# `mailcap` — Mailcap file handling

Source code: Lib/mailcap.py

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.

  Changed in version 3.11: To prevent security issues with shell metacharacters (symbols that have special effects in a shell command line), `findmatch` will refuse to inject ASCII characters other than alphanumerics and `@+=:,./-_` into the returned command line.

  If a disallowed character appears in filename, `findmatch` will always return `(None, None)` as if no entry was found. If such a character appears elsewhere (a value in plist or in MIMEtype), `findmatch` will ignore all mailcap entries which use that value. A `warning` will be raised in either case.

`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'})