Django ModelForm with extra fields that are not in the model

I have done a ModelForm adding some extra fields that are not in the model. I use these fields for some calcualtions when saving the form.

The extra fields appear on the form and they are sent in the POST request when uploading the form. The problem is they are not added to the cleaned_data dictionary when I validate the form. How can I access them?

EDIT 2020 (Django 2 or higher)

In Django 2+ you can add the extra fields like this:

class ProfileForm(forms.ModelForm):
    
    extra_field = forms.ImageField()

    class Meta:
        model = User
        fields = ['username', 'country', 'website', 'biography']

Original answer (Django 1)

It's possible to extend Django ModelForm with extra fields. Imagine you have a custom User model and this ModelForm:

class ProfileForm(forms.ModelForm):

    class Meta:
        model = User
        fields = ['username', 'country', 'website', 'biography']

Now, imagine you want to include an extra field (not present in your User model, lets say an image avatar). Extend your form by doing this:

from django import forms

class AvatarProfileForm(ProfileForm):

    profile_avatar = forms.ImageField()

    class Meta(ProfileForm.Meta):
        fields = ProfileForm.Meta.fields + ('profile_avatar',)

Finally (given that the form has an ImageField), remember to include request.FILES when instantiating the form in your view:

# (view.py)

def edit_profile(request):
    ...
    form = AvatarProfileForm(
        request.POST or None, 
        request.FILES or None, 
        instance=request.user
    )
    ...

Hope it helps. Good luck!

EDIT:

I was getting a "can only concatenate tuple (not "list") to tuple" error in AvatarProfileForm.Meta.fields attribute. Changed it to a tuple and it worked.

I had a very similar problem except it looked like I did all the required thing, but I was getting this error when Django was starting:

django.core.exceptions.FieldError: Unknown field(s) (my_new_field) specified for MyModel

This was a silly mistake from me, I accidentally declared my field using a Widget class:

class MyForm(forms.ModelForm):
    my_new_field = forms.HiddenInput()

Instead of a Field class:

class MyForm(forms.ModelForm):
    my_new_field = forms.CharField(widget=forms.HiddenInput())

Not answering the question at hand here (which is answered well already), but might help others.