class UserLoginForm(forms.Form):
    username = forms.CharField(widget=forms.TextInput(attrs={'class':'form-control','placeholder':'Username'}))
    password = forms.CharField(widget=forms.PasswordInput(attrs={'class':'form-control','placeholder':'Password'}))
youfield = forms.CharField(widget=forms.TextInput(attrs={'class':'form-control','placeholder':'yourfield'}))
    def clean(self, *args, **kwargs):
        username = self.cleaned_data.get("username")
        password = self.cleaned_data.get("password")

        #user_qs = User.objects.filter(username=username)
        #if user_qs.count() == 1:
        #   user = user_qs.first()
        if username and password:
            user = authenticate(username=username, password=password)
            if not user:
                raise forms.ValidationError("This user does not exist")
            if not user.check_password(password):
                raise forms.ValidationError("Incorrect password")
            if not user.is_active:
                raise forms.ValidationError("This user is no longer active")
            return super(UserLoginForm, self).clean(*args, **kwargs)