[Fixed]-Filter manytomanyfield form 'str' object has no attribute 'get'

You’ve made parent_id the first positional argument to the form, so you should pass it as such in the POST block:

form = AddressForm(parent_id, request.POST)

Note that it is better not to change the signature of the form at all, but use kwargs:

def __init__(self, *args, **kwargs):
    parent_id = kwargs.pop('parent_id', None)
    super(AddressForm, self).__init__(*args, **kwargs)

and do:

form = AddressForm(request.POST, parent_id=parent_id)

in the POST block and

form = AddressForm(parent_id=parent_id)

in the else.