赋值前引用的 django 局部变量 't'

Question

我看到这是一个常见错误，我确实查看了其他帖子，但他们没有帮助我。

我收到了错误

Exception Value: local variable 't' referenced before assignment

但是在我的 if 验证中，我的 t 变量在声明它不是的地方声明了 3 行。范围应该适合我的回报。

有问题的功能：

def create(response):
        #response.user
        if response.method == "POST":
                form = CreateNewTrade(response.POST)
                if form.is_valid():
                        n = form.cleaned_data["name"]
                        t = AssetList(name=n)
                        t.save()
                        response.user.assetlist.add(t)

                return HttpResponseRedirect("/userdash/%i" %t.id) #we fail at this t variable

完整代码：

$ cat userdash/views.py

from django.shortcuts import render
from django.http import HttpResponse, HttpResponseRedirect
from .models import AssetList, Items
from .forms import CreateNewTrade

# Create your views here.


#def index(response):
#       return HttpResponse("<h1>Hello Dark World!</h1>")

def userdash(response, id):
        ls = AssetList.objects.get(id=id)
        if response.method == "POST":
                print(response.POST)
                if response.POST.get("save"):
                        for item in ls.items_set.all():
                                if response.POST.get("c" + str(item.id)) == "clicked":
                                        item.sell_asset = True
                                else:
                                        item.sell_asset = False

                                item.save()

                elif response.POST.get("newItem"):
                        txt = response.POST.get("new")
                        if len(txt) > 2: #this validation is retarded and needs to be fixed
                                ls.items_set.create(user_asset=txt, sell_asset=False)
                        else:
                                print("invalid")


        #items = ls.items_set.get(id=1)
        #return HttpResponse("<h1>User Dashboard!</h1><h2>%s</h2><br></br><p>%s</p>" %(ls.name, str(items.user_asset)))
        return render(response, "userdash/list.html", {"ls":ls})

def home(response):
        #pass
        return render(response, "userdash/home.html", {})

def create(response):
        #response.user
        if response.method == "POST":
                form = CreateNewTrade(response.POST)
                if form.is_valid():
                        n = form.cleaned_data["name"]
                        t = AssetList(name=n)
                        t.save()
                        response.user.assetlist.add(t)

                return HttpResponseRedirect("/userdash/%i" %t.id)
        else:
                form = CreateNewTrade()
        return render(response, "userdash/create.html", {"form":form})

def view(response):
        return render(response, "userdash/view.html", {})

$ cat userdash/models.py

from django.db import models
from django.contrib.auth.models import User
# Create your models here.

class AssetList(models.Model):
        user = models.ForeignKey(User, on_delete=models.CASCADE, related_name="assetlist", null=True)
        name = models.CharField(max_length=200)

        def __str__(self):
                return self.name

class Items(models.Model):
        assetlist = models.ForeignKey(AssetList, on_delete=models.CASCADE)
        user_asset = models.CharField(max_length=300)
        sell_asset = models.BooleanField()

        def __str__(self):
                return self.user_asset

$ cat userdash/templates/userdash/view.html

{% extends 'userdash/base.html' %}

{% block title %} View page {% endblock %}
{% load crispy_forms_tags %}


{% block content %}
        {% for td in user.assetlist.all %}
                        <p><a href="/{{td.id}}">{{td.name}}</a></p>
        {% endfor %}

{% endblock %}

错误：

UnboundLocalError at /create/

local variable 't' referenced before assignment

Request Method:     POST
Request URL:    http://192.168.42.14:8081/create/
Django Version:     3.0.5
Exception Type:     UnboundLocalError
Exception Value:    

local variable 't' referenced before assignment

Exception Location:     ./userdash/views.py in create, line 51
Python Executable:  /usr/local/bin/uwsgi
Python Version:     3.7.3
Python Path:    

