Does python optimize modules when they are imported multiple times?

import MyLib
ReallyBigLib = MyLib.SomeModule.ReallyBigLib

import MyLib
import ReallyBigLib

import MyLib
import ReallyBigLib

# module/file a.py
   print "Hello from a.py!"
   import b

   # module/file b.py
   print "Hello from b.py!"
   import a

>>> import b
Hello from b.py!
Hello from a.py!
>>> import a
>>>