Django: Set choices on form attribute MultipleChoiceField

I have the following django form:

class SpecifyColumnsForm(forms.Form):
    columns = forms.MultipleChoiceField(required=False,
    widget=forms.CheckboxSelectMultiple)

Now, I want to specify the choices for this MultipleChoiceField from views.py. How can I do that?

I have tried the following, but it did not work:

        columns_form = SpecifyColumnsForm(request.POST)
        columns_form.choices = (('somestuff', 'spam'),
                                ('otherstuff', 'eggs'),
                                ('banana', 'bar'))

Thanks!