['.',
 '',
 '/home/piggy/Env/lib/python37.zip',
 '/home/piggy/Env/lib/python3.7',
 '/home/piggy/Env/lib/python3.7/lib-dynload',
 '/usr/lib/python3.7',
 '/home/piggy/Env/lib/python3.7/site-packages']

如何分配我的 t 变量以便我的 return 语句接受它？

Answer 1

这是因为您的表单无效。因此，它会直接返回HttpResponseRedirect 部分，其中t 没有定义。

def create(response):
        if response.method == "POST":
                form = CreateNewTrade(response.POST)

                # Because form.is_valid() failed, t will be undefined
                if form.is_valid():
                        n = form.cleaned_data["name"]

                        # Here only, you have assigned value of t
                        t = AssetList(name=n)
                        t.save()
                        response.user.assetlist.add(t)

                # Returns directly to this line, if you see indentation, t is not defined 
                return HttpResponseRedirect("/userdash/%i" %t.id)

请参阅上面的 cmets 以了解发生了什么。因此，可能的错误减少器可能是：

# Define t globaly inside create() method
def create(response):
        t = None
        if response.method == "POST":

现在，您已经在函数内部全局定义了t=None，它不会显示之前显示的错误。但是，仍然会有问题，因为t 是None。所以，它不会有任何属性id。因此，将显示has no attribute DoesNotExist。因此，您必须考虑如果表单无效该怎么办，重定向到哪里。

或者，如果表单无效，您可以使用 else 语句来处理问题。喜欢：

def create(response):
        if response.method == "POST":
                form = CreateNewTrade(response.POST)

                # Because form.is_valid() failed, t will be undefined
                if form.is_valid():
                        n = form.cleaned_data["name"]

                        # Here only, you have assigned value of t
                        t = AssetList(name=n)
                        t.save()
                        response.user.assetlist.add(t)

                        return HttpResponseRedirect("/userdash/%i" %t.id)

        # When if statements doesnot apply, always come to this line unless it goes inside form.is_valid()
        return HttpResponseRedirect("/userdash/")   # redirect to the page whose arguments doesnot depend upon the form subbmission

现在，我希望你明白问题所在。

Answer 2

你的create函数应该返回什么表单无效？

如果表单无效，您将引用不存在的 t 变量的值。您应该在 if 之外创建此变量（如果您想返回默认值），或者如果表单无效，则向用户返回错误。

Answer 3

在这里，您在 if 语句中定义了 t，但它不能保证始终运行，因此有可能没有 t 变量（如果 if 语句失败）。 t 也是 if 语句的局部变量，不能在它之外使用（除非它在 ​​if 语句下）。

`def create(response):
        #response.user
        if response.method == "POST":
                form = CreateNewTrade(response.POST)
                if form.is_valid():
                        n = form.cleaned_data["name"]
                        t = AssetList(name=n) #t created here
                        t.save()
                        response.user.assetlist.add(t)

                return HttpResponseRedirect("/userdash/%i" %t.id) #here we are outside the if statement`

建议修复

`def create(response):
        #response.user
        if response.method == "POST":
                form = CreateNewTrade(response.POST)
                if form.is_valid():
                        n = form.cleaned_data["name"]
                        t = AssetList(name=n) #t created here
                        t.save()
                        response.user.assetlist.add(t)
                        return HttpResponseRedirect("/userdash/%i" %t.id) #here t is defined but your code will not handle if if response.method == "POST" but form is not valid.`

如果你愿意，尝试处理 if 语句失败（对不起，我的标签系统，所以不会接受它作为代码，不知道为什么）

`def create(response):
    #response.user
    if response.method == "POST":
        form = CreateNewTrade(response.POST)
        if form.is_valid():
            n = form.cleaned_data["name"]
            t = AssetList(name=n) #t created here
            t.save()
            response.user.assetlist.add(t)
            return HttpResponseRedirect("/userdash/%i" %t.id) 
        else:
            #handle if inner if fails`