Python网络编程：E-mail服务(五)深入理解email模块的message和MIME类

本文主要介绍Python标准库email的message和MIME类，并分析了相关的实现，能够是读者更好的使用email模块。

Message类是email的核心类， 它是email对象模型中基类，提供了设置和查询邮件头部，访问消息体的核心方法。从概念上讲，Message对象构成了邮件头部(Headers)和消息体(payloads)。头部格式在RFC 2822中进行了定义，每个头部由该项名字和值组成，并由冒号分割。消息体可以是简单消息对象的字符串或多个MIME容器的Message对象组成的多部分邮件。Message类在email.message模块中定义。

Message基类与MIME类的继承关系如下图所示：

MIMEBase作为MIME相关对象基类继承了Message，拥有拥有Message操作邮件头部和邮件体的所有函数。MIME在邮件头部增加了Content-Type和MIME-Version两个头部信息，从下面MIMEBase的实现中可以清楚的看到这一点：

class MIMEBase(message.Message):"""Base class for MIME specializations."""def __init__(self, _maintype, _subtype, **_params):"""This constructor adds a Content-Type: and a MIME-Version: header.The Content-Type: header is taken from the _maintype and _subtypearguments.  Additional parameters for this header are taken from thekeyword arguments."""message.Message.__init__(self)ctype = '%s/%s' % (_maintype, _subtype)self.add_header('Content-Type', ctype, **_params)self['MIME-Version'] = '1.0'

多部分邮件类MIMEMultipart类

MIMEMultipart类用于实现多部分邮件的功能，缺省情况下它会创建Content-Type类型为mulitpart/mixed邮件。

class MIMEMultipart(MIMEBase):"""Base class for MIME multipart/* type messages."""def __init__(self, _subtype='mixed', boundary=None, _subparts=None,**_params):"""Creates a multipart/* type message.By default, creates a multipart/mixed message, with properContent-Type and MIME-Version headers._subtype is the subtype of the multipart content type, defaulting to`mixed'.boundary is the multipart boundary string.  By default it iscalculated as needed._subparts is a sequence of initial subparts for the payload.  Itmust be an iterable object, such as a list.  You can alwaysattach new subparts to the message by using the attach() method.Additional parameters for the Content-Type header are taken from thekeyword arguments (or passed into the _params argument)."""MIMEBase.__init__(self, 'multipart', _subtype, **_params)# Initialise _payload to an empty list as the Message superclass's# implementation of is_multipart assumes that _payload is a list for# multipart messages.self._payload = []if _subparts:for p in _subparts:self.attach(p)if boundary:self.set_boundary(boundary)

在类初始化时，会将_payload初始化为空的列表，因为在Message超类中is_multipart方法假设_payload是一个列表，并用来存放多部分邮件内容，利用attach()方法可以将多部分邮件添加到列表中。这里来看一下超类Message中的相关实现：

class Message:......def is_multipart(self):"""Return True if the message consists of multiple parts."""return isinstance(self._payload, list)## Payload manipulation.#def attach(self, payload):"""Add the given payload to the current payload.The current payload will always be a list of objects after this methodis called.  If you want to set the payload to a scalar object, useset_payload() instead."""if self._payload is None:self._payload = [payload]else:self._payload.append(payload)......

非多部分邮件类MIMENonMultipart类

MIMENonMultipart类与MIMEMultipart类一样，继承自MIMEBase基类，其实现了自定义attach()方法，由于其是非多部分邮件类型实现，用户调用此方法，会抛出MutlipartConversionError异常。

class MIMENonMultipart(MIMEBase):"""Base class for MIME non-multipart type messages."""def attach(self, payload):# The public API prohibits attaching multiple subparts to MIMEBase# derived subtypes since none of them are, by definition, of content# type multipart/*raise errors.MultipartConversionError('Cannot attach additional subparts to non-multipart/*')

MIMENonMultipart类是其它具体MIMEApplication， MIMEText，MIMEImage等其它类的基类，也就是说它们不运行用户使用attach()方法。它们是通过set_payload()方法来实现设置邮件payload功能的。

class Message:......def set_payload(self, payload, charset=None):"""Set the payload to the given value.Optional charset sets the message's default character set.  Seeset_charset() for details."""self._payload = payloadif charset is not None:self.set_charset(charset)......

具体类实现

MIME模块提供了5个MIME具体类，各个具体类都提供了与名称对应的主消息类型的对象的支持，它们都继承了MIMENonMultipart类；关于MIME主类型的知识，可以参考 Python网络编程：E-mail服务(三)MIME详解。这里简单看一下MIMEText的实现：

class MIMEText(MIMENonMultipart):"""Class for generating text/* type MIME documents."""def __init__(self, _text, _subtype='plain', _charset='us-ascii'):"""Create a text/* type MIME document._text is the string for this message object._subtype is the MIME sub content type, defaulting to "plain"._charset is the character set parameter added to the Content-Typeheader.  This defaults to "us-ascii".  Note that as a side-effect, theContent-Transfer-Encoding header will also be set."""MIMENonMultipart.__init__(self, 'text', _subtype,**{'charset': _charset})self.set_payload(_text, _charset)

MIMEText在初始化时，将主类型设置为text类型，并通过set_payload()函数设置邮件体内容。

结合E-mail的核心类Message和MIME相关类的实现，可以更深入的了解通过email标准库编写邮件的机制，高效的实现编写邮件相关的代码。