I have the following code:
>>> class MyClass:
pass
>>> myObj=MyClass()
>>> type(myObj)
<type 'instance'> <==== Why it is not type MyClass ?
>>> type(MyClass)
<type 'classobj'> <=== Why it is not just 'MyClass'?
>>> isinstance(myObj, instance) <==== Why the 'instance' is not defined?
Traceback (most recent call last):
File "<pyshell#91>", line 1, in <module>
isinstance(myObj, instance)
NameError: name 'instance' is not defined
>>> isinstance(myObj, MyClass)
True
>>> myObj.__class__
<class __main__.MyClass at 0x0000000002A44D68> <=== Why different from type(myObj) ?
It seems Python has some extra indirection between a class and its instance type.
I am used to C#. In C#, typeof(MyClass)
will just return the MyClass
.
Below is some comparison between 2.7.6 and 3.4.1.
I am wondering how the ==
operator is implemented in Python.
This is because you're using old-style classes . Instead of doing:
class MyClass:
pass
You need to do:
class MyClass(object):
pass
...in order to use new-style classes. Now, if you do type(myObj)
, you get back <class '__main__.MyClass'>
as expected.
In fact, one of the major reasons why Python introduced new-style classes was exactly because of the problem you observed:
New-style classes were introduced in Python 2.2 to unify classes and types. A new-style class is neither more nor less than a user-defined type. If x is an instance of a new-style class, then
type(x)
is typically the same asx.__class__
(although this is not guaranteed - a new-style class instance is permitted to override the value returned forx.__class__
).The major motivation for introducing new-style classes is to provide a unified object model with a full meta-model. It also has a number of practical benefits, like the ability to subclass most built-in types, or the introduction of “descriptors”, which enable computed properties.
( source )
This is a bit of a kludge, having to extend object
, but thankfully in Python 3, old-style classes were removed altogether so declaring the class using either forms does the same thing.
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