检测序列参数的正确方法？

此外,ABC可以定制以解释异常,例如不将字符串视为序列.这是一个例子:

import abc
import collections

class Atomic(object):
    __metaclass__ = abc.ABCMeta
    @classmethod
    def __subclasshook__(cls, other):
        return not issubclass(other, collections.Sequence) or NotImplemented

Atomic.register(basestring)

注册后,Atomic类可以与isinstance和issubclass一起使用:

assert isinstance("hello", Atomic) == True

这仍然比硬编码列表好得多,因为您只需要在规则中注册例外,并且代码的外部用户可以注册自己的.

请注意,在Python 3中,指定元类的语法已更改,并且basestring删除了抽象超类,这需要使用以下内容:

class Atomic(metaclass=abc.ABCMeta):
    @classmethod
    def __subclasshook__(cls, other):
        return not issubclass(other, collections.Sequence) or NotImplemented

Atomic.register(str)

如果需要,可以编写兼容Python 2.6+ 和 3.x的代码,但这样做需要使用稍微复杂的技术来动态创建所需的抽象基类,从而避免由于元类语法差异导致的语法错误.这与Benjamin Peterson的六个模块的with_metaclass()功能基本相同.

class _AtomicBase(object):
    @classmethod
    def __subclasshook__(cls, other):
        return not issubclass(other, collections.Sequence) or NotImplemented

class Atomic(abc.ABCMeta("NewMeta", (_AtomicBase,), {})):
    pass

try:
    unicode = unicode
except NameError:  # 'unicode' is undefined, assume Python >= 3
    Atomic.register(str)  # str includes unicode in Py3, make both Atomic
    Atomic.register(bytes)  # bytes will also be considered Atomic (optional)
else:
    # basestring is the abstract superclass of both str and unicode types
    Atomic.register(basestring)  # make both types of strings Atomic

在2.6之前的版本中,operator模块中有类型检查器.

>>> import operator
>>> operator.isSequenceType([])
True
>>> operator.isSequenceType(0)
False

def to_sequence(arg):
    ''' 
    determine whether an arg should be treated as a "unit" or a "sequence"
    if it's a unit, return a 1-tuple with the arg
    '''
    def _multiple(x):  
        return hasattr(x,"__iter__")
    if _multiple(arg):  
        return arg
    else:
        return (arg,)

>>> to_sequence("a string")
('a string',)
>>> to_sequence( (1,2,3) )
(1, 2, 3)
>>> to_sequence( xrange(5) )
xrange(5)