# `email.message.Message`: Representing an email message using the `compat32` API

The `Message` class is very similar to the `EmailMessage` class, without the methods added by that class, and with the default behavior of certain other methods being slightly different. We also document here some methods that, while supported by the `EmailMessage` class, are not recommended unless you are dealing with legacy code.

The philosophy and structure of the two classes is otherwise the same.

This document describes the behavior under the default (for `Message`) policy `Compat32`. If you are going to use another policy, you should be using the `EmailMessage` class instead.

An email message consists of headers and a payload. Headers must be RFC 5233 style names and values, where the field name and value are separated by a colon. The colon is not part of either the field name or the field value. The payload may be a simple text message, or a binary object, or a structured sequence of sub-messages each with their own set of headers and their own payload. The latter type of payload is indicated by the message having a MIME type such as multipart/* or message/rfc822.

The conceptual model provided by a `Message` object is that of an ordered dictionary of headers with additional methods for accessing both specialized information from the headers, for accessing the payload, for generating a serialized version of the message, and for recursively walking over the object tree. Note that duplicate headers are supported but special methods must be used to access them.

The `Message` pseudo-dictionary is indexed by the header names, which must be ASCII values. The values of the dictionary are strings that are supposed to contain only ASCII characters; there is some special handling for non-ASCII input, but it doesn't always produce the correct results. Headers are stored and returned in case-preserving form, but field names are matched case-insensitively. There may also be a single envelope header, also known as the Unix-From header or the `From_` header. The payload is either a string or bytes, in the case of simple message objects, or a list of `Message` objects, for MIME container documents (e.g. multipart/* and message/rfc822).

Here are the methods of the `Message` class:

class `email.message.``Message`(policy=compat32)

  If policy is specified (it must be an instance of a `policy` class) use the rules it specifies to update and serialize the representation of the message. If policy is not set, use the `compat32` policy, which maintains backward compatibility with the Python 3.2 version of the email package. For more information see the `policy` documentation.

  在 3.3 版更改: The policy keyword argument was added.

  `as_string`(unixfrom=False, maxheaderlen=0, policy=None)

    Return the entire message flattened as a string. When optional unixfrom is true, the envelope header is included in the returned string. unixfrom defaults to `False`. For backward compatibility reasons, maxheaderlen defaults to `0`, so if you want a different value you must override it explicitly (the value specified for max_line_length in the policy will be ignored by this method). The policy argument may be used to override the default policy obtained from the message instance. This can be used to control some of the formatting produced by the method, since the specified policy will be passed to the `Generator`.

    Flattening the message may trigger changes to the `Message` if defaults need to be filled in to complete the transformation to a string (for example, MIME boundaries may be generated or modified).

    Note that this method is provided as a convenience and may not always format the message the way you want. For example, by default it does not do the mangling of lines that begin with `From` that is required by the unix mbox format. For more flexibility, instantiate a `Generator` instance and use its `flatten()` method directly. For example:

        from io import StringIO
        from email.generator import Generator
        fp = StringIO()
        g = Generator(fp, mangle_from_=True, maxheaderlen=60)
        g.flatten(msg)
        text = fp.getvalue()

    If the message object contains binary data that is not encoded according to RFC standards, the non-compliant data will be replaced by unicode "unknown character" code points. (See also `as_bytes()` and `BytesGenerator`.)

    在 3.4 版更改: the policy keyword argument was added.

  `__str__`()

    Equivalent to `as_string()`. Allows `str(msg)` to produce a string containing the formatted message.

  `as_bytes`(unixfrom=False, policy=None)

    Return the entire message flattened as a bytes object. When optional unixfrom is true, the envelope header is included in the returned string. unixfrom defaults to `False`. The policy argument may be used to override the default policy obtained from the message instance. This can be used to control some of the formatting produced by the method, since the specified policy will be passed to the `BytesGenerator`.

    Flattening the message may trigger changes to the `Message` if defaults need to be filled in to complete the transformation to a string (for example, MIME boundaries may be generated or modified).

    Note that this method is provided as a convenience and may not always format the message the way you want. For example, by default it does not do the mangling of lines that begin with `From` that is required by the unix mbox format. For more flexibility, instantiate a `BytesGenerator` instance and use its `flatten()` method directly. For example:

        from io import BytesIO
        from email.generator import BytesGenerator
        fp = BytesIO()
        g = BytesGenerator(fp, mangle_from_=True, maxheaderlen=60)
        g.flatten(msg)
        text = fp.getvalue()

    3.4 新版功能.

  `__bytes__`()

    Equivalent to `as_bytes()`. Allows `bytes(msg)` to produce a bytes object containing the formatted message.

