Distinguishing between maildir, mbox, and MH files/directories?

[Terry Reedy]

On 8/31/2014 2:45 PM, Tim Chase wrote:
> Tinkering around with a little script, I found myself with the need
> to walk a directory tree and process mail messaged found within.
> Sometimes these end up being mbox files (with multiple messages
> within), sometimes it's a Maildir structure with messages in each
> individual file and extra holding directories, and sometimes it's a
> MH directory.  To complicate matters, there's also the possibility of
> non-{mbox,maildir,mh) files such as binary MUA caches appearing
> alongside these messages.

I know nothing about the format within such file but will make a couple 
of assumptions.

> Python knows how to handle each just fine as long as I tell it what
> type of file to expect.

By instantiating mailbox.mbox or mailbox.Maildir

>  But is there a straight-forward way to
> distinguish them?  (FWIW, the *nix "file" utility is just reporting
> "ASCII text", sometimes "with very long lines", and sometimes
> erroneously flags them as C or C++ files‽).

> All I need is "is it maildir, mbox, mh, or something else" (I don't
> have to get more complex for the "something else") inside an os.walk
> loop.

Simple method: try to parse with mbox and then Maildir and if either 
succeeds, assume that the file was in the corresponding format.

try:
   <process as mbox>
except mailbox.FormatError:
   try:
     <process as Maildir>
   except mailbox.FormatError:
     pass

If a format is detectable in the first line or two, you could try to 
write a detect(path) that would return the corresponding class.  That 
would perhaps be a good addition to the mailbox module.  On the other 
hand, if you are interested in just those two classes, and not any of 
the Maildir subclasses, the above might be good enough. I am assuming 
that FormatError is raised without reading the whole file and doing a 
lot else before detecting the mismatch.

-- 
Terry Jan Reedy