flask 源码专题（四）：wtforms Form实例化流程以及csrf验证

class LoginForm(Form):
    #首先执行后得到的结果是UnboundField()对象
    name=simple.StringField(
        label='用户名',
        validators=[
            validators.DataRequired(message='用户名不能为空'),
        ],
        widget=widgets.TextInput(),
        render_kw={'class': 'form-control'}
    )

    pwd=simple.StringField(
        label='密码',
        validators=[
            validators.DataRequired(message='密码不能为空'),
        ],
        widget=widgets.TextInput(),
        render_kw={'class': 'form-control'}
    )


@user.route('/login',methods=['GET','POST'])
def login():
    if request.method=='GET':
        form=LoginForm()
        print(form)
        return render_template('login.html',form=form)
    else:
        form=LoginForm(request.form)
        if form.validate():

实例化流程

1.执行Field中的__new__方法

我们还没执行到form=LoginForm()时，LoginForm里面所有的字段都已经执行加载完了，里面的字段的值都是Field实例化而来

查看上述 LoginForm 中的 name 字段，可以看到它的值是通过 StringField 类实例化返回的对象。查看 StringField 类，会发现它的整个继承体系中都没有指定 metaclass ，所以实例化时真正返回的对象是由它的 __new__ ，它的 __new__ 方法继承自 wtforms.fields.core.Field ：

 def __new__(cls, *args, **kwargs):
        if '_form' in kwargs and '_name' in kwargs:
            return super(Field, cls).__new__(cls)
        else:
            return UnboundField(cls, *args, **kwargs)

可以知道开始的时候所有的Field对象都是UnboundField()对象，我们所写的Filed实例实际开始是这样的(注释)

class LoginForm(Form):
    # name = UnboundField(StringField, *args, **kwargs) creation_counter=1
    name = simple.StringField(
        label='用户名',
        validators=[
            validators.DataRequired(message='用户名不能为空.'),
            validators.Length(min=6, max=18, message='用户名长度必须大于%(min)d且小于%(max)d')
        ],
        widget=widgets.TextInput(),
        render_kw={'class': 'form-control'}
    )
    # pwd = UnboundField(PasswordField, *args, **kwargs) creation_counter=2
    pwd = simple.PasswordField(
        label='密码',
        validators=[
            validators.DataRequired(message='密码不能为空.'),
            validators.Length(min=8, message='用户名长度必须大于%(min)d'),
            validators.Regexp(regex="^(?=.*[a-z])(?=.*[A-Z])(?=.*d)(?=.*[$@$!%*?&])[A-Za-zd$@$!%*?&]{8,}",
                              message='密码至少8个字符，至少1个大写字母，1个小写字母，1个数字和1个特殊字符')
        ],
        widget=widgets.PasswordInput(),
        render_kw={'class': 'form-control'}
    )

2.执行FormMeta的__call__()方法，读取字段到静态字段 cls._unbound_fields 中； meta类读取到cls._wtforms_meta中

如果一个A类有metaclass，那么该A类创建的时候会执行她的metaclass类中的__init__()方法，实例话这个A类回执行 metaclass中的__call__()方法:

class Form(with_metaclass(FormMeta, BaseForm)):

FormMeta的__call__()方法

def __call__(cls, *args, **kwargs):
        """
        Construct a new `Form` instance.

        Creates the `_unbound_fields` list and the internal `_wtforms_meta`
        subclass of the class Meta in order to allow a proper inheritance
        hierarchy.
        """
        if cls._unbound_fields is None:
            fields = []
            #当前类所有的属性
            for name in dir(cls):
                if not name.startswith('_'):
                    #得到UnboundField()对象
                    unbound_field = getattr(cls, name)
                    #UnboundField()对象默认_formfield为True
                    if hasattr(unbound_field, '_formfield'):
                        fields.append((name, unbound_field))
            # We keep the name as the second element of the sort
            # to ensure a stable sort.
            #根据UnboundField()对象的.creation_counter进行排序
            fields.sort(key=lambda x: (x[1].creation_counter, x[0]))
            #fields=[('name',UnboundField()),('pwd',UnboundField())]
            cls._unbound_fields = fields

        # Create a subclass of the 'class Meta' using all the ancestors.
        if cls._wtforms_meta is None:
            bases = []
            #__mro__代表该类的继承关系
            for mro_class in cls.__mro__:
                if 'Meta' in mro_class.__dict__:
                    bases.append(mro_class.Meta)
            cls._wtforms_meta = type('Meta', tuple(bases), {})
        return type.__call__(cls, *args, **kwargs)

3.执行Form类的构造方法:

def __init__(self, formdata=None, obj=None, prefix='', data=None, meta=None, **kwargs):
        """
        :param formdata:
            Used to pass data coming from the enduser, usually `request.POST` or
            equivalent. formdata should be some sort of request-data wrapper which
            can get multiple parameters from the form input, and values are unicode
            strings, e.g. a Werkzeug/Django/WebOb MultiDict
        :param obj:
            If `formdata` is empty or not provided, this object is checked for
            attributes matching form field names, which will be used for field
            values.
        :param prefix:
            If provided, all fields will have their name prefixed with the
            value.
        :param data:
            Accept a dictionary of data. This is only used if `formdata` and
            `obj` are not present.
        :param meta:
            If provided, this is a dictionary of values to override attributes
            on this form's meta instance.
        :param `**kwargs`:
            If `formdata` is empty or not provided and `obj` does not contain
            an attribute named the same as a field, form will assign the value
            of a matching keyword argument to the field, if one exists.
        """
        meta_obj = self._wtforms_meta()
        if meta is not None and isinstance(meta, dict):
            meta_obj.update_values(meta)
        super(Form, self).__init__(self._unbound_fields, meta=meta_obj, prefix=prefix)#baseForm中的__init__()

