用Python写一个简单的Web框架

一、概述

在Python中,WSGI(Web Server Gateway Interface)定义了Web服务器与Web应用(或Web框架)之间的标准接口。在WSGI的规范下,各种各样的Web服务器和Web框架都可以很好的交互。

由于WSGI的存在,用Python写一个简单的Web框架也变得非常容易。然而,同很多其他的强大软件一样,要实现一个功能丰富、健壮高效的Web框架并非易事;如果您打算这么做,可能使用一个现成的Web框架(如 DjangoTornadoweb.py 等)会是更合适的选择。

本文尝试写一个类似web.py的Web框架。好吧,我承认我夸大其辞了:首先,web.py并不简单;其次,本文只重点实现了 URL调度(URL dispatch)部分。

二、从demo_app开始

首先,作为一个初步体验,我们可以借助 wsgiref.simple_server 来搭建一个简单无比(trivial)的Web应用:

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#!/usr/bin/env python
# -*- coding: utf-8 -*-
 
from wsgiref.simple_server import make_server, demo_app
 
httpd = make_server(, 8086, demo_app)
sa = httpd.socket.getsockname()
print ‘http://{0}:{1}/’.format(*sa)
 
# Respond to requests until process is killed
httpd.serve_forever()


运行脚本:

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$ python code.py
http://0.0.0.0:8086/

打开浏览器,输入http://0.0.0.0:8086/后可以看到:一行”Hello world!” 和 众多环境变量值。

三、WSGI中的application

WSGI中规定:application是一个 可调用对象(callable object),它接受 environstart_response 两个参数,并返回一个 字符串迭代对象

其中,可调用对象 包括 函数方法 或者 具有__call__方法的 实例environ 是一个字典对象,包括CGI风格的环境变量(CGI-style environment variables)和 WSGI必需的变量(WSGI-required variables);start_response 是一个可调用对象,它接受两个 常规参数(status,response_headers)和 一个 默认参数(exc_info);字符串迭代对象 可以是 字符串列表生成器函数 或者 具有__iter__方法的可迭代实例。更多细节参考 Specification Details

The Application/Framework Side 中给出了一个典型的application实现:

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#!/usr/bin/env python
# -*- coding: utf-8 -*-
 
“””application.py”””
 
def simple_app(environ, start_response):
    “””Simplest possible application object”””
    status = ‘200 OK’
    response_headers = [(‘Content-type’, ‘text/plain’)]
    start_response(status, response_headers)
    return [‘Hello world!\n’]

现在用simple_app来替换demo_app:

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#!/usr/bin/env python
# -*- coding: utf-8 -*-
 
“””code.py”””
 
from wsgiref.simple_server import make_server
from application import simple_app as app
 
if __name__ == ‘__main__’:
    httpd = make_server(, 8086, app)
    sa = httpd.socket.getsockname()
    print ‘http://{0}:{1}/’.format(*sa)
 
    # Respond to requests until process is killed
    httpd.serve_forever()

运行脚本code.py后,访问http://0.0.0.0:8086/就可以看到那行熟悉的句子:Hello world!

四、区分URL

倒腾了一阵子后,您会发现不管如何改变URL中的path部分,得到的响应都是一样的。因为simple_app只识别host+port部分。

为了对URL中的path部分进行区分处理,需要修改application.py的实现。

首先,改用 来实现application:

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#!/usr/bin/env python
# -*- coding: utf-8 -*-
 
“””application.py”””
 
class my_app:
    def __init__(self, environ, start_response):
        self.environ = environ
        self.start = start_response
 
    def __iter__(self):
        status = ‘200 OK’
        response_headers = [(‘Content-type’, ‘text/plain’)]
        self.start(status, response_headers)
        yield “Hello world!\n”

然后,增加对URL中path部分的区分处理:

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#!/usr/bin/env python
# -*- coding: utf-8 -*-
 