    3.4 新版功能.

  `is_multipart`()

    Return `True` if the message's payload is a list of sub-`Message` objects, otherwise return `False`. When `is_multipart()` returns `False`, the payload should be a string object (which might be a CTE encoded binary payload). (Note that `is_multipart()` returning `True` does not necessarily mean that "msg.get_content_maintype() == 'multipart'" will return the `True`. For example, `is_multipart` will return `True` when the `Message` is of type `message/rfc822`.)

  `set_unixfrom`(unixfrom)

    Set the message's envelope header to unixfrom, which should be a string.

  `get_unixfrom`()

    返回消息的信封头。如果信封头从未被设置过，默认返回 `None` 。

  `attach`(payload)

    Add the given payload to the current payload, which must be `None` or a list of `Message` objects before the call. After the call, the payload will always be a list of `Message` objects. If you want to set the payload to a scalar object (e.g. a string), use `set_payload()` instead.

    This is a legacy method. On the `EmailMessage` class its functionality is replaced by `set_content()` and the related `make` and `add` methods.

  `get_payload`(i=None, decode=False)

    Return the current payload, which will be a list of `Message` objects when `is_multipart()` is `True`, or a string when `is_multipart()` is `False`. If the payload is a list and you mutate the list object, you modify the message's payload in place.

    With optional argument i, `get_payload()` will return the i-th element of the payload, counting from zero, if `is_multipart()` is `True`. An `IndexError` will be raised if i is less than 0 or greater than or equal to the number of items in the payload. If the payload is a string (i.e. `is_multipart()` is `False`) and i is given, a `TypeError` is raised.

    Optional decode is a flag indicating whether the payload should be decoded or not, according to the Content-Transfer-Encoding header. When `True` and the message is not a multipart, the payload will be decoded if this header's value is `quoted-printable` or `base64`. If some other encoding is used, or Content-Transfer-Encoding header is missing, the payload is returned as-is (undecoded). In all cases the returned value is binary data. If the message is a multipart and the decode flag is `True`, then `None` is returned. If the payload is base64 and it was not perfectly formed (missing padding, characters outside the base64 alphabet), then an appropriate defect will be added to the message's defect property (`InvalidBase64PaddingDefect` or `InvalidBase64CharactersDefect`, respectively).

    When decode is `False` (the default) the body is returned as a string without decoding the Content-Transfer-Encoding. However, for a Content-Transfer-Encoding of 8bit, an attempt is made to decode the original bytes using the `charset` specified by the Content-Type header, using the `replace` error handler. If no `charset` is specified, or if the `charset` given is not recognized by the email package, the body is decoded using the default ASCII charset.

    This is a legacy method. On the `EmailMessage` class its functionality is replaced by `get_content()` and `iter_parts()`.

  `set_payload`(payload, charset=None)

    Set the entire message object's payload to payload. It is the client's responsibility to ensure the payload invariants. Optional charset sets the message's default character set; see `set_charset()` for details.

    This is a legacy method. On the `EmailMessage` class its functionality is replaced by `set_content()`.

  `set_charset`(charset)

    Set the character set of the payload to charset, which can either be a `Charset` instance (see `email.charset`), a string naming a character set, or `None`. If it is a string, it will be converted to a `Charset` instance. If charset is `None`, the `charset` parameter will be removed from the Content-Type header (the message will not be otherwise modified). Anything else will generate a `TypeError`.

    If there is no existing MIME-Version header one will be added. If there is no existing Content-Type header, one will be added with a value of text/plain. Whether the Content-Type header already exists or not, its `charset` parameter will be set to charset.output_charset. If charset.input_charset and charset.output_charset differ, the payload will be re-encoded to the output_charset. If there is no existing Content-Transfer-Encoding header, then the payload will be transfer-encoded, if needed, using the specified `Charset`, and a header with the appropriate value will be added. If a Content-Transfer-Encoding header already exists, the payload is assumed to already be correctly encoded using that Content-Transfer-Encoding and is not modified.

    This is a legacy method. On the `EmailMessage` class its functionality is replaced by the charset parameter of the `email.emailmessage.EmailMessage.set_content()` method.

  `get_charset`()

    Return the `Charset` instance associated with the message's payload.

    This is a legacy method. On the `EmailMessage` class it always returns `None`.

  The following methods implement a mapping-like interface for accessing the message's RFC 2822 headers. Note that there are some semantic differences between these methods and a normal mapping (i.e. dictionary) interface. For example, in a dictionary there are no duplicate keys, but here there may be duplicate message headers. Also, in dictionaries there is no guaranteed order to the keys returned by `keys()`, but in a `Message` object, headers are always returned in the order they appeared in the original message, or were added to the message later. Any header deleted and then re-added are always appended to the end of the header list.

