Flask输入字段特定错误处理：限制字段合法值问题

我正在开发一个爬虫网站，包含三个输入字段（第二个为可选字段）。目前当用户未填写第一个或第三个字段时，系统会显示对应的空字段错误提示；但我需要限制第三个字段仅能输入wohnung或haus，现有代码无法实现该验证——用户输入其他值时，系统仍会提示“Scraping started!”，而非像空字段那样显示错误。

现有代码 #

run_scraper函数 #

def run_scraper(city, subregion, apart_or_house):
    # Ask the user for input
    base_url = ""
    while True:
        if city != "":
            break
        else:
            flash("City name cannot be empty. Please enter a valid city name.", "error")
            return

    while True:
        if apart_or_house == "wohnung" or apart_or_house == 'haus':
            break
        elif apart_or_house == "":
            flash("This field cannot be empty. Please enter what are you buying.", "error")
        else:
            flash("Please enter either 'wohnung' or 'haus'.", "error")
            return

    if subregion:
        base_url = f"https://www.immobilienscout24.de/Suche/de/{city}/{city}/{subregion}/{apart_or_house}-kaufen"
    else:
        base_url = f"https://www.immobilienscout24.de/Suche/de/{city}/{city}/{apart_or_house}-kaufen"

    #Run the scraper script with the provided inputs and base_url
    subprocess.run(['python', 'scraper.py', city, subregion, apart_or_house, base_url])
    session['scraping_finished'] = True
    flash('Scraper has finished!', 'info')
    g.scraping_finished = True

路由代码 #

@app.route('/scrape', methods=['GET', 'POST'])
def scrape():
    form = ScrapingForm()
    if form.validate_on_submit():
        city = request.form.get('city')
        subregion = request.form.get('subregion')
        apart_or_house = request.form.get('apart_or_house')

        if city and apart_or_house:
            g.scraping_finished = False
            threading.Thread(target=run_scraper, args=(city, subregion, apart_or_house)).start()
            flash('Scraping started!', 'success')
        else:
            flash('Please fill all required fields.', 'error')

    if g.get('scraping_finished', False):
        flash('Scraper has finished!', 'info')

    return render_template('scrape.html', form=form)

问题原因 #

原代码的核心问题是校验逻辑与业务逻辑分离错误：

1. 路由仅检查city和apart_or_house非空，就直接启动爬虫线程并弹出“Scraping started!”，完全跳过了第三个字段的有效值校验。
2. run_scraper里的校验逻辑在线程中执行，而Flask的flash、session、g都依赖请求上下文，线程中没有这个上下文，导致校验错误无法正常显示给用户。

解决方案 #

把所有输入校验移到路由的请求主线程中完成，确认输入合法后再启动爬虫线程；同时简化run_scraper，只负责执行爬虫逻辑。

修改后的路由代码 #

@app.route('/scrape', methods=['GET', 'POST'])
def scrape():
    form = ScrapingForm()
    if form.validate_on_submit():
        city = request.form.get('city').strip()
        subregion = request.form.get('subregion', '').strip()
        apart_or_house = request.form.get('apart_or_house').strip()

        # 统一在主线程完成所有输入校验
        error_msg = None
        if not city:
            error_msg = "City name cannot be empty. Please enter a valid city name."
        elif not apart_or_house:
            error_msg = "This field cannot be empty. Please enter what are you buying."
        elif apart_or_house not in ('wohnung', 'haus'):
            error_msg = "Please enter either 'wohnung' or 'haus'."
        
        if error_msg:
            flash(error_msg, 'error')
        else:
            g.scraping_finished = False
            threading.Thread(target=run_scraper, args=(city, subregion, apart_or_house)).start()
            flash('Scraping started!', 'success')

    if g.get('scraping_finished', False):
        flash('Scraper has finished!', 'info')

    return render_template('scrape.html', form=form)

修改后的run_scraper函数 #

def run_scraper(city, subregion, apart_or_house):
    # 直接生成URL，输入已在路由校验过
    base_url = (f"https://www.immobilienscout24.de/Suche/de/{city}/{city}/{subregion}/{apart_or_house}-kaufen"
                if subregion 
                else f"https://www.immobilienscout24.de/Suche/de/{city}/{city}/{apart_or_house}-kaufen")

    # 执行爬虫脚本
    subprocess.run(['python', 'scraper.py', city, subregion, apart_or_house, base_url])
    
    # 线程中操作Flask上下文需要手动激活
    with app.app_context():
        session['scraping_finished'] = True
        g.scraping_finished = True
        flash('Scraper has finished!', 'info')

补充说明 #

• 线程中操作session和g时，必须手动激活应用上下文（with app.app_context()），否则会抛出上下文不存在的错误。
• 如果异步任务更复杂，推荐使用Flask-Executor或Celery这类专门的异步任务工具，比原生线程更稳定。

内容的提问来源于stack exchange，提问作者SolidOpt