def formatWithCall(formatString, mapping):
    """
    Format a string like L{unicode.format}, but:

        - taking only a name mapping; no positional arguments

        - with the additional syntax that an empty set of parentheses
          correspond to a formatting item that should be called, and its result
          C{str}'d, rather than calling C{str} on the element directly as
          normal.

    For example::

        >>> formatWithCall("{string}, {function()}.",
        ...                dict(string="just a string",
        ...                     function=lambda: "a function"))
        'just a string, a function.'

    @param formatString: A PEP-3101 format string.
    @type formatString: L{unicode}

    @param mapping: A L{dict}-like object to format.

    @return: The string with formatted values interpolated.
    @rtype: L{unicode}
    """
    return unicode(
        aFormatter.vformat(formatString, (), CallMapping(mapping))
    )

    Format a string like L{unicode.format}, but:

        - taking only a name mapping; no positional arguments

        - with the additional syntax that an empty set of parentheses
          correspond to a formatting item that should be called, and its result
          C{str}'d, rather than calling C{str} on the element directly as
          normal.

    For example::

        >>> formatWithCall("{string}, {function()}.",
        ...                dict(string="just a string",
        ...                     function=lambda: "a function"))
        'just a string, a function.'

    @param formatString: A PEP-3101 format string.
    @type formatString: L{unicode}

    @param mapping: A L{dict}-like object to format.

    @return: The string with formatted values interpolated.
    @rtype: L{unicode}
    
stackoverflow/venv/lib/python3.6/site-packages/twisted/logger/_format.py