A Python idiom that I don't get

I am trying to get some existing CPython 2.4 code working
under Jython 
(2.1) and I am puzzled by a line in the following function. 
It seems to 
be a Python 2.4 idiom that is opaque to me.

The line is:
     prefix = os.path.commonprefix(filter( bool, lines ))

and I don't understand what that 'bool' is doing.  Or
rather, I think 
that I see what it is doing, but I am not sure - and I
don't much like it.

filter is the built-in filter and it requires a callable
returning a 
bool as the first argument.  It seems that 'bool' without
arguments is a 
callable that always evaluates to True (or at least
non-zero) so this 
'bool' always returns True.  Is this really true (sic) by
intention or 
is it just an implemenation artifact?

I tried replacing 'bool' with 'True' but that won't
work because True is 
not callable.

I replaced 'bool' with 'lambda True: True' as in:
     prefix = os.path.commonprefix(filter( lambda True:
True, lines ))
and that does seem to work - and pass its unit tests.

Have I got this right and can I replace 'bool' with the
lambda expression?

Or is there a clearer way to do this?

Thanks,


Don.

The full function is:

def find_common( lines ):
     """find and return a common prefix to
all the passed lines.
     Should not include trailing spaces
     """

     if not lines: return ""

     # get the common prefix of the non-blank lines. This
isn't the most
     # efficient way of doing this by _far_, but it _is_ the
shortest
     prefix = os.path.commonprefix(filter( bool, lines ))
     if not prefix: return ""

     # this regular expression will match an 'interesting'
line prefix
     matched =
re.match("(\s*\w{1,4}>|\W+)", prefix)
     if not matched: return ""

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