:mod:`email.headerregistry`: Custom Header Objects
.. module:: email.headerregistry :synopsis: Automatic Parsing of headers based on the field name
.. moduleauthor:: R. David Murray <rdmurray@bitdance.com>
.. sectionauthor:: R. David Murray <rdmurray@bitdance.com>
Source code: :source:`Lib/email/headerregistry.py`
.. versionadded:: 3.6 [1]_
Headers are represented by customized subclasses of :class:`str`. The
particular class used to represent a given header is determined by the
:attr:`~email.policy.EmailPolicy.header_factory` of the :mod:`~email.policy` in
effect when the headers are created. This section documents the particular
header_factory implemented by the email package for handling RFC 5322
compliant email messages, which not only provides customized header objects for
various header types, but also provides an extension mechanism for applications
to add their own custom header types.
When using any of the policy objects derived from :data:`~email.policy.EmailPolicy`, all headers are produced by :class:`.HeaderRegistry` and have :class:`.BaseHeader` as their last base class. Each header class has an additional base class that is determined by the type of the header. For example, many headers have the class :class:`.UnstructuredHeader` as their other base class. The specialized second class for a header is determined by the name of the header, using a lookup table stored in the :class:`.HeaderRegistry`. All of this is managed transparently for the typical application program, but interfaces are provided for modifying the default behavior for use by more complex applications.
The sections below first document the header base classes and their attributes, followed by the API for modifying the behavior of :class:`.HeaderRegistry`, and finally the support classes used to represent the data parsed from structured headers.
name and value are passed to BaseHeader from the
:attr:`~email.policy.EmailPolicy.header_factory` call. The string value of
any header object is the value fully decoded to unicode.
This base class defines the following read-only properties:
.. attribute:: name The name of the header (the portion of the field before the ':'). This is exactly the value passed in the :attr:`~email.policy.EmailPolicy.header_factory` call for *name*; that is, case is preserved.
.. attribute:: defects A tuple of :exc:`~email.errors.HeaderDefect` instances reporting any RFC compliance problems found during parsing. The email package tries to be complete about detecting compliance issues. See the :mod:`~email.errors` module for a discussion of the types of defects that may be reported.
.. attribute:: max_count The maximum number of headers of this type that can have the same ``name``. A value of ``None`` means unlimited. The ``BaseHeader`` value for this attribute is ``None``; it is expected that specialized header classes will override this value as needed.
BaseHeader also provides the following method, which is called by the
email library code and should not in general be called by application
programs:
.. method:: fold(*, policy) Return a string containing :attr:`~email.policy.Policy.linesep` characters as required to correctly fold the header according to *policy*. A :attr:`~email.policy.Policy.cte_type` of ``8bit`` will be treated as if it were ``7bit``, since headers may not contain arbitrary binary data. If :attr:`~email.policy.EmailPolicy.utf8` is ``False``, non-ASCII data will be :rfc:`2047` encoded.
BaseHeader by itself cannot be used to create a header object. It
defines a protocol that each specialized header cooperates with in order to
produce the header object. Specifically, BaseHeader requires that
the specialized class provide a :func:`classmethod` named parse. This
method is called as follows:
parse(string, kwds)
kwds is a dictionary containing one pre-initialized key, defects.
defects is an empty list. The parse method should append any detected
defects to this list. On return, the kwds dictionary must contain
values for at least the keys decoded and defects. decoded
should be the string value for the header (that is, the header value fully
decoded to unicode). The parse method should assume that string may
contain content-transfer-encoded parts, but should correctly handle all valid
unicode characters as well so that it can parse un-encoded header values.
BaseHeader's __new__ then creates the header instance, and calls its
init method. The specialized class only needs to provide an init
method if it wishes to set additional attributes beyond those provided by
BaseHeader itself. Such an init method should look like this:
def init(self, /, *args, **kw):
self._myattr = kw.pop('myattr')
super().init(*args, **kw)
That is, anything extra that the specialized class puts in to the kwds
dictionary should be removed and handled, and the remaining contents of
kw (and args) passed to the BaseHeader init method.
An "unstructured" header is the default type of header in RFC 5322. Any header that does not have a specified syntax is treated as unstructured. The classic example of an unstructured header is the :mailheader:`Subject` header.
In RFC 5322, an unstructured header is a run of arbitrary text in the
ASCII character set. RFC 2047, however, has an RFC 5322 compatible
mechanism for encoding non-ASCII text as ASCII characters within a header
value. When a value containing encoded words is passed to the
constructor, the UnstructuredHeader parser converts such encoded words
into unicode, following the RFC 2047 rules for unstructured text. The
parser uses heuristics to attempt to decode certain non-compliant encoded
words. Defects are registered in such cases, as well as defects for issues
such as invalid characters within the encoded words or the non-encoded text.
This header type provides no additional attributes.
RFC 5322 specifies a very specific format for dates within email headers.
The DateHeader parser recognizes that date format, as well as
recognizing a number of variant forms that are sometimes found "in the
wild".
This header type provides the following additional attributes:
.. attribute:: datetime If the header value can be recognized as a valid date of one form or another, this attribute will contain a :class:`~datetime.datetime` instance representing that date. If the timezone of the input date is specified as ``-0000`` (indicating it is in UTC but contains no information about the source timezone), then :attr:`.datetime` will be a naive :class:`~datetime.datetime`. If a specific timezone offset is found (including ``+0000``), then :attr:`.datetime` will contain an aware ``datetime`` that uses :class:`datetime.timezone` to record the timezone offset.
