FM / library

/usr/share/doc/python3-docs/html/_sources/library/

/usr/share/doc/python3-docs/html/_sources/library

Name	Size	Mode	Actions
2to3.rst.txt	15898	0644	edit dl rm
abc.rst.txt	11805	0644	edit dl rm
aifc.rst.txt	7503	0644	edit dl rm
allos.rst.txt	678	0644	edit dl rm
archiving.rst.txt	440	0644	edit dl rm
argparse.rst.txt	77480	0644	edit dl rm
array.rst.txt	10812	0644	edit dl rm
ast.rst.txt	10159	0644	edit dl rm
asynchat.rst.txt	8528	0644	edit dl rm
asyncio-dev.rst.txt	13398	0644	edit dl rm
asyncio-eventloop.rst.txt	33701	0644	edit dl rm
asyncio-eventloops.rst.txt	7660	0644	edit dl rm
asyncio-protocol.rst.txt	25170	0644	edit dl rm
asyncio-queue.rst.txt	4328	0644	edit dl rm
asyncio-stream.rst.txt	14363	0644	edit dl rm
asyncio-subprocess.rst.txt	15544	0644	edit dl rm
asyncio-sync.rst.txt	9078	0644	edit dl rm
asyncio-task.rst.txt	25406	0644	edit dl rm
asyncio.rst.txt	2272	0644	edit dl rm
asyncore.rst.txt	13624	0644	edit dl rm
atexit.rst.txt	3669	0644	edit dl rm
audioop.rst.txt	10712	0644	edit dl rm
base64.rst.txt	10371	0644	edit dl rm
bdb.rst.txt	12985	0644	edit dl rm
binary.rst.txt	654	0644	edit dl rm
binascii.rst.txt	6539	0644	edit dl rm
binhex.rst.txt	1709	0644	edit dl rm
bisect.rst.txt	5398	0644	edit dl rm
builtins.rst.txt	1465	0644	edit dl rm
bz2.rst.txt	8825	0644	edit dl rm
calendar.rst.txt	10988	0644	edit dl rm
cgi.rst.txt	22650	0644	edit dl rm
cgitb.rst.txt	2918	0644	edit dl rm
chunk.rst.txt	5094	0644	edit dl rm
cmath.rst.txt	9326	0644	edit dl rm
cmd.rst.txt	13795	0644	edit dl rm
code.rst.txt	7817	0644	edit dl rm
codecs.rst.txt	75564	0644	edit dl rm
codeop.rst.txt	3021	0644	edit dl rm
collections.abc.rst.txt	12861	0644	edit dl rm
collections.rst.txt	46063	0644	edit dl rm
colorsys.rst.txt	1820	0644	edit dl rm
compileall.rst.txt	8975	0644	edit dl rm
concurrency.rst.txt	673	0644	edit dl rm
concurrent.futures.rst.txt	16910	0644	edit dl rm
concurrent.rst.txt	171	0644	edit dl rm
configparser.rst.txt	48756	0644	edit dl rm
constants.rst.txt	3331	0644	edit dl rm
contextlib.rst.txt	28175	0644	edit dl rm
copy.rst.txt	3378	0644	edit dl rm
copyreg.rst.txt	2182	0644	edit dl rm
crypt.rst.txt	5122	0644	edit dl rm
crypto.rst.txt	411	0644	edit dl rm
csv.rst.txt	19902	0644	edit dl rm
ctypes.rst.txt	88613	0644	edit dl rm
curses.ascii.rst.txt	8920	0644	edit dl rm
curses.panel.rst.txt	2764	0644	edit dl rm
curses.rst.txt	76397	0644	edit dl rm
custominterp.rst.txt	569	0644	edit dl rm
datatypes.rst.txt	751	0644	edit dl rm
datetime.rst.txt	90524	0644	edit dl rm
dbm.rst.txt	14293	0644	edit dl rm
debug.rst.txt	471	0644	edit dl rm
decimal.rst.txt	74869	0644	edit dl rm
development.rst.txt	704	0644	edit dl rm
difflib.rst.txt	29861	0644	edit dl rm
dis.rst.txt	32567	0644	edit dl rm
distribution.rst.txt	452	0644	edit dl rm
distutils.rst.txt	1974	0644	edit dl rm
doctest.rst.txt	71743	0644	edit dl rm
dummy_threading.rst.txt	784	0644	edit dl rm
email.charset.rst.txt	9293	0644	edit dl rm