“””application.py”””
 
class my_app:
    def __init__(self, environ, start_response):
        self.environ = environ
        self.start = start_response
 
    def __iter__(self):
        path = self.environ[‘PATH_INFO’]
        if path == “/”:
            return self.GET_index()
        elif path == “/hello”:
            return self.GET_hello()
        else:
            return self.notfound()
 
    def GET_index(self):
        status = ‘200 OK’
        response_headers = [(‘Content-type’, ‘text/plain’)]
        self.start(status, response_headers)
        yield “Welcome!\n”
 
    def GET_hello(self):
        status = ‘200 OK’
        response_headers = [(‘Content-type’, ‘text/plain’)]
        self.start(status, response_headers)
        yield “Hello world!\n”
 
    def notfound(self):
        status = ‘404 Not Found’
        response_headers = [(‘Content-type’, ‘text/plain’)]
        self.start(status, response_headers)
        yield “Not Found\n”

修改code.py中的from application import simple_app as app,用my_app来替换simple_app后即可体验效果。

五、重构

上面的代码虽然奏效,但是在编码风格和灵活性方面有很多问题,下面逐步对其进行重构。

1、正则匹配URL

消除URL硬编码,增加URL调度的灵活性:

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#!/usr/bin/env python
# -*- coding: utf-8 -*-
 
“””application.py”””
 
import re ##########修改点
 
class my_app:
 
    urls = (
        (“/”, “index”),
        (“/hello/(.*)”, “hello”),
    ) ##########修改点
 
    def __init__(self, environ, start_response):
        self.environ = environ
        self.start = start_response
 
    def __iter__(self): ##########修改点
        path = self.environ[‘PATH_INFO’]
        method = self.environ[‘REQUEST_METHOD’]
 
        for pattern, name in self.urls:
            m = re.match(‘^’ + pattern + ‘$’, path)
            if m:
                # pass the matched groups as arguments to the function
                args = m.groups()
                funcname = method.upper() + ‘_’ + name
                if hasattr(self, funcname):
                    func = getattr(self, funcname)
                    return func(*args)
 
        return self.notfound()
 
    def GET_index(self):
        status = ‘200 OK’
        response_headers = [(‘Content-type’, ‘text/plain’)]
        self.start(status, response_headers)
        yield “Welcome!\n”
 
    def GET_hello(self, name): ##########修改点
        status = ‘200 OK’
        response_headers = [(‘Content-type’, ‘text/plain’)]
        self.start(status, response_headers)
        yield “Hello %s!\n” % name
 
    def notfound(self):
        status = ‘404 Not Found’
        response_headers = [(‘Content-type’, ‘text/plain’)]
        self.start(status, response_headers)
        yield “Not Found\n”

2、DRY

消除GET_*方法中的重复代码,并且允许它们返回字符串:

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#!/usr/bin/env python
# -*- coding: utf-8 -*-
 
“””application.py”””
 
import re
 
class my_app:
 
    urls = (
        (“/”, “index”),
        (“/hello/(.*)”, “hello”),
    )
 
    def __init__(self, environ, start_response): ##########修改点
        self.environ = environ
        self.start = start_response
        self.status = ‘200 OK’
        self._headers = []
 
    def __iter__(self): ##########修改点
        result = self.delegate()
        self.start(self.status, self._headers)
 
        # 将返回值result(字符串 或者 字符串列表)转换为迭代对象
        if isinstance(result, basestring):
            return iter([result])
        else:
            return iter(result)
 
    def delegate(self): ##########修改点
        path = self.environ[‘PATH_INFO’]
        method = self.environ[‘REQUEST_METHOD’]
 
        for pattern, name in self.urls:
            m = re.match(‘^’ + pattern + ‘$’, path)
            if m:
                # pass the matched groups as arguments to the function
                args = m.groups()
                funcname = method.upper() + ‘_’ + name
                if hasattr(self, funcname):
                    func = getattr(self, funcname)
                    return func(*args)
 
        return self.notfound()
 
    def header(self, name, value): ##########修改点
        self._headers.append((name, value))
 
    def GET_index(self): ##########修改点
        self.header(‘Content-type’, ‘text/plain’)
        return “Welcome!\n”
 
    def GET_hello(self, name): ##########修改点
        self.header(‘Content-type’, ‘text/plain’)
        return “Hello %s!\n” % name
 
    def notfound(self): ##########修改点
        self.status = ‘404 Not Found’
        self.header(‘Content-type’, ‘text/plain’)
        return “Not Found\n”

