def explanation_param(self, expr: ast.expr) -> str:
        """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().
        """
        specifier = "py" + str(next(self.variable_counter))
        self.explanation_specifiers[specifier] = expr
        return "%(" + specifier + ")s"
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().
        
src/_pytest/assertion/rewrite.py