email.compat32-message.rst.txt	33493	0644	edit dl rm
email.contentmanager.rst.txt	9112	0644	edit dl rm
email.encoders.rst.txt	2715	0644	edit dl rm
email.errors.rst.txt	4792	0644	edit dl rm
email.examples.rst.txt	1915	0644	edit dl rm
email.generator.rst.txt	13637	0644	edit dl rm
email.header.rst.txt	9188	0644	edit dl rm
email.headerregistry.rst.txt	17957	0644	edit dl rm
email.iterators.rst.txt	2795	0644	edit dl rm
email.message.rst.txt	33008	0644	edit dl rm
email.mime.rst.txt	11721	0644	edit dl rm
email.parser.rst.txt	14092	0644	edit dl rm
email.policy.rst.txt	27022	0644	edit dl rm
email.rst.txt	6785	0644	edit dl rm
email.util.rst.txt	9214	0644	edit dl rm
ensurepip.rst.txt	4987	0644	edit dl rm
enum.rst.txt	32751	0644	edit dl rm
errno.rst.txt	6811	0644	edit dl rm
exceptions.rst.txt	25570	0644	edit dl rm
faulthandler.rst.txt	6136	0644	edit dl rm
fcntl.rst.txt	7152	0644	edit dl rm
filecmp.rst.txt	5623	0644	edit dl rm
fileformats.rst.txt	287	0644	edit dl rm
fileinput.rst.txt	8051	0644	edit dl rm
filesys.rst.txt	961	0644	edit dl rm
fnmatch.rst.txt	3103	0644	edit dl rm
formatter.rst.txt	13242	0644	edit dl rm
fpectl.rst.txt	4180	0644	edit dl rm
fractions.rst.txt	6364	0644	edit dl rm
frameworks.rst.txt	391	0644	edit dl rm
ftplib.rst.txt	17552	0644	edit dl rm
functional.rst.txt	365	0644	edit dl rm
functions.rst.txt	71621	0644	edit dl rm
functools.rst.txt	18503	0644	edit dl rm
gc.rst.txt	9694	0644	edit dl rm
getopt.rst.txt	6557	0644	edit dl rm
getpass.rst.txt	1882	0644	edit dl rm
gettext.rst.txt	25588	0644	edit dl rm
glob.rst.txt	3508	0644	edit dl rm
grp.rst.txt	2417	0644	edit dl rm
gzip.rst.txt	8301	0644	edit dl rm
hashlib.rst.txt	26523	0644	edit dl rm
heapq.rst.txt	13402	0644	edit dl rm
hmac.rst.txt	4020	0644	edit dl rm
html.entities.rst.txt	1319	0644	edit dl rm
html.parser.rst.txt	11275	0644	edit dl rm
html.rst.txt	1304	0644	edit dl rm
http.client.rst.txt	18870	0644	edit dl rm
http.cookiejar.rst.txt	27832	0644	edit dl rm
http.cookies.rst.txt	8645	0644	edit dl rm
http.rst.txt	7096	0644	edit dl rm
http.server.rst.txt	17639	0644	edit dl rm
i18n.rst.txt	408	0644	edit dl rm
idle.rst.txt	25834	0644	edit dl rm
imaplib.rst.txt	20367	0644	edit dl rm
imghdr.rst.txt	2938	0644	edit dl rm
imp.rst.txt	15548	0644	edit dl rm
importlib.rst.txt	53563	0644	edit dl rm
index.rst.txt	2258	0644	edit dl rm
inspect.rst.txt	53928	0644	edit dl rm
internet.rst.txt	992	0644	edit dl rm
intro.rst.txt	2645	0644	edit dl rm
io.rst.txt	40328	0644	edit dl rm
ipaddress.rst.txt	32340	0644	edit dl rm
ipc.rst.txt	649	0644	edit dl rm
itertools.rst.txt	38459	0644	edit dl rm
json.rst.txt	27876	0644	edit dl rm
keyword.rst.txt	617	0644	edit dl rm
language.rst.txt	522	0644	edit dl rm
linecache.rst.txt	2408	0644	edit dl rm
locale.rst.txt	25470	0644	edit dl rm
logging.config.rst.txt	33886	0644	edit dl rm
logging.handlers.rst.txt	42497	0644	edit dl rm
logging.rst.txt	60479	0644	edit dl rm
lzma.rst.txt	17361	0644	edit dl rm
macpath.rst.txt	707	0644	edit dl rm
