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



http://blog.youkuaiyun.com/jinguangliu/article/details/45272863

简介

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。

核心类Message

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基类

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：

[python] view plain copy print ?

1. class MIMEBase(message.Message):  
2.     """Base class for MIME specializations."""  
3.   
4.     def __init__(self, _maintype, _subtype, **_params):  
5.         """This constructor adds a Content-Type: and a MIME-Version: header. 
6.  
7.         The Content-Type: header is taken from the _maintype and _subtype 
8.         arguments.  Additional parameters for this header are taken from the 
9.         keyword arguments. 
10.         """  
11.         message.Message.__init__(self)  
12.         ctype = '%s/%s' % (_maintype, _subtype)  
13.         self.add_header('Content-Type', ctype, **_params)  
14.         self['MIME-Version'] = '1.0'  

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 _subtype
        arguments.  Additional parameters for this header are taken from the
        keyword 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。

[python] view plain copy print ?

1. class MIMEMultipart(MIMEBase):  
2.     """Base class for MIME multipart/* type messages."""  
3.   
4.     def __init__(self, _subtype='mixed', boundary=None, _subparts=None,  
5.                  **_params):  
6.         """Creates a multipart/* type message. 
7.  
8.         By default, creates a multipart/mixed message, with proper 
9.         Content-Type and MIME-Version headers. 
10.  
11.         _subtype is the subtype of the multipart content type, defaulting to 
12.         `mixed'. 
13.  
14.         boundary is the multipart boundary string.  By default it is 
15.         calculated as needed. 
16.  
17.         _subparts is a sequence of initial subparts for the payload.  It 
18.         must be an iterable object, such as a list.  You can always 
19.         attach new subparts to the message by using the attach() method. 
20.  
21.         Additional parameters for the Content-Type header are taken from the 
22.         keyword arguments (or passed into the _params argument). 
23.         """  
24.         MIMEBase.__init__(self, 'multipart', _subtype, **_params)  
25.   
26.         # Initialise _payload to an empty list as the Message superclass's  
27.         # implementation of is_multipart assumes that _payload is a list for  
28.         # multipart messages.  
29.         self._payload = []  
30.   
31.         if _subparts:  
32.             for p in _subparts:  
33.                 self.attach(p)  
34.         if boundary:  
35.             self.set_boundary(boundary)  

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 proper
        Content-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 is
        calculated as needed.

        _subparts is a sequence of initial subparts for the payload.  It
        must be an iterable object, such as a list.  You can always
        attach new subparts to the message by using the attach() method.

        Additional parameters for the Content-Type header are taken from the
        keyword 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：

[python] view plain copy print ?

1. class Message:  
2.     ......  
3.   
4.     def is_multipart(self):  
5.         """Return True if the message consists of multiple parts."""  
6.         return isinstance(self._payload, list)  
7.   
8.     #  
9.     # Payload manipulation.  
10.     #  
11.     def attach(self, payload):  
12.         """Add the given payload to the current payload. 
13.  
14.         The current payload will always be a list of objects after this method 
15.         is called.  If you want to set the payload to a scalar object, use 
16.         set_payload() instead. 
17.         """  
18.         if self._payload is None:  
19.             self._payload = [payload]  
20.         else:  
21.             self._payload.append(payload)  
22.               
23.     ......  

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 method
        is called.  If you want to set the payload to a scalar object, use
        set_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。

[python] view plain copy print ?

1. class MIMENonMultipart(MIMEBase):  
2.     """Base class for MIME non-multipart type messages."""  
3.   
4.     def attach(self, payload):  
5.         # The public API prohibits attaching multiple subparts to MIMEBase  
6.         # derived subtypes since none of them are, by definition, of content  
7.         # type multipart/*  
8.         raise errors.MultipartConversionError(  
9.             'Cannot attach additional subparts to non-multipart/*')  

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。

[python] view plain copy print ?

1. class Message:  
2.     ......  
3.   
4.     def set_payload(self, payload, charset=None):  
5.         """Set the payload to the given value. 
6.  
7.         Optional charset sets the message's default character set.  See 
8.         set_charset() for details. 
9.         """  
10.         self._payload = payload  
11.         if charset is not None:  
12.             self.set_charset(charset)  
13.               
14.     ......  

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.  See
        set_charset() for details.
        """
        self._payload = payload
        if 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：

[python] view plain copy print ?

1. class MIMEText(MIMENonMultipart):  
2.     """Class for generating text/* type MIME documents."""  
3.   
4.     def __init__(self, _text, _subtype='plain', _charset='us-ascii'):  
5.         """Create a text/* type MIME document. 
6.  
7.         _text is the string for this message object. 
8.  
9.         _subtype is the MIME sub content type, defaulting to "plain". 
10.  
11.         _charset is the character set parameter added to the Content-Type 
12.         header.  This defaults to "us-ascii".  Note that as a side-effect, the 
13.         Content-Transfer-Encoding header will also be set. 
14.         """  
15.         MIMENonMultipart.__init__(self, 'text', _subtype,  
16.                                   **{'charset': _charset})  
17.         self.set_payload(_text, _charset)  

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-Type
        header.  This defaults to "us-ascii".  Note that as a side-effect, the
        Content-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。

总结

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。