3、抽象出框架

为了将类my_app抽象成一个独立的框架,需要作出以下修改:

  • 剥离出其中的具体处理细节:urls配置 和 GET_*方法(改成在多个类中实现相应的GET方法)
  • 把方法header实现为类方法(classmethod),以方便外部作为功能函数调用
  • 改用 具有__call__方法的 实例 来实现application

修改后的application.py(最终版本):

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#!/usr/bin/env python
# -*- coding: utf-8 -*-
 
“””application.py”””
 
import re
 
class my_app:
    “””my simple web framework”””
 
    headers = []
 
    def __init__(self, urls=(), fvars={}):
        self._urls = urls
        self._fvars = fvars
 
    def __call__(self, environ, start_response):
        self._status = ‘200 OK’ # 默认状态OK
        del self.headers[:] # 清空上一次的headers
 
        result = self._delegate(environ)
        start_response(self._status, self.headers)
 
        # 将返回值result(字符串 或者 字符串列表)转换为迭代对象
        if isinstance(result, basestring):
            return iter([result])
        else:
            return iter(result)
 
    def _delegate(self, environ):
        path = environ[‘PATH_INFO’]
        method = environ[‘REQUEST_METHOD’]
 
        for pattern, name in self._urls:
            m = re.match(‘^’ + pattern + ‘$’, path)
            if m:
                # pass the matched groups as arguments to the function
                args = m.groups()
                funcname = method.upper() # 方法名大写(如GET、POST)
                klass = self._fvars.get(name) # 根据字符串名称查找类对象
                if hasattr(klass, funcname):
                    func = getattr(klass, funcname)
                    return func(klass(), *args)
 
        return self._notfound()
 
    def _notfound(self):
        self._status = ‘404 Not Found’
        self.header(‘Content-type’, ‘text/plain’)
        return “Not Found\n”
 
    @classmethod
    def header(cls, name, value):
        cls.headers.append((name, value))

对应修改后的code.py(最终版本):

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#!/usr/bin/env python
# -*- coding: utf-8 -*-
 
“””code.py”””
 
from application import my_app
 
urls = (
    (“/”, “index”),
    (“/hello/(.*)”, “hello”),
)
 
wsgiapp = my_app(urls, globals())
 
class index:
    def GET(self):
        my_app.header(‘Content-type’, ‘text/plain’)
        return “Welcome!\n”
 
class hello:
    def GET(self, name):
        my_app.header(‘Content-type’, ‘text/plain’)
        return “Hello %s!\n” % name
 
if __name__ == ‘__main__’:
    from wsgiref.simple_server import make_server
    httpd = make_server(, 8086, wsgiapp)
 
    sa = httpd.socket.getsockname()
    print ‘http://{0}:{1}/’.format(*sa)
 
    # Respond to requests until process is killed
    httpd.serve_forever()

当然,您还可以在code.py中配置更多的URL映射,并实现相应的类来对请求作出响应。

六、参考

本文主要参考了 How to write a web framework in Python(作者 anandology 是web.py代码的两位维护者之一,另一位则是大名鼎鼎却英年早逝的 Aaron Swartz),在此基础上作了一些调整和修改,并掺杂了自己的一些想法。

如果您还觉得意犹未尽,Why so many Python web frameworks? 也是一篇很好的文章,也许它会让您对Python中Web框架的敬畏之心荡然无存:-)

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