mailbox.rst.txt	63055	0644	edit dl rm
mailcap.rst.txt	3691	0644	edit dl rm
markup.rst.txt	679	0644	edit dl rm
marshal.rst.txt	4855	0644	edit dl rm
math.rst.txt	14375	0644	edit dl rm
mimetypes.rst.txt	9839	0644	edit dl rm
misc.rst.txt	247	0644	edit dl rm
mm.rst.txt	431	0644	edit dl rm
mmap.rst.txt	11232	0644	edit dl rm
modulefinder.rst.txt	3240	0644	edit dl rm
modules.rst.txt	355	0644	edit dl rm
msilib.rst.txt	18612	0644	edit dl rm
msvcrt.rst.txt	4392	0644	edit dl rm
multiprocessing.rst.txt	104494	0644	edit dl rm
netdata.rst.txt	339	0644	edit dl rm
netrc.rst.txt	3007	0644	edit dl rm
nis.rst.txt	1993	0644	edit dl rm
nntplib.rst.txt	21821	0644	edit dl rm
numbers.rst.txt	8089	0644	edit dl rm
numeric.rst.txt	696	0644	edit dl rm
operator.rst.txt	18989	0644	edit dl rm
optparse.rst.txt	76996	0644	edit dl rm
os.path.rst.txt	16419	0644	edit dl rm
os.rst.txt	137534	0644	edit dl rm
ossaudiodev.rst.txt	17845	0644	edit dl rm
othergui.rst.txt	2822	0644	edit dl rm
parser.rst.txt	15148	0644	edit dl rm
pathlib.rst.txt	30045	0644	edit dl rm
pdb.rst.txt	19454	0644	edit dl rm
persistence.rst.txt	591	0644	edit dl rm
pickle.rst.txt	37440	0644	edit dl rm
pickletools.rst.txt	3729	0644	edit dl rm
pipes.rst.txt	2557	0644	edit dl rm
pkgutil.rst.txt	8730	0644	edit dl rm
platform.rst.txt	9624	0644	edit dl rm
plistlib.rst.txt	7398	0644	edit dl rm
poplib.rst.txt	8197	0644	edit dl rm
posix.rst.txt	3694	0644	edit dl rm
pprint.rst.txt	14274	0644	edit dl rm
profile.rst.txt	27969	0644	edit dl rm
pty.rst.txt	3123	0644	edit dl rm
pwd.rst.txt	2739	0644	edit dl rm
pyclbr.rst.txt	3297	0644	edit dl rm
pydoc.rst.txt	4688	0644	edit dl rm
pyexpat.rst.txt	28620	0644	edit dl rm
python.rst.txt	475	0644	edit dl rm
py_compile.rst.txt	3851	0644	edit dl rm
queue.rst.txt	7268	0644	edit dl rm
quopri.rst.txt	2571	0644	edit dl rm
random.rst.txt	18503	0644	edit dl rm
re.rst.txt	64936	0644	edit dl rm
readline.rst.txt	11943	0644	edit dl rm
reprlib.rst.txt	5162	0644	edit dl rm
resource.rst.txt	12291	0644	edit dl rm
rlcompleter.rst.txt	2294	0644	edit dl rm
runpy.rst.txt	8285	0644	edit dl rm
sched.rst.txt	4842	0644	edit dl rm
secrets.rst.txt	5930	0644	edit dl rm
select.rst.txt	28175	0644	edit dl rm
selectors.rst.txt	8925	0644	edit dl rm
shelve.rst.txt	8390	0644	edit dl rm
shlex.rst.txt	16441	0644	edit dl rm
shutil.rst.txt	26014	0644	edit dl rm
signal.rst.txt	17218	0644	edit dl rm
site.rst.txt	9628	0644	edit dl rm
smtpd.rst.txt	10806	0644	edit dl rm
smtplib.rst.txt	23439	0644	edit dl rm
sndhdr.rst.txt	1994	0644	edit dl rm
socket.rst.txt	68202	0644	edit dl rm
socketserver.rst.txt	23712	0644	edit dl rm
spwd.rst.txt	2958	0644	edit dl rm
sqlite3.rst.txt	39215	0644	edit dl rm
ssl.rst.txt	96021	0644	edit dl rm
stat.rst.txt	9805	0644	edit dl rm
statistics.rst.txt	15301	0644	edit dl rm
stdtypes.rst.txt	179240	0644	edit dl rm
string.rst.txt	35374	0644	edit dl rm
stringprep.rst.txt	4279	0644	edit dl rm
struct.rst.txt	19581	0644	edit dl rm
