1. 基本语法
isinstance(object, classinfo)
Return true if the object argument is an instance of the classinfo argument, or of a (direct, indirect or virtual) subclass thereof. Also return true if classinfo is a type object (new-style class) and object is an object of that type or of a (direct, indirect or virtual) subclass thereof. If object is not a class instance or an object of the given type, the function always returns false. If classinfo is neither a class object nor a type object, it may be a tuple of class or type objects, or may recursively contain other such tuples (other sequence types are not accepted). If classinfo is not a class, type, or tuple of classes, types, and such tuples, a TypeError exception is raised.
classinfo 处可以是 a class, type, or tuple of classes and types，
如果是 tuple，则满足 tuple 中的任何一个即返回 True
2. 字符串的类型判断。
字符串，分为 str 和 unicode，二者均继承自 basestring
>>> isinstance(u'3.0', unicode)
True
>>> isinstance('3.0', str)
True
>>> isinstance(u'3.0', str)
False
>>> isinstance(u'3.0', str)
False
>>> isinstance(u'3.0', basestring)
True
>>> isinstance('3.0', basestring)
True
3. 数字的类型判断
数字分为 int 和 float，暂未发现二者共同的有效父类。
可以用 (int, float) tuple 来判断是否为数字（int 或 float）
>>> isinstance('3', (int, float))
False
>>> isinstance(3.0, (int, float))
True
>>> isinstance(3, (int, float))
True
>>> isinstance(3.0, float)
True
>>> isinstance(3.0, int)
False
>>> isinstance(3, float)
False
>>> isinstance(3, int)
True
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