Short repr to use when rendering Pipeline graphs.

def graph_repr(self):
        """Short repr to use when rendering Pipeline graphs."""

        # Replace any floating point numbers in the expression
        # with their scientific notation
        final = re.sub(r"[-+]?\d*\.\d+",
                       lambda x: format(float(x.group(0)), '.2E'),
                       self._expr)
        # Graphviz interprets `\l` as "divide label into lines, left-justified"
        return "Expression:\\l  {}\\l".format(
            final,
        )