19.1.6. email.headerregistry: Custom Header Objects

Source code: Lib/email/headerregistry.py

New in version 3.6: [1]

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:


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.


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.


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 headers may not contain arbitrary binary data. If 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 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 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 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 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:


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:


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:


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.


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 joining 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 from the groups attribute of the source header.

class email.headerregistry.SingleAddressHeader

A subclass of AddressHeader that adds one additional attribute:


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:


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


The major version number as an integer


The minor version number as an integer

class email.headerregistry.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.


A dictionary mapping parameter names to parameter values.

class email.headerregistry.ContentTypeHeader

A ParameterizedMIMEHeader class that handles the Content-Type header.


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

class email.headerregistry.ContentDispositionHeader

A ParameterizedMIMEHeader class that handles the Content-Disposition header.


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

class email.headerregistry.ContentTransferEncoding

Handles the Content-Transfer-Encoding header.


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:


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.


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>



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.


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.


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


The domain portion of the address.


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.


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.


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.


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


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.


[1]Oringally added in 3.3 as a provisional module