The decoded value of the header is determined by formatting the
datetime according to the RFC 5322 rules; that is, it is set to:
email.utils.format_datetime(self.datetime)
When creating a DateHeader, value may be
:class:`~datetime.datetime` instance. This means, for example, that
the following code is valid and does what one would expect:
msg['Date'] = datetime(2011, 7, 15, 21)
Because this is a naive datetime it will be interpreted as a UTC
timestamp, and the resulting value will have a timezone of -0000. Much
more useful is to use the :func:`~email.utils.localtime` function from the
:mod:`~email.utils` module:
msg['Date'] = utils.localtime()
This example sets the date header to the current time and date using the current timezone offset.
Address headers are one of the most complex structured header types.
The AddressHeader class provides a generic interface to any address
header.
This header type provides the following additional attributes:
.. attribute:: groups A tuple of :class:`.Group` objects encoding the addresses and groups found in the header value. Addresses that are not part of a group are represented in this list as single-address ``Groups`` whose :attr:`~.Group.display_name` is ``None``.
.. attribute:: addresses A tuple of :class:`.Address` objects encoding all of the individual addresses from the header value. If the header value contains any groups, the individual addresses from the group are included in the list at the point where the group occurs in the value (that is, the list of addresses is "flattened" into a one dimensional list).
The decoded value of the header will have all encoded words decoded to
unicode. :class:`~encodings.idna` encoded domain names are also decoded to
unicode. The decoded value is set by :ref:`joining <meth-str-join>` the
:class:`str` value of the elements of the groups attribute with ',
'.
A list of :class:`.Address` and :class:`.Group` objects in any combination
may be used to set the value of an address header. Group objects whose
display_name is None will be interpreted as single addresses, which
allows an address list to be copied with groups intact by using the list
obtained from the groups attribute of the source header.
A subclass of :class:`.AddressHeader` that adds one additional attribute:
.. attribute:: address The single address encoded by the header value. If the header value actually contains more than one address (which would be a violation of the RFC under the default :mod:`~email.policy`), accessing this attribute will result in a :exc:`ValueError`.
Many of the above classes also have a Unique variant (for example,
UniqueUnstructuredHeader). The only difference is that in the Unique
variant, :attr:`~.BaseHeader.max_count` is set to 1.
There is really only one valid value for the :mailheader:`MIME-Version`
header, and that is 1.0. For future proofing, this header class
supports other valid version numbers. If a version number has a valid value
per RFC 2045, then the header object will have non-None values for
the following attributes:
.. attribute:: version The version number as a string, with any whitespace and/or comments removed.
.. attribute:: major The major version number as an integer
.. attribute:: minor The minor version number as an integer
MIME headers all start with the prefix 'Content-'. Each specific header has a certain value, described under the class for that header. Some can also take a list of supplemental parameters, which have a common format. This class serves as a base for all the MIME headers that take parameters.
.. attribute:: params A dictionary mapping parameter names to parameter values.
A :class:`ParameterizedMIMEHeader` class that handles the :mailheader:`Content-Type` header.
.. attribute:: content_type The content type string, in the form ``maintype/subtype``.
.. attribute:: maintype
.. attribute:: subtype
A :class:`ParameterizedMIMEHeader` class that handles the :mailheader:`Content-Disposition` header.
.. attribute:: content_disposition ``inline`` and ``attachment`` are the only valid values in common use.
Handles the :mailheader:`Content-Transfer-Encoding` header.
.. attribute:: cte Valid values are ``7bit``, ``8bit``, ``base64``, and ``quoted-printable``. See :rfc:`2045` for more information.
The following classes are the classes used to represent data parsed from structured headers and can, in general, be used by an application program to construct structured values to assign to specific headers.
The class used to represent an email address. The general form of an address is:
[display_name] <username@domain>
or:
username@domain
where each part must conform to specific syntax rules spelled out in RFC 5322.
As a convenience addr_spec can be specified instead of username and
domain, in which case username and domain will be parsed from the
addr_spec. An addr_spec must be a properly RFC quoted string; if it is
not Address will raise an error. Unicode characters are allowed and
will be property encoded when serialized. However, per the RFCs, unicode is
not allowed in the username portion of the address.
.. attribute:: display_name The display name portion of the address, if any, with all quoting removed. If the address does not have a display name, this attribute will be an empty string.
.. attribute:: username The ``username`` portion of the address, with all quoting removed.
.. attribute:: domain The ``domain`` portion of the address.
.. attribute:: addr_spec The ``username@domain`` portion of the address, correctly quoted for use as a bare address (the second form shown above). This attribute is not mutable.
.. method:: __str__() The ``str`` value of the object is the address quoted according to :rfc:`5322` rules, but with no Content Transfer Encoding of any non-ASCII characters.
To support SMTP (RFC 5321), Address handles one special case: if
username and domain are both the empty string (or None), then
the string value of the Address is <>.
The class used to represent an address group. The general form of an address group is:
display_name: [address-list];
As a convenience for processing lists of addresses that consist of a mixture
of groups and single addresses, a Group may also be used to represent
single addresses that are not part of a group by setting display_name to
None and providing a list of the single address as addresses.
.. attribute:: display_name The ``display_name`` of the group. If it is ``None`` and there is exactly one ``Address`` in ``addresses``, then the ``Group`` represents a single address that is not in a group.
.. attribute:: addresses A possibly empty tuple of :class:`.Address` objects representing the addresses in the group.
.. method:: __str__() The ``str`` value of a ``Group`` is formatted according to :rfc:`5322`, but with no Content Transfer Encoding of any non-ASCII characters. If ``display_name`` is none and there is a single ``Address`` in the ``addresses`` list, the ``str`` value will be the same as the ``str`` of that single ``Address``.
Footnotes
| [1] | Originally added in 3.3 as a :term:`provisional module <provisional package>` |