Django set field value after a form is initialized

Since you're not passing in POST data, I'll assume that what you are trying to do is set an initial value that will be displayed in the form. The way you do this is with the initial keyword.

form = CustomForm(initial={'Email': GetEmailString()})

See the Django Form docs for more explanation.

If you are trying to change a value after the form was submitted, you can use something like:

if form.is_valid():
    form.cleaned_data['Email'] = GetEmailString()

Check the referenced docs above for more on using cleaned_data

If you've already initialized the form, you can use the initial property of the field. For example,

form = CustomForm()
form.fields["Email"].initial = GetEmailString()

If you want to do it within the form's __init__ method for some reason, you can manipulate the initial dict:

class MyForm(forms.Form):
    my_field = forms.CharField(max_length=255)

    def __init__(self, *args, **kwargs):
        super(MyForm, self).__init__(*args, **kwargs)
        self.initial['my_field'] = 'Initial value'

Something like Nigel Cohen's would work if you were adding data to a copy of the collected set of form data:

form = FormType(request.POST)
if request.method == "POST":
    formcopy = form(request.POST.copy())
    formcopy.data['Email'] = GetEmailString()