subprocess.rst.txt	45793	0644	edit dl rm
sunau.rst.txt	7390	0644	edit dl rm
superseded.rst.txt	258	0644	edit dl rm
symbol.rst.txt	975	0644	edit dl rm
symtable.rst.txt	4964	0644	edit dl rm
sys.rst.txt	58273	0644	edit dl rm
sysconfig.rst.txt	8749	0644	edit dl rm
syslog.rst.txt	4302	0644	edit dl rm
tabnanny.rst.txt	2007	0644	edit dl rm
tarfile.rst.txt	31749	0644	edit dl rm
telnetlib.rst.txt	7899	0644	edit dl rm
tempfile.rst.txt	13857	0644	edit dl rm
termios.rst.txt	3751	0644	edit dl rm
test.rst.txt	25764	0644	edit dl rm
text.rst.txt	584	0644	edit dl rm
textwrap.rst.txt	10519	0644	edit dl rm
threading.rst.txt	39257	0644	edit dl rm
time.rst.txt	32967	0644	edit dl rm
timeit.rst.txt	13000	0644	edit dl rm
tk.rst.txt	1639	0644	edit dl rm
tkinter.rst.txt	33027	0644	edit dl rm
tkinter.scrolledtext.rst.txt	1255	0644	edit dl rm
tkinter.tix.rst.txt	22653	0644	edit dl rm
tkinter.ttk.rst.txt	58489	0644	edit dl rm
token.rst.txt	2607	0644	edit dl rm
tokenize.rst.txt	10002	0644	edit dl rm
trace.rst.txt	6914	0644	edit dl rm
traceback.rst.txt	17854	0644	edit dl rm
tracemalloc.rst.txt	22089	0644	edit dl rm
tty.rst.txt	1097	0644	edit dl rm
turtle.rst.txt	71221	0644	edit dl rm
types.rst.txt	9999	0644	edit dl rm
typing.rst.txt	34332	0644	edit dl rm
undoc.rst.txt	780	0644	edit dl rm
unicodedata.rst.txt	5762	0644	edit dl rm
unittest.mock-examples.rst.txt	46206	0644	edit dl rm
unittest.mock.rst.txt	85991	0644	edit dl rm
unittest.rst.txt	93304	0644	edit dl rm
unix.rst.txt	446	0644	edit dl rm
urllib.error.rst.txt	2199	0644	edit dl rm
urllib.parse.rst.txt	27332	0644	edit dl rm
urllib.request.rst.txt	59360	0644	edit dl rm
urllib.robotparser.rst.txt	2969	0644	edit dl rm
urllib.rst.txt	466	0644	edit dl rm
uu.rst.txt	2387	0644	edit dl rm
uuid.rst.txt	8760	0644	edit dl rm
venv.rst.txt	20621	0644	edit dl rm
warnings.rst.txt	20152	0644	edit dl rm
wave.rst.txt	6870	0644	edit dl rm
weakref.rst.txt	21328	0644	edit dl rm
webbrowser.rst.txt	9783	0644	edit dl rm
windows.rst.txt	272	0644	edit dl rm
winreg.rst.txt	24073	0644	edit dl rm
winsound.rst.txt	5134	0644	edit dl rm
wsgiref.rst.txt	33047	0644	edit dl rm
xdrlib.rst.txt	8078	0644	edit dl rm
xml.dom.minidom.rst.txt	10198	0644	edit dl rm
xml.dom.pulldom.rst.txt	5186	0644	edit dl rm
xml.dom.rst.txt	39507	0644	edit dl rm
xml.etree.elementtree.rst.txt	43871	0644	edit dl rm
xml.rst.txt	6071	0644	edit dl rm
xml.sax.handler.rst.txt	15399	0644	edit dl rm
xml.sax.reader.rst.txt	12133	0644	edit dl rm
xml.sax.rst.txt	7159	0644	edit dl rm
xml.sax.utils.rst.txt	3901	0644	edit dl rm
xmlrpc.client.rst.txt	23071	0644	edit dl rm
xmlrpc.rst.txt	475	0644	edit dl rm
xmlrpc.server.rst.txt	14990	0644	edit dl rm
zipapp.rst.txt	17392	0644	edit dl rm
zipfile.rst.txt	24005	0644	edit dl rm
zipimport.rst.txt	5805	0644	edit dl rm
zlib.rst.txt	13912	0644	edit dl rm
_dummy_thread.rst.txt	762	0644	edit dl rm
_thread.rst.txt	6886	0644	edit dl rm
__future__.rst.txt	5245	0644	edit dl rm
__main__.rst.txt	904	0644	edit dl rm

