[Django]-Django set field value after a form is initialized

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()

If you have initialized the form like this

form = CustomForm()

then the correct way as of Jan 2019, is to use .initial to replace the data. This will replace the data in the intial dict that goes along with the form. It also works if you have initialized using some instance such as form = CustomForm(instance=instance)

To replace data in the form, you need to

form.initial['Email'] = GetEmailString()

Generalizing this it would be,

form.initial['field_name'] = new_value

Just change your Form.data field:

class ChooseProjectForm(forms.Form):
    project = forms.ModelChoiceField(queryset=project_qs)
    my_projects = forms.BooleanField()

    def __init__(self, *args, **kwargs):
        super(ChooseProjectForm, self).__init__(*args, **kwargs)
        self.data = self.data.copy()  # IMPORTANT, self.data is immutable
        # any condition:
        if self.data.get('my_projects'):
            my_projects = self.fields['project'].queryset.filter(my=True)
            self.fields['project'].queryset = my_projects
            self.fields['project'].initial = my_projects.first().pk
            self.fields['project'].empty_label = None  # disable "-----"
            self.data.update(project=my_projects.first().pk)  # Update Form data
            self.fields['project'].widget = forms.HiddenInput()  # Hide if you want

To throw yet another way into the mix: this works too, with a bit more modern notation. It just works around the fact that a QueryDict is immutable.

>>> the_form.data = {**f.data.dict(), 'some_field': 47}
>>> the_form['some_field'].as_widget()
'<input type="hidden" name="some_field" value="47"
        class="field-some_field" id="id_some_field">'

in widget use ‘value’ attr.
Example:

username = forms.CharField(
    required=False,
    widget=forms.TextInput(attrs={'readonly': True, 'value': 'CONSTANT_VALUE'}),
)

In your case it seems that you are trying to create a new empty (unbound) form and set an initial value that will be displayed in this form.

In this case it is better to instantiate the form with the initial value (instead of trying to change the value after the form is initialized).

There are multiple options how to do this but the simplest one would be the one presented in Grant’s answer. i.e.:

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

Another way to do this, if you have already initialised a form (with or without data), and you need to add further data before displaying it:

form = Form(request.POST.form)
form.data['Email'] = GetEmailString()