# documentação Django 2.0.8

# Código fonte para django.forms.boundfield

    import datetime
    import warnings

    from django.forms.utils import flatatt, pretty_name
    from django.forms.widgets import Textarea, TextInput
    from django.utils.deprecation import RemovedInDjango21Warning
    from django.utils.functional import cached_property
    from django.utils.html import conditional_escape, format_html, html_safe
    from django.utils.inspect import func_accepts_kwargs, func_supports_parameter
    from django.utils.safestring import mark_safe
    from django.utils.translation import gettext_lazy as _

    __all__ = ('BoundField',)

    @html_safe
    class BoundField:
        "A Field plus data"
        def __init__(self, form, field, name):
            self.form = form
            self.field = field
            self.name = name
            self.html_name = form.add_prefix(name)
            self.html_initial_name = form.add_initial_prefix(name)
            self.html_initial_id = form.add_initial_prefix(self.auto_id)
            if self.field.label is None:
                self.label = pretty_name(name)
            else:
                self.label = self.field.label
            self.help_text = field.help_text or ''

        def __str__(self):
            """Render this field as an HTML widget."""
            if self.field.show_hidden_initial:
                return self.as_widget() + self.as_hidden(only_initial=True)
            return self.as_widget()

        @cached_property
        def subwidgets(self):
            """
            Most widgets yield a single subwidget, but others like RadioSelect and
            CheckboxSelectMultiple produce one subwidget for each choice.

            This property is cached so that only one database query occurs when
            rendering ModelChoiceFields.
            """
            id_ = self.field.widget.attrs.get('id') or self.auto_id
            attrs = {'id': id_} if id_ else {}
            attrs = self.build_widget_attrs(attrs)
            return [
                BoundWidget(self.field.widget, widget, self.form.renderer)
                for widget in self.field.widget.subwidgets(self.html_name, self.value(), attrs=attrs)
            ]

        def __bool__(self):
            # BoundField evaluates to True even if it doesn't have subwidgets.
            return True

        def __iter__(self):
            return iter(self.subwidgets)

        def __len__(self):
            return len(self.subwidgets)

        def __getitem__(self, idx):
            # Prevent unnecessary reevaluation when accessing BoundField's attrs
            # from templates.
            if not isinstance(idx, (int, slice)):
                raise TypeError
            return self.subwidgets[idx]

        @property
        def errors(self):
            """
            Return an ErrorList (empty if there are no errors) for this field.
            """
            return self.form.errors.get(self.name, self.form.error_class())

        def as_widget(self, widget=None, attrs=None, only_initial=False):
            """
            Render the field by rendering the passed widget, adding any HTML
            attributes passed as attrs. If a widget isn't specified, use the
            field's default widget.
            """
            if not widget:
                widget = self.field.widget

            if self.field.localize:
                widget.is_localized = True

            attrs = attrs or {}
            attrs = self.build_widget_attrs(attrs, widget)
            auto_id = self.auto_id
            if auto_id and 'id' not in attrs and 'id' not in widget.attrs:
                if not only_initial:
                    attrs['id'] = auto_id
                else:
                    attrs['id'] = self.html_initial_id

            if not only_initial:
                name = self.html_name
            else:
                name = self.html_initial_name

            kwargs = {}
            if func_supports_parameter(widget.render, 'renderer') or func_accepts_kwargs(widget.render):
                kwargs['renderer'] = self.form.renderer
            else:
                warnings.warn(
                    'Add the `renderer` argument to the render() method of %s. '
                    'It will be mandatory in Django 2.1.' % widget.__class__,
                    RemovedInDjango21Warning, stacklevel=2,
                )
            return widget.render(
                name=name,
                value=self.value(),
                attrs=attrs,
                **kwargs
            )

        def as_text(self, attrs=None, **kwargs):
            """
            Return a string of HTML for representing this as an <input type="text">.
            """
            return self.as_widget(TextInput(), attrs, **kwargs)

        def as_textarea(self, attrs=None, **kwargs):
            """Return a string of HTML for representing this as a <textarea>."""
            return self.as_widget(Textarea(), attrs, **kwargs)

        def as_hidden(self, attrs=None, **kwargs):
            """
            Return a string of HTML for representing this as an <input type="hidden">.
            """
            return self.as_widget(self.field.hidden_widget(), attrs, **kwargs)

        @property
        def data(self):
            """
            Return the data for this BoundField, or None if it wasn't given.
            """
            return self.field.widget.value_from_datadict(self.form.data, self.form.files, self.html_name)

        def value(self):
            """
            Return the value for this BoundField, using the initial value if
            the form is not bound or the data otherwise.
            """
            data = self.initial
            if self.form.is_bound:
                data = self.field.bound_data(self.data, data)
            return self.field.prepare_value(data)

        def label_tag(self, contents=None, attrs=None, label_suffix=None):
            """
            Wrap the given contents in a <label>, if the field has an ID attribute.
            contents should be mark_safe'd to avoid HTML escaping. If contents
            aren't given, use the field's HTML-escaped label.

