Understanding Form.errors and Non-Field Errors

from django import forms
from django.core.exceptions import ValidationError

class LoginForm(forms.Form):
    username = forms.CharField()
    password = forms.CharField(widget=forms.PasswordInput)
    
    def clean(self):
        cleaned_data = super().clean()
        username = cleaned_data.get('username')
        password = cleaned_data.get('password')
        if username == 'admin' and password != 'secret123':
            raise ValidationError('Invalid credentials for admin user')
        return cleaned_data

form = LoginForm(data={'username': 'admin', 'password': 'wrong'})
form.is_valid()
print('__all__' in form.errors)