mirror of https://github.com/python/cpython.git
217 lines
9.3 KiB
ReStructuredText
217 lines
9.3 KiB
ReStructuredText
:mod:`email` Package Architecture
|
|
=================================
|
|
|
|
Overview
|
|
--------
|
|
|
|
The email package consists of three major components:
|
|
|
|
Model
|
|
An object structure that represents an email message, and provides an
|
|
API for creating, querying, and modifying a message.
|
|
|
|
Parser
|
|
Takes a sequence of characters or bytes and produces a model of the
|
|
email message represented by those characters or bytes.
|
|
|
|
Generator
|
|
Takes a model and turns it into a sequence of characters or bytes. The
|
|
sequence can either be intended for human consumption (a printable
|
|
unicode string) or bytes suitable for transmission over the wire. In
|
|
the latter case all data is properly encoded using the content transfer
|
|
encodings specified by the relevant RFCs.
|
|
|
|
Conceptually the package is organized around the model. The model provides both
|
|
"external" APIs intended for use by application programs using the library,
|
|
and "internal" APIs intended for use by the Parser and Generator components.
|
|
This division is intentionally a bit fuzzy; the API described by this
|
|
documentation is all a public, stable API. This allows for an application
|
|
with special needs to implement its own parser and/or generator.
|
|
|
|
In addition to the three major functional components, there is a third key
|
|
component to the architecture:
|
|
|
|
Policy
|
|
An object that specifies various behavioral settings and carries
|
|
implementations of various behavior-controlling methods.
|
|
|
|
The Policy framework provides a simple and convenient way to control the
|
|
behavior of the library, making it possible for the library to be used in a
|
|
very flexible fashion while leveraging the common code required to parse,
|
|
represent, and generate message-like objects. For example, in addition to the
|
|
default :rfc:`5322` email message policy, we also have a policy that manages
|
|
HTTP headers in a fashion compliant with :rfc:`2616`. Individual policy
|
|
controls, such as the maximum line length produced by the generator, can also
|
|
be controlled individually to meet specialized application requirements.
|
|
|
|
|
|
The Model
|
|
---------
|
|
|
|
The message model is implemented by the :class:`~email.message.Message` class.
|
|
The model divides a message into the two fundamental parts discussed by the
|
|
RFC: the header section and the body. The `Message` object acts as a
|
|
pseudo-dictionary of named headers. Its dictionary interface provides
|
|
convenient access to individual headers by name. However, all headers are kept
|
|
internally in an ordered list, so that the information about the order of the
|
|
headers in the original message is preserved.
|
|
|
|
The `Message` object also has a `payload` that holds the body. A `payload` can
|
|
be one of two things: data, or a list of `Message` objects. The latter is used
|
|
to represent a multipart MIME message. Lists can be nested arbitrarily deeply
|
|
in order to represent the message, with all terminal leaves having non-list
|
|
data payloads.
|
|
|
|
|
|
Message Lifecycle
|
|
-----------------
|
|
|
|
The general lifecycle of a message is:
|
|
|
|
Creation
|
|
A `Message` object can be created by a Parser, or it can be
|
|
instantiated as an empty message by an application.
|
|
|
|
Manipulation
|
|
The application may examine one or more headers, and/or the
|
|
payload, and it may modify one or more headers and/or
|
|
the payload. This may be done on the top level `Message`
|
|
object, or on any sub-object.
|
|
|
|
Finalization
|
|
The Model is converted into a unicode or binary stream,
|
|
or the model is discarded.
|
|
|
|
|
|
|
|
Header Policy Control During Lifecycle
|
|
--------------------------------------
|
|
|
|
One of the major controls exerted by the Policy is the management of headers
|
|
during the `Message` lifecycle. Most applications don't need to be aware of
|
|
this.
|
|
|
|
A header enters the model in one of two ways: via a Parser, or by being set to
|
|
a specific value by an application program after the Model already exists.
|
|
Similarly, a header exits the model in one of two ways: by being serialized by
|
|
a Generator, or by being retrieved from a Model by an application program. The
|
|
Policy object provides hooks for all four of these pathways.
|
|
|
|
The model storage for headers is a list of (name, value) tuples.
|
|
|
|
The Parser identifies headers during parsing, and passes them to the
|
|
:meth:`~email.policy.Policy.header_source_parse` method of the Policy. The
|
|
result of that method is the (name, value) tuple to be stored in the model.
|
|
|
|
When an application program supplies a header value (for example, through the
|
|
`Message` object `__setitem__` interface), the name and the value are passed to
|
|
the :meth:`~email.policy.Policy.header_store_parse` method of the Policy, which
|
|
returns the (name, value) tuple to be stored in the model.
