19.1.5. "email.headerregistry": Custom Header Objects
*****************************************************

New in version 3.3: as a *provisional module*.

**Source code:** Lib/email/headerregistry.py

Note: The headerregistry module has been included in the standard
  library on a *provisional basis*. Backwards incompatible changes (up
  to and including removal of the module) may occur if deemed
  necessary by the core developers.

======================================================================

Headers are represented by customized subclasses of "str".  The
particular class used to represent a given header is determined by the
"header_factory" of the "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 "EmailPolicy", all
headers are produced by "HeaderRegistry" and have "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 "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 "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
"HeaderRegistry", and finally the support classes used to represent
the data parsed from structured headers.

class email.headerregistry.BaseHeader(name, value)

   *name* and *value* are passed to "BaseHeader" from the
   "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:

   name

      The name of the header (the portion of the field before the
      ‘:’).  This is exactly the value passed in the "header_factory"
      call for *name*; that is, case is preserved.

   defects

      A tuple of "HeaderDefect" instances reporting any RFC compliance
      problems found during parsing.  The email package tries to be
      complete about detecting compliance issues.  See the "errors"
      module for a discussion of the types of defects that may be
      reported.

   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:

   fold(*, policy)

      Return a string containing "linesep" characters as required to
      correctly fold the header according to *policy*.  A "cte_type"
      of "8bit" will be treated as if it were "7bit", since strings
      may not contain binary data.

   "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 "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 transport 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 email.headerregistry.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 *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 back in to
   the original 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 email.headerregistry.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:

   datetime

      If the header value can be recognized as a valid date of one
      form or another, this attribute will contain a "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
      "datetime" will be a naive "datetime".  If a specific timezone
      offset is found (including *+0000*), then "datetime" will
      contain an aware "datetime" that uses "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 "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 "localtime()" function from the
   "utils" module:

      msg['Date'] = utils.localtime()

   This example sets the date header to the current time and date
   using the current timezone offset.

class email.headerregistry.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:

   groups

      A tuple of "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 "display_name" is "None".

   addresses

      A tuple of "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.  "idna" encoded domain names are also decoded
   to unicode.  The "decoded" value is set by "join"ing the "str"
   value of the elements of the "groups" attribute with "', '".

   A list of "Address" and "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 "groups" attribute of the source header.

class email.headerregistry.SingleAddressHeader

   A subclass of "AddressHeader" that adds one additional 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 "policy"), accessing this
      attribute will result in a "ValueError".

Many of the above classes also have a "Unique" variant (for example,
"UniqueUnstructuredHeader").  The only difference is that in the
"Unique" variant, "max_count" is set to 1.

class email.headerregistry.MIMEVersionHeader

   There is really only one valid value for the *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:

   version

      The version number as a string, with any whitespace and/or
      comments removed.

   major

      The major version number as an integer

   minor

      The minor version number as an integer

class email.headerregistry.ParameterizedMIMEHeader

   MOME 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.

   params

      A dictionary mapping parameter names to parameter values.

class email.headerregistry.ContentTypeHeader

   A "ParameterizedMIMEHeader" class that handles the *Content-Type*
   header.

   content_type

      The content type string, in the form "maintype/subtype".

   maintype

   subtype

class email.headerregistry.ContentDispositionHeader

   A "ParameterizedMIMEHeader" class that handles the *Content-
   Disposition* header.

   content-disposition

      "inline" and "attachment" are the only valid values in common
      use.

class email.headerregistry.ContentTransferEncoding

   Handles the *Content-Transfer-Encoding* header.

   cte

      Valid values are "7bit", "8bit", "base64", and "quoted-
      printable".  See **RFC 2045** for more information.

class email.headerregistry.HeaderRegistry(base_class=BaseHeader, default_class=UnstructuredHeader, use_default_map=True)

   This is the factory used by "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:

   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*.

   __getitem__(name)

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

   __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 email.headerregistry.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.

   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.

   username

      The "username" portion of the address, with all quoting removed.

   domain

      The "domain" portion of the address.

   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.

   __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 email.headerregistry.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*.

   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.

   addresses

      A possibly empty tuple of "Address" objects representing the
      addresses in the group.

   __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".
