class ModelForm

    def _bound_items(self):
        """Yield (name, bf) pairs, where bf is a BoundField object."""
        for name in self.fields:
            yield name, self[name]

    def _clean_fields(self):
        for name, bf in self._bound_items():
            field = bf.field
            try:
                self.cleaned_data[name] = field._clean_bound_field(bf)
                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, exclude several types of fields from model
        validation. See tickets #12507, #12521, #12553.
        """
        exclude = set()
        # 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.add(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.add(f.name)
            elif self._meta.exclude and field in self._meta.exclude:
                exclude.add(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:
                exclude.add(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.add(f.name)
        return exclude

    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
        # private_fields here. (GenericRelation was previously a fake
        # m2m field).
        for f in chain(opts.many_to_many, opts.private_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 _widget_data_value(self, widget, html_name):
        # 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.
        return widget.value_from_datadict(self.data, self.files, html_name)

    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 it's None, treat the errors 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. An "error" can be
        either a simple string or an instance of ValidationError with its
        message attribute set and a "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", renderer=self.renderer
                    )
                else:
                    self._errors[field] = self.error_class(
                        renderer=self.renderer,
                        field_id=self[field].auto_id,
                    )
            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 an 'initial' prefix for checking dynamic initial values."""
        return "initial-%s" % self.add_prefix(field_name)

    def add_prefix(self, field_name):
        """
        Return 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_div(self):
        """Render as <div> elements."""
        return self.render(self.template_name_div)

    def as_p(self):
        """Render as <p> elements."""
        return self.render(self.template_name_p)

    def as_table(self):
        """Render as <tr> elements excluding the surrounding <table> tag."""
        return self.render(self.template_name_table)

    def as_ul(self):
        """Render as <li> elements excluding the surrounding <ul> tag."""
        return self.render(self.template_name_ul)

    def full_clean(self):
        """
        Clean all of self.data and populate self._errors and self.cleaned_data.
        """
        self._errors = ErrorDict(renderer=self.renderer)
        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 get_initial_for_field(self, field, field_name):
        """
        Return initial data for field on form. Use initial data from the form
        or the field, in that order. Evaluate callable values.
        """
        value = self.initial.get(field_name, field.initial)
        if callable(value):
            value = value()
        # If this is an auto-generated default date, nix the microseconds
        # for standardized handling. See #22502.
        if (
            isinstance(value, (datetime.datetime, datetime.time))
            and not field.widget.supports_microseconds
        ):
            value = value.replace(microsecond=0)
        return value

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

    def has_error(self, field, code=None):
        return field in self.errors and (
            code is None
            or any(error.code == code for error in self.errors.as_data()[field])
        )

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

    def is_valid(self):
        """Return True if the form has no errors, or False otherwise."""
        return self.is_bound and not self.errors

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

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

        field_order is a list of field names specifying the order. Append
        fields not included in the list in the default order for backward
        compatibility with subclasses not overriding field_order. If
        field_order is None, keep all fields in the order defined in the class.
        Ignore unknown fields in field_order to allow disabling fields in form
        subclasses without redefining ordering.
        """
        if field_order is None:
            return
        fields = {}
        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 render(self, template_name=None, context=None, renderer=None):
        renderer = renderer or self.renderer
        template = template_name or self.template_name
        context = context or self.get_context()
        return mark_safe(renderer.render(template, context))

    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_constraints(self):
        """
        Call the instance's validate_constraints() method and update the form's
        validation errors if any were raised.
        """
        exclude = self._get_validation_exclusions()
        try:
            self.instance.validate_constraints(exclude=exclude)
        except ValidationError as e:
            self._update_errors(e)

    def validate_unique(self):
        """
        Call the instance's validate_unique() method and update 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):
        """
        Return 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]