Null field in select dropdown

How do I get rid of the Null choice in a select dropdown? For example, I have the following model --

class Network(models.Model):
    type = models.CharField(max_length=10, choices = [('closed','closed'),('open','open')])

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When I use the form in the template, I get three choices in the dropdown:

-------
Closed
Open

How do I get rid of this Null choice? So the select dropdown will only have the Closed and Open options?

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From the documentation (via):

By default the widget used by ModelChoiceField will have an empty choice at the top of the list. You can change the text of this label (which is "---------" by default) with the empty_label attribute, or you can disable the empty label entirely by setting empty_label to None

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Set blank=False in your model.

From the documentation:

Unless blank=False is set on the field along with a default then a label containing "---------" will be rendered with the select box. To override this behavior, add a tuple to choices containing None; e.g. (None, 'Your String For Display'). Alternatively, you can use an empty string instead of None where this makes sense - such as on a CharField.

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choices = [("","-- Select an option --"),('closed','closed'),('open','open')]