|
|
|
|
When an application program retrieves a header (through any of the dict or list
|
|
interfaces of `Message`), the name and value are passed to the
|
|
:meth:`~email.policy.Policy.header_fetch_parse` method of the Policy to
|
|
obtain the value returned to the application.
|
|
|
|
When a Generator requests a header during serialization, the name and value are
|
|
passed to the :meth:`~email.policy.Policy.fold` method of the Policy, which
|
|
returns a string containing line breaks in the appropriate places. The
|
|
:meth:`~email.policy.Policy.cte_type` Policy control determines whether or
|
|
not Content Transfer Encoding is performed on the data in the header. There is
|
|
also a :meth:`~email.policy.Policy.binary_fold` method for use by generators
|
|
that produce binary output, which returns the folded header as binary data,
|
|
possibly folded at different places than the corresponding string would be.
|
|
|
|
|
|
Handling Binary Data
|
|
--------------------
|
|
|
|
In an ideal world all message data would conform to the RFCs, meaning that the
|
|
parser could decode the message into the idealized unicode message that the
|
|
sender originally wrote. In the real world, the email package must also be
|
|
able to deal with badly formatted messages, including messages containing
|
|
non-ASCII characters that either have no indicated character set or are not
|
|
valid characters in the indicated character set.
|
|
|
|
Since email messages are *primarily* text data, and operations on message data
|
|
are primarily text operations (except for binary payloads of course), the model
|
|
stores all text data as unicode strings. Un-decodable binary inside text
|
|
data is handled by using the `surrogateescape` error handler of the ASCII
|
|
codec. As with the binary filenames the error handler was introduced to
|
|
handle, this allows the email package to "carry" the binary data received
|
|
during parsing along until the output stage, at which time it is regenerated
|
|
in its original form.
|
|
|
|
This carried binary data is almost entirely an implementation detail. The one
|
|
place where it is visible in the API is in the "internal" API. A Parser must
|
|
do the `surrogateescape` encoding of binary input data, and pass that data to
|
|
the appropriate Policy method. The "internal" interface used by the Generator
|
|
to access header values preserves the `surrogateescaped` bytes. All other
|
|
interfaces convert the binary data either back into bytes or into a safe form
|
|
(losing information in some cases).
|
|
|
|
|
|
Backward Compatibility
|
|
----------------------
|
|
|
|
The :class:`~email.policy.Policy.Compat32` Policy provides backward
|
|
compatibility with version 5.1 of the email package. It does this via the
|
|
following implementation of the four+1 Policy methods described above:
|
|
|
|
header_source_parse
|
|
Splits the first line on the colon to obtain the name, discards any spaces
|
|
after the colon, and joins the remainder of the line with all of the
|
|
remaining lines, preserving the linesep characters to obtain the value.
|
|
Trailing carriage return and/or linefeed characters are stripped from the
|
|
resulting value string.
|
|
|
|
header_store_parse
|
|
Returns the name and value exactly as received from the application.
|
|
|
|
header_fetch_parse
|
|
If the value contains any `surrogateescaped` binary data, return the value
|
|
as a :class:`~email.header.Header` object, using the character set
|
|
`unknown-8bit`. Otherwise just returns the value.
|
|
|
|
fold
|
|
Uses :class:`~email.header.Header`'s folding to fold headers in the
|
|
same way the email5.1 generator did.
|
|
|
|
binary_fold
|
|
Same as fold, but encodes to 'ascii'.
|
|
|
|
|
|
New Algorithm
|
|
-------------
|
|
|
|
header_source_parse
|
|
Same as legacy behavior.
|
|
|
|
header_store_parse
|
|
Same as legacy behavior.
|
|
|
|
header_fetch_parse
|
|
If the value is already a header object, returns it. Otherwise, parses the
|
|
value using the new parser, and returns the resulting object as the value.
|
|
`surrogateescaped` bytes get turned into unicode unknown character code
|
|
points.
|
|
|
|
fold
|
|
Uses the new header folding algorithm, respecting the policy settings.
|
|
surrogateescaped bytes are encoded using the ``unknown-8bit`` charset for
|
|
``cte_type=7bit`` or ``8bit``. Returns a string.
|
|
|
|
At some point there will also be a ``cte_type=unicode``, and for that
|
|
policy fold will serialize the idealized unicode message with RFC-like
|
|
folding, converting any surrogateescaped bytes into the unicode
|
|
unknown character glyph.
|
|
|
|
binary_fold
|
|
Uses the new header folding algorithm, respecting the policy settings.
|
|
surrogateescaped bytes are encoded using the `unknown-8bit` charset for
|
|
``cte_type=7bit``, and get turned back into bytes for ``cte_type=8bit``.
|
|
Returns bytes.
|
|
|
|
At some point there will also be a ``cte_type=unicode``, and for that
|
|
policy binary_fold will serialize the message according to :rfc:``5335``.
|