.user is None:
return redirect(url_for('auth.login'))
return view(**kwargs)
return wrapped_viewView相对还是比较单薄，大型的项目都会分模块进行开发，所以flask还有blueprint的概念。下面是示例项目flaskr中的 auth.py :
import functools
from flask import (
Blueprint, flash, g, redirect, render_template, request, session, url_for
)
...
bp = Blueprint('auth', __name__, url_prefix='/auth')
@bp.route('/register', methods=('GET', 'POST'))
def register():
...
@bp.route('/login', methods=('GET', 'POST'))
def login():
...
@bp.route('/logout')
def logout():
...这里定义了一个名称叫做auth的蓝图，里面定义了3个方法: register , login 和 logout 。蓝图在app的__init__.py中注册:
def create_app():
app = ...
# existing code omitted
from . import auth
app.register_blueprint(auth.bp)
return app在flask项目中还有名为blog的蓝图，提供博客文章的增删改查方法:
bp = Blueprint("blog", __name__)
@bp.route("/")
def index():
...
@bp.route("/create", methods=("GET", "POST"))
@login_required
def create():
...
采用这种方式，就可以很方便的分模块进行程序开发。
了解了bluerpint的使用方法后，我们再看看其实现原理。
class Blueprint(_PackageBoundObject):
def __init__(
self,
name,
import_name,
static_folder=None,
static_url_path=None,
template_folder=None,
url_prefix=None,
subdomain=None,
url_defaults=None,
root_path=None,
cli_group=_sentinel,
):
_PackageBoundObject.__init__(
self, import_name, template_folder, root_path=root_path
)
self.name = name
self.url_prefix = url_prefix
self.subdomain = subdomain
self.static_folder = static_folder
self.static_url_path = static_url_path
self.deferred_functions = []
if url_defaults is None:
url_defaults = {}
self.url_values_defaults = url_defaults
self.cli_group = cli_group上面Blueprint的构造函数中显示：
蓝图的route装饰器:
def route(self, rule, **options):
def decorator(f):
endpoint = options.pop("endpoint", f.__name__)
self.add_url_rule(rule, endpoint, f, **options)
return f
return decorator
def add_url_rule(self, rule, endpoint=None, view_func=None, **options):
...
self.record(lambda s: s.add_url_rule(rule, endpoint, view_func, **options))
def record(self, func):
self.deferred_functions.append(func)这里主要的疑问在添加视图函数时候的lambda函数的参数 s 是什么？继续看看蓝图的注册:
# app的方法
def register_blueprint(self, blueprint, **options):
self.blueprints[blueprint.name] = blueprint
self._blueprint_order.append(blueprint)
first_registration = True
blueprint.register(self, options, first_registration)
# blueprint的方法
def register(self, app, options, first_registration=False):
self._got_registered_once = True
state = self.make_setup_state(app, options, first_registration)
for deferred in self.deferred_functions:
deferred(state)
...make_setup_stat创建BlueprintSetupState对象, 然后执行蓝图route添加到deferred_functions的方法。这个方法就是前面的lambda函数，前面的 s 就是state对象.
class BlueprintSetupState(object):
def __init__(self, blueprint, app, options, first_registration):
#: a reference to the current application
self.app = app
self.blueprint = blueprint
def add_url_rule(self, rule, endpoint=None, view_func=None, **options):
"""A helper method to register a rule (and optionally a view function)
to the application. The endpoint is automatically prefixed with the
blueprint's name.
"""
if self.url_prefix is not None:
if rule:
rule = "/".join((self.url_prefix.rstrip("/"), rule.lstrip("/")))
else:
rule = self.url_prefix
options.setdefault("subdomain", self.subdomain)
if endpoint is None:
endpoint = _endpoint_from_view_func(view_func)
defaults = self.url_defaults
if "defaults" in options:
defaults = dict(defaults, **options.pop("defaults"))
self.app.add_url_rule(
rule,
"%s.%s" % (self.blueprint.name, endpoint),
view_func,
defaults=defaults,
**options
)BlueprintSetupState中建立了app和blueprint的关联，并且使用app的add_url_rule方法，把blueprint的视图函数注册进入app。
flask是一个 micro 框架，但是也(至少)可以支持中型项目。我们可以利用Blueprint和View功能进行模块化: View可以很好的区分URL上的http-method；Blueprint可以很好的定义子域名和URL前缀等。
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