Should all Python classes extend object?

In Python 2, not inheriting from object will create an old-style class, which, amongst other effects, causes type to give different results:

>>> class Foo: pass
... 
>>> type(Foo())
<type 'instance'>

vs.

>>> class Bar(object): pass
... 
>>> type(Bar())
<class '__main__.Bar'>

Also the rules for multiple inheritance are different in ways that I won't even try to summarize here. All good documentation that I've seen about MI describes new-style classes.

Finally, old-style classes have disappeared in Python 3, and inheritance from object has become implicit. So, always prefer new style classes unless you need backward compat with old software.

In Python 3, classes extend object implicitly, whether you say so yourself or not.

In Python 2, there's old-style and new-style classes. To signal a class is new-style, you have to inherit explicitly from object. If not, the old-style implementation is used.

You generally want a new-style class. Inherit from object explicitly. Note that this also applies to Python 3 code that aims to be compatible with Python 2.