  These semantic differences are intentional and are biased toward maximal convenience.

  还请留意，无论在什么情况下，消息当中的任何信封头字段都不会包含在映射接口当中。

  In a model generated from bytes, any header values that (in contravention of the RFCs) contain non-ASCII bytes will, when retrieved through this interface, be represented as `Header` objects with a charset of unknown-8bit.

  `__len__`()

    返回头字段的总数，重复的也计算在内。

  `__contains__`(name)

    Return `True` if the message object has a field named name. Matching is done case-insensitively and name should not include the trailing colon. Used for the `in` operator, e.g.:

        if 'message-id' in myMessage:
           print('Message-ID:', myMessage['message-id'])

  `__getitem__`(name)

    Return the value of the named header field. name should not include the colon field separator. If the header is missing, `None` is returned; a `KeyError` is never raised.

    Note that if the named field appears more than once in the message's headers, exactly which of those field values will be returned is undefined. Use the `get_all()` method to get the values of all the extant named headers.

  `__setitem__`(name, val)

    Add a header to the message with field name name and value val. The field is appended to the end of the message's existing fields.

    请注意，这个方法 既不会 覆盖 也不会 删除任何字段名重名的已有字段。如果你确实想保证新字段是整个信息头当中唯一拥有 name 字段名的字段，你需要先把旧字段删除。例如：

        del msg['subject']
        msg['subject'] = 'Python roolz!'

  `__delitem__`(name)

    删除信息头当中字段名匹配 name 的所有字段。如果匹配指定名称的字段没有找到，也不会抛出任何异常。

  `keys`()

    以列表形式返回消息头中所有的字段名。

  `values`()

    以列表形式返回消息头中所有的字段值。

  `items`()

    以二元元组的列表形式返回消息头中所有的字段名和字段值。

  `get`(name, failobj=None)

    Return the value of the named header field. This is identical to `__getitem__()` except that optional failobj is returned if the named header is missing (defaults to `None`).

  Here are some additional useful methods:

  `get_all`(name, failobj=None)

    返回字段名为 name 的所有字段值的列表。如果信息内不存在匹配的字段，返回 failobj （其默认值为 `None` ）。

  `add_header`(_name, _value, **_params)

    高级头字段设定。这个方法与 `__setitem__()` 类似，不过你可以使用关键字参数为字段提供附加参数。 _name 是字段名， _value 是字段 主 值。

    For each item in the keyword argument dictionary _params, the key is taken as the parameter name, with underscores converted to dashes (since dashes are illegal in Python identifiers). Normally, the parameter will be added as `key="value"` unless the value is `None`, in which case only the key will be added. If the value contains non-ASCII characters, it can be specified as a three tuple in the format `(CHARSET, LANGUAGE, VALUE)`, where `CHARSET` is a string naming the charset to be used to encode the value, `LANGUAGE` can usually be set to `None` or the empty string (see RFC 2231 for other possibilities), and `VALUE` is the string value containing non-ASCII code points. If a three tuple is not passed and the value contains non-ASCII characters, it is automatically encoded in RFC 2231 format using a `CHARSET` of `utf-8` and a `LANGUAGE` of `None`.

    Here's an example:

        msg.add_header('Content-Disposition', 'attachment', filename='bud.gif')

    会添加一个形如下文的头字段：

        Content-Disposition: attachment; filename="bud.gif"

    An example with non-ASCII characters:

        msg.add_header('Content-Disposition', 'attachment',
                       filename=('iso-8859-1', '', 'Fußballer.ppt'))

    Which produces

        Content-Disposition: attachment; filename*="iso-8859-1''Fu%DFballer.ppt"

  `replace_header`(_name, _value)

    Replace a header. Replace the first header found in the message that matches _name, retaining header order and field name case. If no matching header was found, a `KeyError` is raised.

  `get_content_type`()

    Return the message's content type. The returned string is coerced to lower case of the form maintype/subtype. If there was no Content-Type header in the message the default type as given by `get_default_type()` will be returned. Since according to RFC 2045, messages always have a default type, `get_content_type()` will always return a value.

    RFC 2045 defines a message's default type to be text/plain unless it appears inside a multipart/digest container, in which case it would be message/rfc822. If the Content-Type header has an invalid type specification, RFC 2045 mandates that the default type be text/plain.

