how to remove 50000 elements from a 100000 list?

[Ju Hui]

I want to remove about 50000 elements from a list,which has 100000
elements.
sample code like below:

>>> a=range(10)
>>> b=range(4)
>>> for x in b:
...     a.remove(x)
...
>>> a
[4, 5, 6, 7, 8, 9]

when a and b is small size, it will finished quickly, but when a and b
have many elements.
such as:
>>> a=range(100000)
>>> b=range(50000)
>>> for x in b:
...     a.remove(x)
...
it will very slowly. Shall I change to another data structure and choos
a better arithmetic?
any suggestion is welcome.
thanks a lot!