Question

Some code.

In [1]: A = type('B', (), {})

In [2]: a = A()

In [3]: b = B()
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)

/home/shabda/<ipython console> in <module>()

NameError: name 'B' is not defined

What does first argument to type doing here?

Answer 1

It’s setting the __name__ property of the created class.

When you say:

class B(object):

two things happen with that ‘B’:

• The name ‘B’ is assigned the class. This is just like if you’d said “B = ...“.

• The __name__ property of the class is set to ‘B’.

When you invoke the type constructor manually only the latter half is done. If you don’t assign the result to ‘B’ then B will remain set (or unset) as it was before.

Note that a similar result could be obtained by saying:

class B(object):
    pass
A = B
del B

Now A refers to a class that calls itself ‘B’, and B doesn’t refer to anything.