Edit: /usr/share/doc/python3-docs/html/_sources/library/email.headerregistry.rst.txt (17957B)

: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.

.. class:: BaseHeader(name, value)

*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.

.. class:: UnstructuredHeader

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.

.. class:: DateHeader

: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.

.. class:: AddressHeader

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 :attr:`~str.join`\ ing 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.

.. class:: SingleAddressHeader

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.

.. class:: MIMEVersionHeader

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

.. class:: ParameterizedMIMEHeader

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.

.. class:: ContentTypeHeader

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

.. class:: ContentDispositionHeader

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.

.. class:: ContentTransferEncoding

Handles the :mailheader:`Content-Transfer-Encoding` header.

.. attribute:: cte

Valid values are ``7bit``, ``8bit``, ``base64``, and
      ``quoted-printable``.  See :rfc:`2045` for more information.

.. class:: HeaderRegistry(base_class=BaseHeader, \
                          default_class=UnstructuredHeader, \
                          use_default_map=True)

This is the factory used by :class:`~email.policy.EmailPolicy` by default.
    ``HeaderRegistry`` builds the class used to create a header instance
    dynamically, using *base_class* and a specialized class retrieved from a
    registry that it holds.  When a given header name does not appear in the
    registry, the class specified by *default_class* is used as the specialized
    class.  When *use_default_map* is ``True`` (the default), the standard
    mapping of header names to classes is copied in to the registry during
    initialization.  *base_class* is always the last class in the generated
    class's ``__bases__`` list.

The default mappings are:

:subject:         UniqueUnstructuredHeader
      :date:            UniqueDateHeader
      :resent-date:     DateHeader
      :orig-date:       UniqueDateHeader
      :sender:          UniqueSingleAddressHeader
      :resent-sender:   SingleAddressHeader
      :to:              UniqueAddressHeader
      :resent-to:       AddressHeader
      :cc:              UniqueAddressHeader
      :resent-cc:       AddressHeader
      :from:            UniqueAddressHeader
      :resent-from:     AddressHeader
      :reply-to:        UniqueAddressHeader

``HeaderRegistry`` has the following methods:

.. method:: map_to_type(self, name, cls)

*name* is the name of the header to be mapped.  It will be converted to
       lower case in the registry.  *cls* is the specialized class to be used,
       along with *base_class*, to create the class used to instantiate headers
       that match *name*.

.. method:: __getitem__(name)

Construct and return a class to handle creating a *name* header.

.. method:: __call__(name, value)

Retrieves the specialized header associated with *name* from the
       registry (using *default_class* if *name* does not appear in the
       registry) and composes it with *base_class* to produce a class,
       calls the constructed class's constructor, passing it the same
       argument list, and finally returns the class instance created thereby.

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.

.. class:: Address(display_name='', username='', domain='', addr_spec=None)

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 ``<>``.

.. class:: Group(display_name=None, addresses=None)

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``.

.. rubric:: Footnotes

.. [1] Originally added in 3.3 as a :term:`provisional module <provisional
       package>`

cmd: