class BaseFormSet

    def _construct_form(self, i, **kwargs):
        """Instantiate and return the i-th form instance in a formset."""
        defaults = {
            'auto_id': self.auto_id,
            'prefix': self.add_prefix(i),
            'error_class': self.error_class,
            # Don't render the HTML 'required' attribute as it may cause
            # incorrect validation for extra, optional, and deleted
            # forms in the formset.
            'use_required_attribute': False,
        }
        if self.is_bound:
            defaults['data'] = self.data
            defaults['files'] = self.files
        if self.initial and 'initial' not in kwargs:
            try:
                defaults['initial'] = self.initial[i]
            except IndexError:
                pass
        # Allow extra forms to be empty, unless they're part of
        # the minimum forms.
        if i >= self.initial_form_count() and i >= self.min_num:
            defaults['empty_permitted'] = True
        defaults.update(kwargs)
        form = self.form(**defaults)
        self.add_fields(form, i)
        return form

    def _should_delete_form(self, form):
        """Return whether or not the form was marked for deletion."""
        return form.cleaned_data.get(DELETION_FIELD_NAME, False)

    def add_fields(self, form, index):
        """A hook for adding extra fields on to each form instance."""
        if self.can_order:
            # Only pre-fill the ordering field for initial forms.
            if index is not None and index < self.initial_form_count():
                form.fields[ORDERING_FIELD_NAME] = IntegerField(label=_('Order'), initial=index + 1, required=False)
            else:
                form.fields[ORDERING_FIELD_NAME] = IntegerField(label=_('Order'), required=False)
        if self.can_delete:
            form.fields[DELETION_FIELD_NAME] = BooleanField(label=_('Delete'), required=False)

    def add_prefix(self, index):
        return '%s-%s' % (self.prefix, index)

    def as_p(self):
        "Return this formset rendered as HTML <p>s."
        forms = ' '.join(form.as_p() for form in self)
        return mark_safe(str(self.management_form) + '\n' + forms)

    def as_table(self):
        "Return this formset rendered as HTML <tr>s -- excluding the <table></table>."
        # XXX: there is no semantic division between forms here, there
        # probably should be. It might make sense to render each form as a
        # table row with each field as a td.
        forms = ' '.join(form.as_table() for form in self)
        return mark_safe(str(self.management_form) + '\n' + forms)

    def as_ul(self):
        "Return this formset rendered as HTML <li>s."
        forms = ' '.join(form.as_ul() for form in self)
        return mark_safe(str(self.management_form) + '\n' + forms)

    def clean(self):
        """
        Hook for doing any extra formset-wide cleaning after Form.clean() has
        been called on every form. Any ValidationError raised by this method
        will not be associated with a particular form; it will be accessible
        via formset.non_form_errors()
        """
        pass

    def full_clean(self):
        """
        Clean all of self.data and populate self._errors and
        self._non_form_errors.
        """
        self._errors = []
        self._non_form_errors = self.error_class()
        empty_forms_count = 0

        if not self.is_bound:  # Stop further processing.
            return
        for i in range(0, self.total_form_count()):
            form = self.forms[i]
            # Empty forms are unchanged forms beyond those with initial data.
            if not form.has_changed() and i >= self.initial_form_count():
                empty_forms_count += 1
            # Accessing errors calls full_clean() if necessary.
            # _should_delete_form() requires cleaned_data.
            form_errors = form.errors
            if self.can_delete and self._should_delete_form(form):
                continue
            self._errors.append(form_errors)
        try:
            if (self.validate_max and
                    self.total_form_count() - len(self.deleted_forms) > self.max_num) or \
                    self.management_form.cleaned_data[TOTAL_FORM_COUNT] > self.absolute_max:
                raise ValidationError(ngettext(
                    "Please submit %d or fewer forms.",
                    "Please submit %d or fewer forms.", self.max_num) % self.max_num,
                    code='too_many_forms',
                )
            if (self.validate_min and
                    self.total_form_count() - len(self.deleted_forms) - empty_forms_count < self.min_num):
                raise ValidationError(ngettext(
                    "Please submit %d or more forms.",
                    "Please submit %d or more forms.", self.min_num) % self.min_num,
                    code='too_few_forms')
            # Give self.clean() a chance to do cross-form validation.
            self.clean()
        except ValidationError as e:
            self._non_form_errors = self.error_class(e.error_list)

    def get_form_kwargs(self, index):
        """
        Return additional keyword arguments for each individual formset form.

        index will be None if the form being constructed is a new empty
        form.
        """
        return self.form_kwargs.copy()

    def has_changed(self):
        """Return True if data in any form differs from initial."""
        return any(form.has_changed() for form in self)

    def initial_form_count(self):
        """Return the number of forms that are required in this FormSet."""
        if self.is_bound:
            return self.management_form.cleaned_data[INITIAL_FORM_COUNT]
        else:
            # Use the length of the initial data if it's there, 0 otherwise.
            initial_forms = len(self.initial) if self.initial else 0
        return initial_forms

    def is_multipart(self):
        """
        Return True if the formset needs to be multipart, i.e. it
        has FileInput, or False otherwise.
        """
        if self.forms:
            return self.forms[0].is_multipart()
        else:
            return self.empty_form.is_multipart()

    def is_valid(self):
        """Return True if every form in self.forms is valid."""
        if not self.is_bound:
            return False
        # We loop over every form.errors here rather than short circuiting on the
        # first failure to make sure validation gets triggered for every form.
        forms_valid = True
        # This triggers a full clean.
        self.errors
        for i in range(0, self.total_form_count()):
            form = self.forms[i]
            if self.can_delete and self._should_delete_form(form):
                # This form is going to be deleted so any of its errors
                # shouldn't cause the entire formset to be invalid.
                continue
            forms_valid &= form.is_valid()
        return forms_valid and not self.non_form_errors()

    def non_form_errors(self):
        """
        Return an ErrorList of errors that aren't associated with a particular
        form -- i.e., from formset.clean(). Return an empty ErrorList if there
        are none.
        """
        if self._non_form_errors is None:
            self.full_clean()
        return self._non_form_errors

    def total_error_count(self):
        """Return the number of errors across all forms in the formset."""
        return len(self.non_form_errors()) +\
            sum(len(form_errors) for form_errors in self.errors)

    def total_form_count(self):
        """Return the total number of forms in this FormSet."""
        if self.is_bound:
            # return absolute_max if it is lower than the actual total form
            # count in the data; this is DoS protection to prevent clients
            # from forcing the server to instantiate arbitrary numbers of
            # forms
            return min(self.management_form.cleaned_data[TOTAL_FORM_COUNT], self.absolute_max)
        else:
            initial_forms = self.initial_form_count()
            total_forms = max(initial_forms, self.min_num) + self.extra
            # Allow all existing related objects/inlines to be displayed,
            # but don't allow extra beyond max_num.
            if initial_forms > self.max_num >= 0:
                total_forms = initial_forms
            elif total_forms > self.max_num >= 0:
                total_forms = self.max_num
        return total_forms