赋值前引用的 django 局部变量 't'

Question

I saw this was a common error and I did look through other post, but they did not help me.

I am getting the error

Exception Value: local variable 't' referenced before assignment

But my t variable is declared 3 lines above where it is saying it is not, inside my if validation. Scope should be fine for my return.

function in question:

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

complete code:

$ 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 %}

error:

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']

How do I assign my t variable so my return statement will accept it?

Answer 1

It is because your form is not valid. So, it will directly go to the return HttpResponseRedirect part, where t is not defined.

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)

See comments above to understand what's going on. So, possible error reducer could be:

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

Now, you have defined t=None globally inside function, it will not show the error it was showing before. But, there will still be problem because t is None . So, it won't have any attribute id . Hence, has no attribute DoesNotExist will be shown. Therefore, you have to think that what to do if form is not valid, where to redirect.

Or, you can deal with the problem with else statement if form is not valid. Like:

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

Now, I hope you understood what was the problem.

Answer 2

What should your create function return is the form is invalid?

If the form is invalid you're referencing the value of a t variable that doesn't exists. You should create this variable outside the if (if you want to return a default value) or return an error to the user if the form is invalid.

Answer 3

here you defined t inside the if statement but it doesn't guarantee to always run so there is a chance that there is no t variable(if the if statement failed). t is also a local variable of the if statement and can't be used outside of it(unless its something under the if statement).

`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`

suggested fix

`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.`

try handling the if statement fails if you want(sorry for my tabbing system, SO wont accept it as a code not sure why)

`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`