class YourForm(forms.Form):
    your_field = forms.BooleanField(initial=True)

class MyForm(forms.Form):
    your_field = forms.BooleanField()

    def __init__(self, *args, **kwargs):
        super(MyForm, self).__init__(*args, **kwargs)
        self.fields['your_field'].initial = True

form = MyForm(initial={'your_field': True})