class ModelForm

    def _clean_fields(self):
        for name, field in self.fields.items():
            # value_from_datadict() gets the data from the data dictionaries.
            # Each widget type knows how to retrieve its own data, because some
            # widgets split data over several HTML fields.
            if field.disabled:
                value = self.initial.get(name, field.initial)
            else:
                value = field.widget.value_from_datadict(self.data, self.files, self.add_prefix(name))
            try:
                if isinstance(field, FileField):
                    initial = self.initial.get(name, field.initial)
                    value = field.clean(value, initial)
                else:
                    value = field.clean(value)
                self.cleaned_data[name] = value
                if hasattr(self, 'clean_%s' % name):
                    value = getattr(self, 'clean_%s' % name)()
                    self.cleaned_data[name] = value
            except ValidationError as e:
                self.add_error(name, e)

    def _clean_form(self):
        try:
            cleaned_data = self.clean()
        except ValidationError as e:
            self.add_error(None, e)
        else:
            if cleaned_data is not None:
                self.cleaned_data = cleaned_data

    def _get_validation_exclusions(self):
        """
        For backwards-compatibility, several types of fields need to be
        excluded from model validation. See the following tickets for
        details: #12507, #12521, #12553
        """
        exclude = []
        # Build up a list of fields that should be excluded from model field
        # validation and unique checks.
        for f in self.instance._meta.fields:
            field = f.name
            # Exclude fields that aren't on the form. The developer may be
            # adding these values to the model after form validation.
            if field not in self.fields:
                exclude.append(f.name)

            # Don't perform model validation on fields that were defined
            # manually on the form and excluded via the ModelForm's Meta
            # class. See #12901.
            elif self._meta.fields and field not in self._meta.fields:
                exclude.append(f.name)
            elif self._meta.exclude and field in self._meta.exclude:
                exclude.append(f.name)

            # Exclude fields that failed form validation. There's no need for
            # the model fields to validate them as well.
            elif field in self._errors.keys():
                exclude.append(f.name)

            # Exclude empty fields that are not required by the form, if the
            # underlying model field is required. This keeps the model field
            # from raising a required error. Note: don't exclude the field from
            # validation if the model field allows blanks. If it does, the blank
            # value may be included in a unique check, so cannot be excluded
            # from validation.
            else:
                form_field = self.fields[field]
                field_value = self.cleaned_data.get(field)
                if not f.blank and not form_field.required and field_value in form_field.empty_values:
                    exclude.append(f.name)
        return exclude

    def _html_output(self, normal_row, error_row, row_ender, help_text_html, errors_on_separate_row):
        "Helper function for outputting HTML. Used by as_table(), as_ul(), as_p()."
        top_errors = self.non_field_errors()  # Errors that should be displayed above all fields.
        output, hidden_fields = [], []

        for name, field in self.fields.items():
            html_class_attr = ''
            bf = self[name]
            # Escape and cache in local variable.
            bf_errors = self.error_class([conditional_escape(error) for error in bf.errors])
            if bf.is_hidden:
                if bf_errors:
                    top_errors.extend(
                        [_('(Hidden field %(name)s) %(error)s') % {'name': name, 'error': force_text(e)}
                         for e in bf_errors])
                hidden_fields.append(six.text_type(bf))
            else:
                # Create a 'class="..."' attribute if the row should have any
                # CSS classes applied.
                css_classes = bf.css_classes()
                if css_classes:
                    html_class_attr = ' class="%s"' % css_classes

                if errors_on_separate_row and bf_errors:
                    output.append(error_row % force_text(bf_errors))

                if bf.label:
                    label = conditional_escape(force_text(bf.label))
                    label = bf.label_tag(label) or ''
                else:
                    label = ''

                if field.help_text:
                    help_text = help_text_html % force_text(field.help_text)
                else:
                    help_text = ''

                output.append(normal_row % {
                    'errors': force_text(bf_errors),
                    'label': force_text(label),
                    'field': six.text_type(bf),
                    'help_text': help_text,
                    'html_class_attr': html_class_attr,
                    'css_classes': css_classes,
                    'field_name': bf.html_name,
                })

        if top_errors:
            output.insert(0, error_row % force_text(top_errors))

