'Return a new named %-formatting placeholder for expr.
This creates a %-formatting placeholder for expr in the
current formatting context, e.g. ``%(py0)s``.  The placeholder
and expr are placed in the current format context so that it
can be used on the next call to .pop_format_context().'

def explanation_param(self, expr):

    specifier = ('py' + str(next(self.variable_counter)))
    self.explanation_specifiers[specifier] = expr
    return (('%(' + specifier) + ')s')
