Django异步项目（Uvicorn）POST构建触发sync_to_async类型错误

• python==3.11.3
• asgiref==3.6.0
• Django==4.1.8

我正在开发一个基于Uvicorn的Django异步项目，书籍构建功能本身逻辑正常，但触发构建的POST请求执行时，虽然书籍已成功创建，却抛出如下错误：

TypeError at /builds/ sync_to_async can only be applied to sync functions.

刷新页面后显示正常，相关代码如下：

import re
from subprocess import check_output

from asgiref.sync import sync_to_async
from django.contrib.auth import get_user_model
from django.contrib.auth.mixins import LoginRequiredMixin
from django.db.utils import IntegrityError
from django.shortcuts import get_object_or_404, render, redirect
from django.views.generic import View

from crayon.books.models import Book
from crayon.builds.models import Build
from crayon.core.mixin import AsyncViewMixin

class AsyncViewMixin:
    async def __call__(self):
        return super().__call__(self)

def build_book(book_id: int, user_id: int) -> str:
    global document, build
    # step 1: Get the book
    book = Book.objects.get(id=book_id, creator_id=user_id)
    # step 2: compile the book's contents
    build = Build.objects.create(builder_id=user_id, book_id=book_id)
    build.status = Build.STATUS.IN_PROGRESS
    build.save()
    document_list = []
    for chapter in book.chapter_set.all():
        document_list.append(f"# {chapter.title}")
        # If a "reges" finds an markdown H1
        if re.search(r"\n\#\s", chapter.body) or re.match("\#\s", chapter.body):
            chapter.body = add_pound_to_headers(chapter.body)
        document_list.append(chapter.body)
    document = "\n\n".join(document_list)
    # step 3: render as something using pandoc
    # Pandoc -o book.html input.md
    with open("input.md", "w") as f:
        f.write(document)
    commands = ["pandoc", "-o", "book.html", "input.md"]
    output = check_output(commands)
    return output

class BuildView(LoginRequiredMixin, AsyncViewMixin, View):
    def get(self, request, *args, **kwargs):
        # If without ID then shows list of builds for user
        # If with ID then show details
        return render(request, "builds/build.html", {})

    def post(self, request, *args, **kwargs):
        # Step 1: get the book
        book = get_object_or_404(Book, id=self.request.POST["book_id"], creator=request.user.id)
        # accept a form request asking for a build
        sync_to_async(build_book(book_id=book.id, user_id=request.user.id))
        return redirect("books:update", pk=request.POST["book_id"])

核心错误 #

你调用sync_to_async时犯了关键错误：直接执行build_book(...)并把函数返回值传给了sync_to_async，但sync_to_async要求传入的是同步函数对象，而不是函数执行后的结果（这里返回的是字符串），因此触发类型错误。同时你的AsyncViewMixin实现不正确，无法支撑异步视图的执行逻辑，post方法也未处理异步操作的等待逻辑。

修复步骤 #

1. 修正AsyncViewMixin实现
   让Mixin正确处理异步视图的调用流程：

   import asyncio
   
   class AsyncViewMixin:
       async def __call__(self, request, *args, **kwargs):
           # 处理异步GET/POST方法
           if hasattr(self, 'get') and asyncio.iscoroutinefunction(self.get):
               return await self.get(request, *args, **kwargs)
           elif hasattr(self, 'post') and asyncio.iscoroutinefunction(self.post):
               return await self.post(request, *args, **kwargs)
           # 兼容同步方法
           return super().__call__(request, *args, **kwargs)
   

2. 改造post方法为异步并正确使用sync_to_async
   将post改为异步方法，用sync_to_async包裹函数对象并传入参数，最后await执行：

   class BuildView(LoginRequiredMixin, AsyncViewMixin, View):
       # 保留原有get方法不变
       def get(self, request, *args, **kwargs):
           return render(request, "builds/build.html", {})
   
       async def post(self, request, *args, **kwargs):
           book = get_object_or_404(Book, id=request.POST["book_id"], creator=request.user)
           # 正确用法：先包裹函数对象，再传入参数并await
           await sync_to_async(build_book)(book_id=book.id, user_id=request.user.id)
           return redirect("books:update", pk=request.POST["book_id"])
   

3. 可选优化建议

   • 移除build_book中的全局变量document和build，改用局部变量，避免并发场景下的变量污染
   • 不要直接使用当前目录的input.md，建议用临时文件或用户专属目录，防止多用户并发时文件被覆盖
   • 为check_output添加异常捕获，处理pandoc未安装或执行失败的情况

后台执行构建（可选） #

如果不需要等待构建完成就返回响应，可以用asyncio.create_task将任务丢到后台执行：

import asyncio

async def post(self, request, *args, **kwargs):
    book = get_object_or_404(Book, id=request.POST["book_id"], creator=request.user)
    asyncio.create_task(sync_to_async(build_book)(book_id=book.id, user_id=request.user.id))
    return redirect("books:update", pk=request.POST["book_id"])

内容的提问来源于stack exchange，提问作者Houdaeyfha Alsalih