类属性及对象属性

class Field(object):
    widget = TextInput  # Default widget to use when rendering this type of Field.
    hidden_widget = HiddenInput  # Default widget to use when rendering this as "hidden".
    default_validators = []  # Default set of validators
    # Add an 'invalid' entry to default_error_message if you want a specific
    # field error message not raised by the field validators.
    default_error_messages = {
        'required': _('This field is required.'),
    }
    empty_values = list(validators.EMPTY_VALUES)

    # Tracks each time a Field instance is created. Used to retain order.
    creation_counter = 0

    def __init__(self, required=True, widget=None, label=None, initial=None,
                 help_text='', error_messages=None, show_hidden_initial=False,
                 validators=(), localize=False, disabled=False, label_suffix=None):
        # required -- Boolean that specifies whether the field is required.
        #             True by default.
        # widget -- A Widget class, or instance of a Widget class, that should
        #           be used for this Field when displaying it. Each Field has a
        #           default Widget that it'll use if you don't specify this. In
        #           most cases, the default widget is TextInput.
        # label -- A verbose name for this field, for use in displaying this
        #          field in a form. By default, Django will use a "pretty"
        #          version of the form field name, if the Field is part of a
        #          Form.
        # initial -- A value to use in this Field's initial display. This value
        #            is *not* used as a fallback if data isn't given.
        # help_text -- An optional string to use as "help text" for this Field.
        # error_messages -- An optional dictionary to override the default
        #                   messages that the field will raise.
        # show_hidden_initial -- Boolean that specifies if it is needed to render a
        #                        hidden widget with initial value after widget.
        # validators -- List of additional validators to use
        # localize -- Boolean that specifies if the field should be localized.
        # disabled -- Boolean that specifies whether the field is disabled, that
        #             is its widget is shown in the form but not editable.
        # label_suffix -- Suffix to be added to the label. Overrides
        #                 form's label_suffix.
        self.required, self.label, self.initial = required, label, initial
        self.show_hidden_initial = show_hidden_initial
        self.help_text = help_text
        self.disabled = disabled
        self.label_suffix = label_suffix
        widget = widget or self.widget
        if isinstance(widget, type):
            widget = widget()
        else:
            widget = copy.deepcopy(widget)

        # Trigger the localization machinery if needed.
        self.localize = localize
        if self.localize:
            widget.is_localized = True

        # Let the widget know whether it should display as required.
        widget.is_required = self.required

        # Hook into self.widget_attrs() for any Field-specific HTML attributes.
        extra_attrs = self.widget_attrs(widget)
        if extra_attrs:
            widget.attrs.update(extra_attrs)

        self.widget = widget

        # Increase the creation counter, and save our local copy.
        self.creation_counter = Field.creation_counter
        Field.creation_counter += 1

        messages = {}
        for c in reversed(self.__class__.__mro__):
            messages.update(getattr(c, 'default_error_messages', {}))
        messages.update(error_messages or {})
        self.error_messages = messages

        self.validators = list(itertools.chain(self.default_validators, validators))

        super(Field, self).__init__()