Django表单DecimalField空字符串无法转换为float该如何处理？

当用户未填写bf字段时，浏览器提交的GET参数中会包含bf=（键存在，值为空字符串），而非不存在bf键，所以request.GET.get('bf', 0)不会触发默认值0，而是拿到空字符串''，转换float时就会触发报错。

同时你原有逻辑中的if bf is False判断永远无法触发：因为bf会被转换为float类型，数值0和布尔值False是完全不同的类型，空值场景下不会进入自动计算体脂的分支。

推荐方案：使用Django表单内置能力 #

你已经定义了Form类，直接用表单自带的校验和空值处理能力即可，同时还能自动完成字段类型转换、参数合法性校验，安全性和可维护性更高，修改后视图代码如下：

def cmr(request):
    # 实例化表单绑定GET参数
    form = CMForm(request.GET)
    # 校验参数合法性，校验失败可以自行添加错误跳转逻辑
    if not form.is_valid():
        pass
    cleaned = form.cleaned_data
    a = float(cleaned['age'])
    # 非必填字段未填时cleaned_data对应值为None
    bf = cleaned['bf']
    # 未填bf时走自动计算逻辑，不需要转0
    auto_calc_bf = bf is None
    bf = float(bf) if bf is not None else 0
    s = cleaned['sex']
    g = 1 if s == 'female' else 0
    w = float(cleaned['weight'])
    h = float(cleaned['height'])
    act = cleaned['activeness']
    u = cleaned['units']
    # BooleanField未勾选时自动返回False，不需要判断'on'
    p = cleaned['pregnant']
    lac = cleaned['lactating']
    
    dci = 0
    bmrm = 10*w+6.25*h-5*a+5
    bmrf = 10*w+6.25*h-5*a-161
    bmi = round(w/(h/100)**2, 1)
    # 改为判断是否需要自动计算体脂
    if auto_calc_bf:
        bf = round(-44.988 + (0.503 * a) + (10.689 * g) + (3.172 * bmi) - (0.026 * bmi**2) + (0.181 * bmi * g) - (0.02 * bmi * a) - (0.005 * bmi**2 * g) + (0.00021 * bmi**2 * a), 1)
    tbf = w*(bf/100)
    ffm = w*(1-(bf/100))
    ffmi = ffm/(h/100)**2
    nffmi = ffmi+6.1*(1.8-(h/100))
    if p:
        dci = 300
    if lac:
        dci = dci+500
    if s == 'male':
        bmr = dci+bmrm
        dcis = bmrm*1.2
        dcil = bmrm*1.375
        dcim = bmrm*1.55
        dciv = bmrm*1.725
        dcie = bmrm*1.9
    else:
        bmr = dci+bmrf
        dcis = dci+bmrf*1.2
        dcil = dci+bmrf*1.375
        dcim = dci+bmrf*1.55
        dciv = dci+bmrf*1.725
        dcie = dci+bmrf*1.9

    context = {'dci': round(int(dci)),
               'p': p,
               'lac': lac,
               'bmr': int(bmr),
               'dcis': int(dcis),
               'dcil': int(dcil),
               'dcim': int(dcim),
               'dciv': int(dciv),
               'dcie': int(dcie),
               'act': act,
               'bmi': bmi,
               'bf': bf,
               'tbf': round(tbf, 1),
               'ffm': round(ffm, 1),
               'ffmi': round(ffmi, 1),
               'nffmi': round(nffmi, 1),
               }
    return render(request, "calorie_maintenance_result.html", context)

快速修复方案：仅修改bf取值逻辑 #

如果不想改动现有整体逻辑，只需要把bf取值的代码改为如下即可：

bf_raw = request.GET.get('bf', '').strip()
bf = float(bf_raw) if bf_raw else 0
# 同时把原来的if bf is False判断改为if bf == 0，保证自动计算逻辑可以触发

内容的提问来源于stack exchange，提问作者Ángel Guzmán