        if hidden_fields:  # Insert any hidden fields in the last row.
            str_hidden = ''.join(hidden_fields)
            if output:
                last_row = output[-1]
                # Chop off the trailing row_ender (e.g. '</td></tr>') and
                # insert the hidden fields.
                if not last_row.endswith(row_ender):
                    # This can happen in the as_p() case (and possibly others
                    # that users write): if there are only top errors, we may
                    # not be able to conscript the last row for our purposes,
                    # so insert a new, empty row.
                    last_row = (normal_row % {
                        'errors': '',
                        'label': '',
                        'field': '',
                        'help_text': '',
                        'html_class_attr': html_class_attr,
                        'css_classes': '',
                        'field_name': '',
                    })
                    output.append(last_row)
                output[-1] = last_row[:-len(row_ender)] + str_hidden + row_ender
            else:
                # If there aren't any rows in the output, just append the
                # hidden fields.
                output.append(str_hidden)
        return mark_safe('\n'.join(output))

    def _save_m2m(self):
        """
        Save the many-to-many fields and generic relations for this form.
        """
        cleaned_data = self.cleaned_data
        exclude = self._meta.exclude
        fields = self._meta.fields
        opts = self.instance._meta
        # Note that for historical reasons we want to include also
        # virtual_fields here. (GenericRelation was previously a fake
        # m2m field).
        for f in chain(opts.many_to_many, opts.virtual_fields):
            if not hasattr(f, 'save_form_data'):
                continue
            if fields and f.name not in fields:
                continue
            if exclude and f.name in exclude:
                continue
            if f.name in cleaned_data:
                f.save_form_data(self.instance, cleaned_data[f.name])

    def _update_errors(self, errors):
        # Override any validation error messages defined at the model level
        # with those defined at the form level.
        opts = self._meta

        # Allow the model generated by construct_instance() to raise
        # ValidationError and have them handled in the same way as others.
        if hasattr(errors, 'error_dict'):
            error_dict = errors.error_dict
        else:
            error_dict = {NON_FIELD_ERRORS: errors}

        for field, messages in error_dict.items():
            if (field == NON_FIELD_ERRORS and opts.error_messages and
                    NON_FIELD_ERRORS in opts.error_messages):
                error_messages = opts.error_messages[NON_FIELD_ERRORS]
            elif field in self.fields:
                error_messages = self.fields[field].error_messages
            else:
                continue

            for message in messages:
                if (isinstance(message, ValidationError) and
                        message.code in error_messages):
                    message.message = error_messages[message.code]

        self.add_error(None, errors)

    def add_error(self, field, error):
        """
        Update the content of `self._errors`.

        The `field` argument is the name of the field to which the errors
        should be added. If its value is None the errors will be treated as
        NON_FIELD_ERRORS.

        The `error` argument can be a single error, a list of errors, or a
        dictionary that maps field names to lists of errors. What we define as
        an "error" can be either a simple string or an instance of
        ValidationError with its message attribute set and what we define as
        list or dictionary can be an actual `list` or `dict` or an instance
        of ValidationError with its `error_list` or `error_dict` attribute set.

        If `error` is a dictionary, the `field` argument *must* be None and
        errors will be added to the fields that correspond to the keys of the
        dictionary.
        """
        if not isinstance(error, ValidationError):
            # Normalize to ValidationError and let its constructor
            # do the hard work of making sense of the input.
            error = ValidationError(error)

        if hasattr(error, 'error_dict'):
            if field is not None:
                raise TypeError(
                    "The argument `field` must be `None` when the `error` "
                    "argument contains errors for multiple fields."
                )
            else:
                error = error.error_dict
        else:
            error = {field or NON_FIELD_ERRORS: error.error_list}

        for field, error_list in error.items():
            if field not in self.errors:
                if field != NON_FIELD_ERRORS and field not in self.fields:
                    raise ValueError(
                        "'%s' has no field named '%s'." % (self.__class__.__name__, field))
                if field == NON_FIELD_ERRORS:
                    self._errors[field] = self.error_class(error_class='nonfield')
                else:
                    self._errors[field] = self.error_class()
            self._errors[field].extend(error_list)
            if field in self.cleaned_data:
                del self.cleaned_data[field]

    def add_initial_prefix(self, field_name):
        """
        Add a 'initial' prefix for checking dynamic initial values
        """
        return 'initial-%s' % self.add_prefix(field_name)

    def add_prefix(self, field_name):
        """
        Returns the field name with a prefix appended, if this Form has a
        prefix set.

