wikipedia Type introspection

In computing, type introspection(类型反省) is the ability of a program to examine the type or properties of an object at runtime. Some programming languages possess this capability.

Introspection should not be confused with reflection, which goes a step further and is the ability for a program to manipulate the values, meta-data, properties and/or functions of an object at runtime. Some programming languages - e.g. Java, Python and Go - also possess that capability.

Examples

Python

The most common method of introspection in Python is using the dir function to detail the attributes of an object. For example:

class Foo(object):
    def __init__(self, val):
        self.x = val
    def bar(self):
        return self.x

...

>>> dir(Foo(5))
['__class__', '__delattr__', '__dict__', '__doc__', '__getattribute__',
'__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__',
'__repr__', '__setattr__', '__str__', '__weakref__', 'bar', 'x']

Also, the built-in functions type and isinstance can be used to determine what an object is while hasattr can determine what an object does. For example:

>>> a = Foo(10)
>>> b = Bar(11)
>>> type(a)
<type 'Foo'>
>>> isinstance(a, Foo)
True
>>> isinstance(a, type(a))
True
>>> isinstance(a, type(b))
False
>>> hasattr(a, 'bar')
True

In Python 2 but not Python 3, declaring class Foo instead of class Foo(object) will result in type returning the generic instance type instead of the class.[5]

NOTE: 参见youdao/programming-language-python/python2to3，其中总结了python2和python3之间的差异