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Character Encoding Detection

RFC 3023 defines the interaction between :abbr:`XML (Extensible Markup Language)` and :abbr:`HTTP (Hypertext Transfer Protocol)` as it relates to character encoding. :abbr:`XML (Extensible Markup Language)` and :abbr:`HTTP (Hypertext Transfer Protocol)` have different ways of specifying character encoding and different defaults in case no encoding is specified, and determining which value takes precedence depends on a variety of factors.

Introduction to Character Encoding

In :abbr:`XML (Extensible Markup Language)`, the character encoding is optional and may be given in the :abbr:`XML (Extensible Markup Language)` declaration in the first line of the document, like this:

<?xml version="1.0" encoding="utf-8"?>

If no encoding is given, :abbr:`XML (Extensible Markup Language)` supports the use of a Byte Order Mark to identify the document as some flavor of UTF-32, UTF-16, or UTF-8. Section F of the XML specification outlines the process for determining the character encoding based on unique properties of the Byte Order Mark in the first two to four bytes of the document.

If no encoding is specified and no Byte Order Mark is present, :abbr:`XML (Extensible Markup Language)` defaults to UTF-8.

:abbr:`HTTP (Hypertext Transfer Protocol)` uses :abbr:`MIME` to define a method of specifying the character encoding, as part of the Content-Type :abbr:`HTTP (Hypertext Transfer Protocol)` header, which looks like this:

Content-Type: text/html; charset="utf-8"

If no charset is specified, :abbr:`HTTP (Hypertext Transfer Protocol)` defaults to iso-8859-1, but only for text/* media types. For other media types, the default encoding is undefined, which is where :abbr:`RFC (Request For Comments)` 3023 comes in.

According to :abbr:`RFC (Request For Comments)` 3023, if the media type given in the Content-Type :abbr:`HTTP (Hypertext Transfer Protocol)` header is application/xml, application/xml-dtd, application/xml-external-parsed-entity, or any one of the subtypes of application/xml such as application/atom+xml or application/rss+xml or even application/rdf+xml, then the encoding is

  1. the encoding given in the charset parameter of the Content-Type :abbr:`HTTP (Hypertext Transfer Protocol)` header, or
  2. the encoding given in the encoding attribute of the :abbr:`XML (Extensible Markup Language)` declaration within the document, or
  3. utf-8.

On the other hand, if the media type given in the Content-Type :abbr:`HTTP (Hypertext Transfer Protocol)` header is text/xml, text/xml-external-parsed-entity, or a subtype like text/AnythingAtAll+xml, then the encoding attribute of the :abbr:`XML (Extensible Markup Language)` declaration within the document is ignored completely, and the encoding is

  1. the encoding given in the charset parameter of the Content-Type :abbr:`HTTP (Hypertext Transfer Protocol)` header, or
  2. us-ascii.

Handling Incorrectly-Declared Encodings

:program:`Universal Feed Parser` initially uses the rules specified in :abbr:`RFC (Request For Comments)` 3023 to determine the character encoding of the feed. If parsing succeeds, then that's that. If parsing fails, :program:`Universal Feed Parser` sets the bozo bit to 1 and sets bozo_exception to feedparser.CharacterEncodingOverride. Then it tries to reparse the feed with the following character encodings:

  1. the encoding specified in the :abbr:`XML (Extensible Markup Language)` declaration
  2. the encoding sniffed from the first four bytes of the document (as per Section F)
  3. the encoding auto-detected by the chardet, if installed
  4. utf-8
  5. windows-1252

If the character encoding can not be determined, :program:`Universal Feed Parser` sets the bozo bit to 1 and sets bozo_exception to feedparser.CharacterEncodingUnknown. In this case, parsed values will be strings, not Unicode strings.

Handling Incorrectly-Declared Media Types

:abbr:`RFC (Request For Comments)` 3023 only applies when the feed is served over :abbr:`HTTP (Hypertext Transfer Protocol)` with a Content-Type that declares the feed to be some kind of :abbr:`XML (Extensible Markup Language)`. However, some web servers are severely misconfigured and serve feeds with a Content-Type of text/plain, application/octet-stream, or some completely bogus media type.

:program:`Universal Feed Parser` will attempt to parse such feeds, but it will set the bozo bit to 1 and set bozo_exception to feedparser.NonXMLContentType.

.. seealso::

    * `RFC 3023 <http://www.ietf.org/rfc/rfc3023.txt>`_

    * `Section F of the XML specification <http://www.w3.org/TR/REC-xml/#sec-guessing-no-ext-info>`_

    * `On the well-formedness of XML documents served as text/plain <http://www.imc.org/atom-syntax/mail-archive/msg05575.html>`_