        Subclasses may wish to override.
        """
        return '%s-%s' % (self.prefix, field_name) if self.prefix else field_name

    def as_p(self):
        "Returns this form rendered as HTML <p>s."
        return self._html_output(
            normal_row='<p%(html_class_attr)s>%(label)s %(field)s%(help_text)s</p>',
            error_row='%s',
            row_ender='</p>',
            help_text_html=' <span class="helptext">%s</span>',
            errors_on_separate_row=True)

    def as_table(self):
        "Returns this form rendered as HTML <tr>s -- excluding the <table></table>."
        return self._html_output(
            normal_row='<tr%(html_class_attr)s><th>%(label)s</th><td>%(errors)s%(field)s%(help_text)s</td></tr>',
            error_row='<tr><td colspan="2">%s</td></tr>',
            row_ender='</td></tr>',
            help_text_html='<br /><span class="helptext">%s</span>',
            errors_on_separate_row=False)

    def as_ul(self):
        "Returns this form rendered as HTML <li>s -- excluding the <ul></ul>."
        return self._html_output(
            normal_row='<li%(html_class_attr)s>%(errors)s%(label)s %(field)s%(help_text)s</li>',
            error_row='<li>%s</li>',
            row_ender='</li>',
            help_text_html=' <span class="helptext">%s</span>',
            errors_on_separate_row=False)

    def full_clean(self):
        """
        Cleans all of self.data and populates self._errors and
        self.cleaned_data.
        """
        self._errors = ErrorDict()
        if not self.is_bound:  # Stop further processing.
            return
        self.cleaned_data = {}
        # If the form is permitted to be empty, and none of the form data has
        # changed from the initial data, short circuit any validation.
        if self.empty_permitted and not self.has_changed():
            return

        self._clean_fields()
        self._clean_form()
        self._post_clean()

    def has_changed(self):
        """
        Returns True if data differs from initial.
        """
        return bool(self.changed_data)

    def has_error(self, field, code=None):
        if code is None:
            return field in self.errors
        if field in self.errors:
            for error in self.errors.as_data()[field]:
                if error.code == code:
                    return True
        return False

    def hidden_fields(self):
        """
        Returns a list of all the BoundField objects that are hidden fields.
        Useful for manual form layout in templates.
        """
        return [field for field in self if field.is_hidden]

    def is_multipart(self):
        """
        Returns True if the form needs to be multipart-encoded, i.e. it has
        FileInput. Otherwise, False.
        """
        for field in self.fields.values():
            if field.widget.needs_multipart_form:
                return True
        return False

    def is_valid(self):
        """
        Returns True if the form has no errors. Otherwise, False. If errors are
        being ignored, returns False.
        """
        return self.is_bound and not self.errors

    def non_field_errors(self):
        """
        Returns an ErrorList of errors that aren't associated with a particular
        field -- i.e., from Form.clean(). Returns an empty ErrorList if there
        are none.
        """
        return self.errors.get(NON_FIELD_ERRORS, self.error_class(error_class='nonfield'))

    def order_fields(self, field_order):
        """
        Rearranges the fields according to field_order.

        field_order is a list of field names specifying the order. Fields not
        included in the list are appended in the default order for backward
        compatibility with subclasses not overriding field_order. If field_order
        is None, all fields are kept in the order defined in the class.
        Unknown fields in field_order are ignored to allow disabling fields in
        form subclasses without redefining ordering.
        """
        if field_order is None:
            return
        fields = OrderedDict()
        for key in field_order:
            try:
                fields[key] = self.fields.pop(key)
            except KeyError:  # ignore unknown fields
                pass
        fields.update(self.fields)  # add remaining fields in original order
        self.fields = fields

    def save(self, commit=True):
        """
        Save this form's self.instance object if commit=True. Otherwise, add
        a save_m2m() method to the form which can be called after the instance
        is saved manually at a later time. Return the model instance.
        """
        if self.errors:
            raise ValueError(
                "The %s could not be %s because the data didn't validate." % (
                    self.instance._meta.object_name,
                    'created' if self.instance._state.adding else 'changed',
                )
            )
        if commit:
            # If committing, save the instance and the m2m data immediately.
            self.instance.save()
            self._save_m2m()
        else:
            # If not committing, add a method to the form to allow deferred
            # saving of m2m data.
            self.save_m2m = self._save_m2m
        return self.instance

    def validate_unique(self):
        """
        Calls the instance's validate_unique() method and updates the form's
        validation errors if any were raised.
        """
        exclude = self._get_validation_exclusions()
        try:
            self.instance.validate_unique(exclude=exclude)
        except ValidationError as e:
            self._update_errors(e)

    def visible_fields(self):
        """
        Returns a list of BoundField objects that aren't hidden fields.
        The opposite of the hidden_fields() method.
        """
        return [field for field in self if not field.is_hidden]