class Form

    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 _post_clean(self):
        """
        An internal hook for performing additional cleaning after form cleaning
        is complete. Used for model validation in model forms.
        """
        pass

    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 clean(self):
        """
        Hook for doing any extra form-wide cleaning after Field.clean() has been
        called on every field. Any ValidationError raised by this method will
        not be associated with a particular field; it will have a special-case
        association with the field named '__all__'.
        """
        return self.cleaned_data

    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 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]