Django Form源码分析之BaseForm验证逻辑

引言

在Django中，Form的主要功能分为输入验证以及在模板中的展示。
首先看一下Form的源码定义：

class Form(six.with_metaclass(DeclarativeFieldsMetaclass, BaseForm)):
    "A collection of Fields, plus their associated data."
    # This is a separate class from BaseForm in order to abstract the way
    # self.fields is specified. This class (Form) is the one that does the
    # fancy metaclass stuff purely for the semantic sugar -- it allows one
    # to define a form using declarative syntax.
    # BaseForm itself has no way of designating self.fields.

因为主要涉及的是BaseForm的验证逻辑分析，因此我在本篇博客中不会关心fields里面的数据如何从请求中得到，我会假设数据经过处理已经传达到了fields上面。（模板的输出格式规范也不会涉及太多）
同时，我会在引言中给出BaseForm的初始化，方便查阅。

def __init__(self, data=None, files=None, auto_id='id_%s', prefix=None,
                 initial=None, error_class=ErrorList, label_suffix=None,
                 empty_permitted=False, field_order=None):
        self.is_bound = data is not None or files is not None
        self.data = data or {}
        self.files = files or {}
        self.auto_id = auto_id
        if prefix is not None:
            self.prefix = prefix
        self.initial = initial or {}
        self.error_class = error_class
        # Translators: This is the default suffix added to form field labels
        self.label_suffix = label_suffix if label_suffix is not None else _(':')
        self.empty_permitted = empty_permitted
        self._errors = None  # Stores the errors after clean() has been called.

        # The base_fields class attribute is the *class-wide* definition of
        # fields. Because a particular *instance* of the class might want to
        # alter self.fields, we create self.fields here by copying base_fields.
        # Instances should always modify self.fields; they should not modify