Username: disallow @ in usernames.

myauth/migrations/0002_auto_20200515_0801.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.0.3 on 2020-05-15 08:01
+
+from django.db import migrations, models
+import myauth.models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('myauth', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='user',
+            name='username',
+            field=models.CharField(error_messages={'unique': 'A user with that username already exists.'}, help_text='Required. 150 characters or fewer. Letters, digits and ./+/-/_ only.', max_length=150, unique=True, validators=[myauth.models.UnicodeUsernameValidator()], verbose_name='username'),
+        ),
+    ]

myauth/models.py

@@ -1,5 +1,30 @@
 from django.contrib.auth.models import AbstractUser
+from django.core import validators
+from django.db import models
+from django.utils.deconstruct import deconstructible
+from django.utils.translation import gettext_lazy as _
+
+
+@deconstructible
+class UnicodeUsernameValidator(validators.RegexValidator):
+    regex = r'^[\w.+-]+\Z'
+    message = _(
+        'Enter a valid username. This value may contain only letters, '
+        'numbers, and ./+/-/_ characters.'
+    )
+    flags = 0
 
 
 class User(AbstractUser):
-    pass
+    username_validator = UnicodeUsernameValidator()
+
+    username = models.CharField(
+        _('username'),
+        max_length=150,
+        unique=True,
+        help_text=_('Required. 150 characters or fewer. Letters, digits and ./+/-/_ only.'),
+        validators=[username_validator],
+        error_messages={
+            'unique': _("A user with that username already exists."),
+        },
+    )