Flexible doc adding

def trim(docstring):
    """Remove insignificant indentation from a docstring.

    This implementation is taken directly from PEP-0257.
    """
    if not docstring:
        return ''
    # Convert tabs to spaces (following the normal Python rules)
    # and split into a list of lines:
    lines = docstring.expandtabs().splitlines()
    # Determine minimum indentation (first line doesn't count):
    indent = sys.maxint
    for line in lines[1:]:
        stripped = line.lstrip()
        if stripped:
            indent = min(indent, len(line) - len(stripped))
    # Remove indentation (first line is special):
    trimmed = [lines[0].strip()]
    if indent < sys.maxint:
        for line in lines[1:]:
            trimmed.append(line[indent:].rstrip())
    # Strip off trailing and leading blank lines:
    while trimmed and not trimmed[-1]:
        trimmed.pop()
    while trimmed and not trimmed[0]:
        trimmed.pop(0)
    # Return a single string:
    return '\n'.join(trimmed)


class WithDoc:

    def __new__(cls, *args):
        if not args:
            raise ValueError('Not enough args')
        if len(args) > 2:
            raise ValueError('Too many args')
        if len(args) == 2:
            obj, doc = args
            return cls(doc)(obj)
        else:
            return super().__new__(cls)

    def __init__(self, doc):
        self.doc = trim(doc)

    def __call__(self, obj):
        existing = trim(obj.__doc__)
        if existing:
            obj.__doc__ = self.doc + '\n\n' + existing
        else:
            obj.__doc__ = self.doc
        return obj