Python中yield为何无需二次调用即可恢复？上下文管理器解析

先看你提供的代码：

# file: your_app/views.py
import contextlib
import inspect

from airium import Airium
from django.http import HttpResponse


@contextlib.contextmanager
def basic_body(a: Airium, useful_name: str = ''):
    """Works like a Django/Ninja template."""

    a('<!DOCTYPE html>')
    with a.html(lang='en'):
        with a.head():
            a.meta(charset='utf-8')
            a.meta(content='width=device-width, initial-scale=1', name='viewport')
            # do not use CSS from this URL in a production, it's just for an educational purpose
            a.link(href='https://unpkg.com/@picocss/pico@1.4.1/css/pico.css', rel='stylesheet')
            a.title(_t=f'Hello World')

        with a.body():
            with a.div():
                with a.nav(klass='container-fluid'):
                    with a.ul():
                        with a.li():
                            with a.a(klass='contrast', href='./'):
                                a.strong(_t="⌨ Foo Bar")
                    with a.ul():
                        with a.li():
                            a.a(klass='contrast', href='#', **{'data-theme-switcher': 'auto'}, _t='Auto')
                        with a.li():
                            a.a(klass='contrast', href='#', **{'data-theme-switcher': 'light'}, _t='Light')
                        with a.li():
                            a.a(klass='contrast', href='#', **{'data-theme-switcher': 'dark'}, _t='Dark')

                with a.header(klass='container'):
                    with a.hgroup():
                        a.h1(_t=f"You're on the {useful_name}")
                        a.h2(_t="It's a page made by our automatons with a power of steam engines.")

            with a.main(klass='container'):
                yield  # This is the point where main content gets inserted

            with a.footer(klass='container'):
                with a.small():
                    margin = 'margin: auto 10px;'
                    a.span(_t='© Airium HTML generator example', style=margin)

            # do not use JS from this URL in a production, it's just for an educational purpose
            a.script(src='https://picocss.com/examples/js/minimal-theme-switcher.js')


def index(request) -> HttpResponse:
    a = Airium()
    with basic_body(a, f'main page: {request.path}'):
        with a.article():
            a.h3(_t="Hello World from Django running Airium")
            with a.p().small():
                a("This bases on ")
                with a.a(href="https://picocss.com/examples/company/"):
                    a("Pico.css / Company example")

            with a.p():
                a("Instead of a HTML template, airium has been used.")
                a("The whole body is generated by a template "
                  "and the article code looks like that:")

            with a.code().pre():
                a(inspect.getsource(index))

    return HttpResponse(bytes(a))  # from django.http import HttpResponse

你疑惑的点在于：明明yield是生成器的语法，为什么这里不需要手动调用生成器方法（比如next()）就能恢复执行？下面是具体原理：

核心原因：@contextlib.contextmanager封装了生成器的执行流程 #

这个装饰器的作用就是把带yield的函数，自动转换成符合Python上下文协议的对象，帮你隐藏了生成器的手动调用逻辑。

具体执行步骤拆解 #

1. 进入with语句时：
   当执行with basic_body(a, ...):，Python会触发上下文管理器的__enter__方法——这个方法内部会调用生成器的next()，让basic_body从开头执行，直到碰到yield语句后暂停，把控制权交给with块内的代码（也就是你编写的填充main区域的article内容）。

2. with块代码执行完毕后：
   当with块里的所有代码执行完成，Python会触发上下文管理器的__exit__方法——这个方法内部会再次调用生成器的next()，让basic_body恢复执行，跑完yield之后的所有代码（也就是生成footer、主题切换脚本的部分），最终完成完整的HTML结构。

和普通生成器的区别 #

普通生成器函数调用后返回生成器对象，必须手动调用next()或send()才能推进执行；但@contextlib.contextmanager帮你做了封装：

• __enter__方法负责启动生成器到yield暂停；
• __exit__方法负责恢复生成器执行剩余代码。

结合你的代码理解 #

在basic_body中，yield正好卡在<main>标签内部：

• 进入with块时，函数已经生成了HTML头部、导航、header部分，停在<main>内部等待填充；
• with块里的代码往Airium对象里添加主体内容；
• with块结束后，函数继续生成footer和脚本，最终拼接出完整的HTML页面。

内容的提问来源于stack exchange，提问作者arennuit