        for name, field in iteritems(self._fields):
            # Set all the fields to attributes so that they obscure the class
            # attributes with the same names.
            setattr(self, name, field)#在继续回到Form的构造方法中循环_fields，为对象设置属性
        self.process(formdata, obj, data=data, **kwargs)

 super(Form, self).__init__(self._unbound_fields, meta=meta_obj, prefix=prefix)

a.先执行 baseForm中的__init__()

def __init__(self, fields, prefix='', meta=DefaultMeta()):
        """
        :param fields:
            A dict or sequence of 2-tuples of partially-constructed fields.
        :param prefix:
            If provided, all fields will have their name prefixed with the
            value.
        :param meta:
            A meta instance which is used for configuration and customization
            of WTForms behaviors.
        """
        if prefix and prefix[-1] not in '-_;:/.':
            prefix += '-'

        self.meta = meta
        self._prefix = prefix
        self._errors = None
        self._fields = OrderedDict()

        if hasattr(fields, 'items'):
            fields = fields.items()

        translations = self._get_translations()
        extra_fields = []
        if meta.csrf:
            self._csrf = meta.build_csrf(self)
            extra_fields.extend(self._csrf.setup_form(self))

        for name, unbound_field in itertools.chain(fields, extra_fields):
            options = dict(name=name, prefix=prefix, translations=translations)
            field = meta.bind_field(self, unbound_field, options)#从中执行UnboundField中的bind()方法：
            self._fields[name] = field #将上一行代码返回值添加到 self._fields[name] 中

  #将fields和extra_fields链接起来
        for name, unbound_field in itertools.chain(fields, extra_fields):
            options = dict(name=name, prefix=prefix, translations=translations)
            field = meta.bind_field(self, unbound_field, options)
            self._fields[name] = field

b.执行UnboundField中的bind()方法：

class UnboundField(object):
    _formfield = True
    creation_counter = 0

    def __init__(self, field_class, *args, **kwargs):
        UnboundField.creation_counter += 1
        self.field_class = field_class
        self.args = args
        self.kwargs = kwargs
        self.creation_counter = UnboundField.creation_counter

    def bind(self, form, name, prefix='', translations=None, **kwargs):
        kw = dict(
            self.kwargs,
            _form=form,
            _prefix=prefix,
            _name=name,
            _translations=translations,
            **kwargs
        )
        return self.field_class(*self.args, **kw)

    def __repr__(self):
        return '<UnboundField(%s, %r, %r)>' % (self.field_class.__name__, self.args, self.kwargs)

在bind方法中我们可以看到，由于字段中的__new__方法，实例化时：name = simple.StringField(label='用户名')，创建的是UnboundField(cls, *args, **kwargs)，当执行完bind之后，就变成执行 wtforms.fields.core.StringField()，

c.再回到BaseForm中的__init__()中，将返回值添加到 self._fields[name] 中，既:

 _fields = {
                    name: wtforms.fields.core.StringField(),
                }

d.执行玩BaseForm的__init__()后，在继续回到Form的构造方法中循环_fields，为对象设置属性

 for name, field in iteritems(self._fields):
            # Set all the fields to attributes so that they obscure the class
            # attributes with the same names.
            setattr(self, name, field)

e.执行process，为字段设置默认值：self.process(formdata, obj, data=data, **kwargs),再循环执行每个字段的process方法，为每个字段设置值：

for name, field, in iteritems(self._fields):
                if obj is not None and hasattr(obj, name):
                    field.process(formdata, getattr(obj, name))
                elif name in kwargs:
                    field.process(formdata, kwargs[name])
                else:
                    field.process(formdata)

f.执行每个字段的process方法，为字段的data和字段的raw_data赋值

Field的process

def process(self, formdata, data=unset_value):
        """
        Process incoming data, calling process_data, process_formdata as needed,
        and run filters.

        If `data` is not provided, process_data will be called on the field's
        default.

        Field subclasses usually won't override this, instead overriding the
        process_formdata and process_data methods. Only override this for
        special advanced processing, such as when a field encapsulates many
        inputs.
        """
        self.process_errors = []
        if data is unset_value:
            try:
                data = self.default()
            except TypeError:
                data = self.default

        self.object_data = data

        try:
            self.process_data(data)
        except ValueError as e:
            self.process_errors.append(e.args[0])

        if formdata:
            try:
                if self.name in formdata:
                    self.raw_data = formdata.getlist(self.name)
                else:
                    self.raw_data = []
                self.process_formdata(self.raw_data)
            except ValueError as e:
                self.process_errors.append(e.args[0])

        try:
            for filter in self.filters:
                self.data = filter(self.data)
        except ValueError as e:
            self.process_errors.append(e.args[0])

 4.验证流程

a. 执行form的validate方法，获取钩子方法
            def validate(self):
                extra = {}
                for name in self._fields:
                    inline = getattr(self.__class__, 'validate_%s' % name, None)
                    if inline is not None:
                        extra[name] = [inline]
        
                return super(Form, self).validate(extra)
        b. 循环每一个字段，执行字段的 validate 方法进行校验(参数传递了钩子函数)
            def validate(self, extra_validators=None):
                self._errors = None
                success = True
                for name, field in iteritems(self._fields):
                    if extra_validators is not None and name in extra_validators:
                        extra = extra_validators[name]
                    else:
                        extra = tuple()
                    if not field.validate(self, extra):
                        success = False
                return success
        c. 每个字段进行验证时候
            字段的pre_validate 【预留的扩展】
            字段的_run_validation_chain，对正则和字段的钩子函数进行校验
            字段的post_validate【预留的扩展】