ValueError: too many values to unpack (expected 2)

class Mymeta(type):
    def __new__(cls, class_name, class_bases, class_attrs):
        print(--->, cls)  # ---> <class __main__.Mymeta>
        print(--->, class_name)  # ---> Chinese
        print(--->, class_bases)  # ---> (<class object>,)
        print(--->, class_attrs)  # Chinese, country: china, skin: yello, __init__: ....
        print(class_attrs.items())
        update_attrs = {}
        for key, value in class_attrs:
            if not callable(value) and not key.startswith(__):
                update_attrs[key.upper()] = value
            else:
                update_attrs[key] = value

        return type.__new__(cls, class_name, class_bases, update_attrs)


class Chinese(object, metaclass=Mymeta):
    country = china
    skin = yello

class Mymeta(type): def __new__(cls, class_name, class_bases, class_attrs): print(--->, cls) # ---> print(--->, class_name) # ---> Chinese print(--->, class_bases) # ---> ( ,) print(--->, class_attrs) # Chinese, country: china, skin: yello, __init__: .... print(class_attrs.items()) update_attrs = {} for key, value in class_attrs: if not callable(value) and not key.startswith(__): update_attrs[key.upper()] = value else: update_attrs[key] = value return type.__new__(cls, class_name, class_bases, update_attrs) class Chinese(object, metaclass=Mymeta): country = china skin = yello

返回了下面的错误：

for key, value in class_attrs:
ValueError: too many values to unpack (expected 2)

for key, value in class_attrs: ValueError: too many values to unpack (expected 2)

原因是字典这个是一个迭代器对象，参考官方文档找到下列说明，字典只支持Key的遍历,，如果想对key，value，则可以使用items方法。 The “implicit” iteration that dictionaries support only iterates over keys.

python只支持对于key的遍历，所以不能使用for k,v这种形式，这个时候会提示ValueError: too many values to unpack， 正确代码如下：

for key, value in class_attrs.items():

for key, value in class_attrs.items():

class Mymeta(type): def __new__(cls, class_name, class_bases, class_attrs): print(--->, cls) # ---> print(--->, class_name) # ---> Chinese print(--->, class_bases) # ---> ( ,) print(--->, class_attrs) # Chinese, country: china, skin: yello, __init__: .... print(class_attrs.items()) update_attrs = {} for key, value in class_attrs: if not callable(value) and not key.startswith(__): update_attrs[key.upper()] = value else: update_attrs[key] = value return type.__new__(cls, class_name, class_bases, update_attrs) class Chinese(object, metaclass=Mymeta): country = china skin = yello 返回了下面的错误： for key, value in class_attrs: ValueError: too many values to unpack (expected 2) 原因是字典这个是一个迭代器对象，参考官方文档找到下列说明，字典只支持Key的遍历,，如果想对key，value，则可以使用items方法。 The “implicit” iteration that dictionaries support only iterates over keys. python只支持对于key的遍历，所以不能使用for k,v这种形式，这个时候会提示ValueError: too many values to unpack， 正确代码如下： for key, value in class_attrs.items():