  `get_content_maintype`()

    返回信息的主要内容类型。准确来说，此方法返回的是 `get_content_type()` 方法所返回的形如 maintype/subtype 的字符串当中的 maintype 部分。

  `get_content_subtype`()

    返回信息的子内容类型。准确来说，此方法返回的是 `get_content_type()` 方法所返回的形如 maintype/subtype 的字符串当中的 subtype 部分。

  `get_default_type`()

    返回默认的内容类型。绝大多数的信息，其默认内容类型都是 text/plain 。作为 multipart/digest 容器内子部分的信息除外，它们的默认内容类型是 message/rfc822 。

  `set_default_type`(ctype)

    Set the default content type. ctype should either be text/plain or message/rfc822, although this is not enforced. The default content type is not stored in the Content-Type header.

  `get_params`(failobj=None, header='content-type', unquote=True)

    Return the message's Content-Type parameters, as a list. The elements of the returned list are 2-tuples of key/value pairs, as split on the `'='` sign. The left hand side of the `'='` is the key, while the right hand side is the value. If there is no `'='` sign in the parameter the value is the empty string, otherwise the value is as described in `get_param()` and is unquoted if optional unquote is `True` (the default).

    Optional failobj is the object to return if there is no Content-Type header. Optional header is the header to search instead of Content-Type.

    This is a legacy method. On the `EmailMessage` class its functionality is replaced by the params property of the individual header objects returned by the header access methods.

  `get_param`(param, failobj=None, header='content-type', unquote=True)

    Return the value of the Content-Type header's parameter param as a string. If the message has no Content-Type header or if there is no such parameter, then failobj is returned (defaults to `None`).

    Optional header if given, specifies the message header to use instead of Content-Type.

    Parameter keys are always compared case insensitively. The return value can either be a string, or a 3-tuple if the parameter was RFC 2231 encoded. When it's a 3-tuple, the elements of the value are of the form `(CHARSET, LANGUAGE, VALUE)`. Note that both `CHARSET` and `LANGUAGE` can be `None`, in which case you should consider `VALUE` to be encoded in the `us-ascii` charset. You can usually ignore `LANGUAGE`.

    If your application doesn't care whether the parameter was encoded as in RFC 2231, you can collapse the parameter value by calling `email.utils.collapse_rfc2231_value()`, passing in the return value from `get_param()`. This will return a suitably decoded Unicode string when the value is a tuple, or the original string unquoted if it isn't. For example:

        rawparam = msg.get_param('foo')
        param = email.utils.collapse_rfc2231_value(rawparam)

    In any case, the parameter value (either the returned string, or the `VALUE` item in the 3-tuple) is always unquoted, unless unquote is set to `False`.

    This is a legacy method. On the `EmailMessage` class its functionality is replaced by the params property of the individual header objects returned by the header access methods.

  `set_param`(param, value, header='Content-Type', requote=True, charset=None, language='', replace=False)

    Set a parameter in the Content-Type header. If the parameter already exists in the header, its value will be replaced with value. If the Content-Type header as not yet been defined for this message, it will be set to text/plain and the new parameter value will be appended as per RFC 2045.

    Optional header specifies an alternative header to Content-Type, and all parameters will be quoted as necessary unless optional requote is `False` (the default is `True`).

    If optional charset is specified, the parameter will be encoded according to RFC 2231. Optional language specifies the RFC 2231 language, defaulting to the empty string. Both charset and language should be strings.

    如果 replace 为 `False` （默认值），该头字段会被移动到所有头字段的末尾。如果 replace 为 `True` ，字段会被原地更新。

    在 3.4 版更改: 添加了 `replace` 关键字。

  `del_param`(param, header='content-type', requote=True)

    Remove the given parameter completely from the Content-Type header. The header will be re-written in place without the parameter or its value. All values will be quoted as necessary unless requote is `False` (the default is `True`). Optional header specifies an alternative to Content-Type.

  `set_type`(type, header='Content-Type', requote=True)

    Set the main type and subtype for the Content-Type header. type must be a string in the form maintype/subtype, otherwise a `ValueError` is raised.

    This method replaces the Content-Type header, keeping all the parameters in place. If requote is `False`, this leaves the existing header's quoting as is, otherwise the parameters will be quoted (the default).

    An alternative header can be specified in the header argument. When the Content-Type header is set a MIME-Version header is also added.

    This is a legacy method. On the `EmailMessage` class its functionality is replaced by the `make_` and `add_` methods.

