django: is_valid解析

is_valid方法属于BaseForm类，返回self.is_bound和not self.errors。

@html_safe
class BaseForm:
    """
    The main implementation of all the Form logic. Note that this class is
    different than Form. See the comments by the Form class for more info. Any
    improvements to the form API should be made to this class, not to the Form
    class.
    """
    default_renderer = None
    field_order = None
    prefix = None
    use_required_attribute = True

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

self.is_bound方法中，data不为空或者files不为空即为真

self.is_bound = data is not None or files is not None

@property
def errors(self):
    """Return an ErrorDict for the data provided for the form."""
    if self._errors is None:
        self.full_clean()
    return self._errors

self._errors = None  # Stores the errors after clean() has been called.

def full_clean(self):
    """
    Clean all of self.data and populate 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 _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.get_initial_for_field(field, name)
        else:
            value = field.widget.value_from_datadict(self.data, self.files, self.add_prefix(name))
        try:
            if isinstance(field, FileField):
                initial = self.get_initial_for_field(field, name)
                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 _post_clean(self):
    """
    An internal hook for performing additional cleaning after form cleaning
    is complete. Used for model validation in model forms.
    """
    pass

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