19.3. "mailcap" — Mailcap file handling
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**Source code:** Lib/mailcap.py

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Mailcap files are used to configure how MIME-aware applications such
as mail readers and Web browsers react to files with different MIME
types. (The name “mailcap” is derived from the phrase “mail
capability”.)  For example, a mailcap file might contain a line like
"video/mpeg; xmpeg %s".  Then, if the user encounters an email message
or Web document with the MIME type *video/mpeg*, "%s" will be replaced
by a filename (usually one belonging to a temporary file) and the
**xmpeg** program can be automatically started to view the file.

The mailcap format is documented in **RFC 1524**, “A User Agent
Configuration Mechanism For Multimedia Mail Format Information,” but
is not an Internet standard.  However, mailcap files are supported on
most Unix systems.

mailcap.findmatch(caps, MIMEtype, key='view', filename='/dev/null', plist=[])

   Return a 2-tuple; the first element is a string containing the
   command line to be executed (which can be passed to "os.system()"),
   and the second element is the mailcap entry for a given MIME type.
   If no matching MIME type can be found, "(None, None)" is returned.

   *key* is the name of the field desired, which represents the type
   of activity to be performed; the default value is ‘view’, since in
   the  most common case you simply want to view the body of the MIME-
   typed data.  Other possible values might be ‘compose’ and ‘edit’,
   if you wanted to create a new body of the given MIME type or alter
   the existing body data.  See **RFC 1524** for a complete list of
   these fields.

   *filename* is the filename to be substituted for "%s" in the
   command line; the default value is "'/dev/null'" which is almost
   certainly not what you want, so usually you’ll override it by
   specifying a filename.

   *plist* can be a list containing named parameters; the default
   value is simply an empty list.  Each entry in the list must be a
   string containing the parameter name, an equals sign ("'='"), and
   the parameter’s value.  Mailcap entries can contain  named
   parameters like "%{foo}", which will be replaced by the value of
   the parameter named ‘foo’.  For example, if the command line
   "showpartial %{id} %{number} %{total}" was in a mailcap file, and
   *plist* was set to "['id=1', 'number=2', 'total=3']", the resulting
   command line would be "'showpartial 1 2 3'".

   In a mailcap file, the “test” field can optionally be specified to
   test some external condition (such as the machine architecture, or
   the window system in use) to determine whether or not the mailcap
   line applies.  "findmatch()" will automatically check such
   conditions and skip the entry if the check fails.

mailcap.getcaps()

   Returns a dictionary mapping MIME types to a list of mailcap file
   entries. This dictionary must be passed to the "findmatch()"
   function.  An entry is stored as a list of dictionaries, but it
   shouldn’t be necessary to know the details of this representation.

   The information is derived from all of the mailcap files found on
   the system. Settings in the user’s mailcap file "$HOME/.mailcap"
   will override settings in the system mailcap files "/etc/mailcap",
   "/usr/etc/mailcap", and "/usr/local/etc/mailcap".

An example usage:

   >>> import mailcap
   >>> d = mailcap.getcaps()
   >>> mailcap.findmatch(d, 'video/mpeg', filename='tmp1223')
   ('xmpeg tmp1223', {'view': 'xmpeg %s'})
