Why is creating a class in Python so much slower than instantiating a class?

Question

I found that creation of a class is way slower than instantiation of a class.

>>> from timeit import Timer as T >>> def calc(n): ...     return T("class Haha(object): pass").timeit(n)  <<After several these 'calc' things, at least one of them have a big number, eg. 100000>>  >>> calc(9000) 15.947055101394653 >>> calc(9000) 17.39099097251892 >>> calc(9000) 18.824054956436157 >>> calc(9000) 20.33335590362549 

Yeah, create 9000 classes took 16 secs, and becomes even slower in the subsequent calls.

And this:

>>> T("type('Haha', b, d)", "b = (object, ); d = {}").timeit(9000) 

gives similar results.

But instantiation don't suffer:

>>> T("Haha()", "class Haha(object): pass").timeit(5000000) 0.8786070346832275 

5000000 instances in less than one sec.

What makes the creation this expensive?

And why the creation process become slower?

EDIT:

How to reproduce:

start a fresh python process, the initial several "calc(10000)"s give a number of 0.5 on my machine. And try some bigger values, calc(100000), it can't end in even 10secs, interrupt it, and calc(10000), gives a 15sec.

EDIT:

Additional fact:

If you gc.collect() after 'calc' becomes slow, you can get the 'normal' speed at beginning, but the timing will increasing in subsequent calls

>>> from a import calc >>> calc(10000) 0.4673938751220703 >>> calc(10000) 0.4300072193145752 >>> calc(10000) 0.4270968437194824 >>> calc(10000) 0.42754602432250977 >>> calc(10000) 0.4344758987426758 >>> calc(100000) ^CTraceback (most recent call last):   File "<stdin>", line 1, in <module>   File "a.py", line 3, in calc     return T("class Haha(object): pass").timeit(n)   File "/usr/lib/python2.7/timeit.py", line 194, in timeit     timing = self.inner(it, self.timer)   File "<timeit-src>", line 6, in inner KeyboardInterrupt >>> import gc >>> gc.collect() 234204 >>> calc(10000) 0.4237039089202881 >>> calc(10000) 1.5998330116271973 >>> calc(10000) 4.136359930038452 >>> calc(10000) 6.625348806381226 

Answer 1

This might give you the intuition:

>>> class Haha(object): pass ... >>> sys.getsizeof(Haha) 904 >>> sys.getsizeof(Haha()) 64 

Class object is much more complex and expensive structure than an instance of that class.