Why ModelForm.Meta.fields and exclude are not honoured for custom fields?

​Selecting fields to use states that only form fields that are listed in Meta.fields or are not listed in Meta.exclude are used and "Any fields not included in a form by the above logic will not be set by the form’s save() method.".


But when I add a custom field, it is displayed and saved even when not included in fields or when excluded. Eg.:

class TestForm(forms.ModelForm):
    conditional_field = forms.TextField()

    class Meta:
        model = MyModel
        exclude = ('conditional_field',)

Still displays "conditional_field".


The reason I wanted to use this, is conditionally toggle the field in __init__.

(Even if that is not the correct way to approach this, it still is unexpected behaviour)