import ast

class CustomError(Exception):
    def __init__(self, msg, node = None):
        self.msg = msg
        if node is not None:
            self.at(node)

    def at(self, node):
        self.msg = f'Error at {node.lineno}:{node.col_offset+1}-' \
            f'{node.end_lineno}:{node.end_col_offset+1} ' \
            f'"{ast.unparse(node)}"\n{self.msg}'
        return self

def stringify_subst(subst):
    if isinstance(subst, str):
        return subst
    elements = [f"{key}: {str(value)}" for key, value in subst.items()]
    return "{" + ', '.join(elements) + "}"