  `get_filename`(failobj=None)

    返回信息头当中 Content-Disposition 字段当中名为 `filename` 的参数值。如果该字段当中没有此参数，该方法会退而寻找 Content-Type 字段当中的 `name` 参数值。如果这个也没有找到，或者这些个字段压根就不存在，返回 failobj 。返回的字符串永远按照 `email.utils.unquote()` 方法去除引号。

  `get_boundary`(failobj=None)

    返回信息头当中 Content-Type 字段当中名为 `boundary` 的参数值。如果字段当中没有此参数，或者这些个字段压根就不存在，返回 failobj 。返回的字符串永远按照 `email.utils.unquote()` 方法去除引号。

  `set_boundary`(boundary)

    将 Content-Type 头字段的 `boundary` 参数设置为 boundary 。 `set_boundary()` 方法永远都会在必要的时候为 boundary 添加引号。如果信息对象中没有 Content-Type 头字段，抛出 `HeaderParseError` 异常。

    Note that using this method is subtly different than deleting the old Content-Type header and adding a new one with the new boundary via `add_header()`, because `set_boundary()` preserves the order of the Content-Type header in the list of headers. However, it does not preserve any continuation lines which may have been present in the original Content-Type header.

  `get_content_charset`(failobj=None)

    返回 Content-Type 头字段中的 `charset` 参数，强制小写。如果字段当中没有此参数，或者这个字段压根不存在，返回 failobj 。

    Note that this method differs from `get_charset()` which returns the `Charset` instance for the default encoding of the message body.

  `get_charsets`(failobj=None)

    返回一个包含了信息内所有字符集名字的列表。如果信息是 multipart 类型的，那么列表当中的每一项都对应其负载的子部分的字符集名字。否则，该列表是一个长度为1的列表。

    Each item in the list will be a string which is the value of the `charset` parameter in the Content-Type header for the represented subpart. However, if the subpart has no Content-Type header, no `charset` parameter, or is not of the text main MIME type, then that item in the returned list will be failobj.

  `get_content_disposition`()

    如果信息的 Content-Disposition 头字段存在，返回其字段值；否则返回 `None` 。返回的值均为小写，不包含参数。如果信息遵循 RFC 2183 标准，则返回值只可能在 inline 、 attachment 和 `None` 之间选择。

    3.5 新版功能.

  `walk`()

    `walk()` 方法是一个多功能生成器。它可以被用来以深度优先顺序遍历信息对象树的所有部分和子部分。一般而言， `walk()` 会被用作 `for` 循环的迭代器，每一次迭代都返回其下一个子部分。

    以下例子会打印出一封具有多部分结构之信息的每个部分的 MIME 类型。

        >>> for part in msg.walk():
        ...     print(part.get_content_type())
        multipart/report
        text/plain
        message/delivery-status
        text/plain
        text/plain
        message/rfc822
        text/plain

    `walk` 会遍历所有 `is_multipart()` 方法返回 `True` 的部分之子部分，哪怕 `msg.get_content_maintype() == 'multipart'` 返回的是 `False` 。使用 `_structure` 除错帮助函数可以帮助我们在下面这个例子当中看清楚这一点：

        >>> for part in msg.walk():
        ...     print(part.get_content_maintype() == 'multipart',
        ...           part.is_multipart())
        True True
        False False
        False True
        False False
        False False
        False True
        False False
        >>> _structure(msg)
        multipart/report
            text/plain
            message/delivery-status
                text/plain
                text/plain
            message/rfc822
                text/plain

    在这里， `message` 的部分并非 `multiparts` ，但是它们真的包含子部分！ `is_multipart()` 返回 `True` ， `walk` 也深入进这些子部分中。

  `Message` objects can also optionally contain two instance attributes, which can be used when generating the plain text of a MIME message.

  `preamble`

    The format of a MIME document allows for some text between the blank line following the headers, and the first multipart boundary string. Normally, this text is never visible in a MIME-aware mail reader because it falls outside the standard MIME armor. However, when viewing the raw text of the message, or when viewing the message in a non-MIME aware reader, this text can become visible.

    The preamble attribute contains this leading extra-armor text for MIME documents. When the `Parser` discovers some text after the headers but before the first boundary string, it assigns this text to the message's preamble attribute. When the `Generator` is writing out the plain text representation of a MIME message, and it finds the message has a preamble attribute, it will write this text in the area between the headers and the first boundary. See `email.parser` and `email.generator` for details.

    Note that if the message object has no preamble, the preamble attribute will be `None`.

  `epilogue`

    The epilogue attribute acts the same way as the preamble attribute, except that it contains text that appears between the last boundary and the end of the message.

    You do not need to set the epilogue to the empty string in order for the `Generator` to print a newline at the end of the file.

  `defects`

    The defects attribute contains a list of all the problems found when parsing this message. See `email.errors` for a detailed description of the possible parsing defects.