            If attrs are given, use them as HTML attributes on the <label> tag.

            label_suffix overrides the form's label_suffix.
            """
            contents = contents or self.label
            if label_suffix is None:
                label_suffix = (self.field.label_suffix if self.field.label_suffix is not None
                                else self.form.label_suffix)
            # Only add the suffix if the label does not end in punctuation.
            # Translators: If found as last label character, these punctuation
            # characters will prevent the default label_suffix to be appended to the label
            if label_suffix and contents and contents[-1] not in _(':?.!'):
                contents = format_html('{}{}', contents, label_suffix)
            widget = self.field.widget
            id_ = widget.attrs.get('id') or self.auto_id
            if id_:
                id_for_label = widget.id_for_label(id_)
                if id_for_label:
                    attrs = dict(attrs or {}, **{'for': id_for_label})
                if self.field.required and hasattr(self.form, 'required_css_class'):
                    attrs = attrs or {}
                    if 'class' in attrs:
                        attrs['class'] += ' ' + self.form.required_css_class
                    else:
                        attrs['class'] = self.form.required_css_class
                attrs = flatatt(attrs) if attrs else ''
                contents = format_html('<label{}>{}</label>', attrs, contents)
            else:
                contents = conditional_escape(contents)
            return mark_safe(contents)

        def css_classes(self, extra_classes=None):
            """
            Return a string of space-separated CSS classes for this field.
            """
            if hasattr(extra_classes, 'split'):
                extra_classes = extra_classes.split()
            extra_classes = set(extra_classes or [])
            if self.errors and hasattr(self.form, 'error_css_class'):
                extra_classes.add(self.form.error_css_class)
            if self.field.required and hasattr(self.form, 'required_css_class'):
                extra_classes.add(self.form.required_css_class)
            return ' '.join(extra_classes)

        @property
        def is_hidden(self):
            """Return True if this BoundField's widget is hidden."""
            return self.field.widget.is_hidden

        @property
        def auto_id(self):
            """
            Calculate and return the ID attribute for this BoundField, if the
            associated Form has specified auto_id. Return an empty string otherwise.
            """
            auto_id = self.form.auto_id  # Boolean or string
            if auto_id and '%s' in str(auto_id):
                return auto_id % self.html_name
            elif auto_id:
                return self.html_name
            return ''

        @property
        def id_for_label(self):
            """
            Wrapper around the field widget's `id_for_label` method.
            Useful, for example, for focusing on this field regardless of whether
            it has a single widget or a MultiWidget.
            """
            widget = self.field.widget
            id_ = widget.attrs.get('id') or self.auto_id
            return widget.id_for_label(id_)

        @cached_property
        def initial(self):
            data = self.form.get_initial_for_field(self.field, self.name)
            # If this is an auto-generated default date, nix the microseconds for
            # standardized handling. See #22502.
            if (isinstance(data, (datetime.datetime, datetime.time)) and
                    not self.field.widget.supports_microseconds):
                data = data.replace(microsecond=0)
            return data

        def build_widget_attrs(self, attrs, widget=None):
            if not widget:
                widget = self.field.widget
            attrs = dict(attrs)  # Copy attrs to avoid modifying the argument.
            if widget.use_required_attribute(self.initial) and self.field.required and self.form.use_required_attribute:
                attrs['required'] = True
            if self.field.disabled:
                attrs['disabled'] = True
            return attrs

    @html_safe
    class BoundWidget:
        """
        A container class used for iterating over widgets. This is useful for
        widgets that have choices. For example, the following can be used in a
        template:

        {% for radio in myform.beatles %}
          <label for="{{ radio.id_for_label }}">
            {{ radio.choice_label }}
            <span class="radio">{{ radio.tag }}</span>
          </label>
        {% endfor %}
        """
        def __init__(self, parent_widget, data, renderer):
            self.parent_widget = parent_widget
            self.data = data
            self.renderer = renderer

        def __str__(self):
            return self.tag(wrap_label=True)

        def tag(self, wrap_label=False):
            context = {'widget': self.data, 'wrap_label': wrap_label}
            return self.parent_widget._render(self.template_name, context, self.renderer)

        @property
        def template_name(self):
            if 'template_name' in self.data:
                return self.data['template_name']
            return self.parent_widget.template_name

        @property
        def id_for_label(self):
            return 'id_%s_%s' % (self.data['name'], self.data['index'])

        @property
        def choice_label(self):
            return self.data['label']