django - Is_valid メソッドを使用して 2 回目に false を返す

Question

ポスト関数の単体テストを作成しますが、2回目は常に失敗します.hereは私のforms.pyです

from django import forms
from django.contrib.auth.models import User
from django.utils import timezone
from models import Post

class PostForm(forms.ModelForm):
    class Meta:
        model = Post

ここに私のmodels.pyがあります

from django.db import models
from django.contrib.auth.models import User


class Post(models.Model):
    post_author = models.ForeignKey(User)
    created_at = models.DateTimeField()
    update_at = models.DateTimeField()
    post_title = models.TextField(blank=True)
    activity_address = models.TextField(blank=True)
    post_content = models.TextField()
    post_type = models.CharField(max_length=50, choices=[('activity', 'activity', ), ('blog', 'blog')])
    post_status = models.CharField(max_length=50, choices=[('open', 'open'), ('locked', 'locked') ])

ここにpostlib.pyがあります

from models import Post
from forms import PostForm
from django.utils import timezone

def create(data):
    if not data['created_at']:
        data['created_at'] = timezone.now()
    if not data['update_at']:
        data['update_at'] = timezone.now()
    post_form = PostForm(data=data)
    if post_form.is_valid():
        post = post_form.save()
    else:
        post = None
    return post

およびユニットテストコード

from django.test import TestCase
from django.utils import timezone
from Publish.forms import PostForm
from Publish.models import Post
from Publish import postlib
from django.contrib import auth
from django.contrib.auth.models import User

class PublishTest(TestCase):
    def setUp(self):
        User.objects.create_user(username='test', email='test@test.com', password='abc')
        self.user = auth.authenticate(username="test", password='abc')
        self.data = {'post_author': '1',
                'post_title': 'test',
                'post_content': 'nothing',
                'activity_address': 'New York',
                'created_at': '',
                'update_at': '',
                'post_type': 'activity',
                'post_status': 'open'}

    def test_a(self):
        post = postlib.create(self.data)
        self.assertNotEqual(post, None)

    def test_b(self):
        post = postlib.create(self.data)
        self.assertNotEqual(post, None)

単体テストでは、必ず test_a または test_b のいずれかで失敗し、失敗の理由は form.is_valid() です。Is_valid() メソッドはいつか失敗しますが、同じデータを使用しています。

PS: post_form エラーは 'Select a valid choice. That choice is not one of the available choices.'