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If you're just using a vanilla form (not a ModelForm), you can set a Field initial value (http://docs.djangoproject.com/en/dev/ref/forms/fields/#django.forms.Field.initial) like
classMyForm(forms.Form):
my_field = forms.BooleanField(initial=True)
If you're using a ModelForm, you can set a default value on the model field (http://docs.djangoproject.com/en/dev/ref/models/fields/#default), which will apply to the resulting ModelForm.
Finally, if you want to dynamically choose at runtime whether or not your field will be selected by default, you can use the initial parameter to the form when you initialize it:
form =MyForm(initial={'my_field':True})