Django forms: default value if user leaves field blank

If you're using a ModelForm simply follow Dan's advice.

If however you're simply using a Form then you may have to specify how to deal with validation. Example Documentation

class YourForm(forms.Form):
    ...

    def clean_field(self):
        data = self.cleaned_data['field']
        if not data:
            data = 'default value'

        return data

Set a default value in the model.

class YourModel(models.Model):
    this_field = models.TextField(default="default_value")